- Open Access
Offspring pay sooner, parents pay later: experimental manipulation of body mass reveals trade-offs between immune function, reproduction and survival
© Hegemann et al.; licensee BioMed Central Ltd. 2013
- Received: 11 October 2013
- Accepted: 13 December 2013
- Published: 17 December 2013
Life-history theory predicts that organisms trade off survival against reproduction. However, the time scales on which various consequences become evident and the physiology mediating the cost of reproduction remain poorly understood. Yet, explaining not only which mechanisms mediate this trade-off, but also how fast or slow the mechanisms act, is crucial for an improved understanding of life-history evolution. We investigated three time scales on which an experimental increase in body mass could affect this trade-off: within broods, within season and between years. We handicapped adult skylarks (Alauda arvensis) by attaching extra weight during first broods to both adults of a pair. We measured body mass, immune function and return rates in these birds. We also measured nest success, feeding rates, diet composition, nestling size, nestling immune function and recruitment rates.
When nestlings of first broods fledged, parent body condition had not changed, but experimental birds experienced higher nest failure. Depending on the year, immune parameters of nestlings from experimental parents were either higher or lower than of control nestlings. Later, when parents were feeding their second brood, the balance between self-maintenance and nest success had shifted. Control and experimental adults differed in immune function, while mass and immune function of their nestlings did not differ. Although weights were removed after breeding, immune measurements during the second brood had the capacity to predict return rates to the next breeding season. Among birds that returned the next year, body condition and reproductive performance a year after the experiment did not differ between treatment groups.
We conclude that the balance between current reproduction and survival shifts from affecting nestlings to affecting parents as the reproductive season progresses. Furthermore, immune function is apparently one physiological mechanism involved in this trade-off. By unravelling a physiological mechanism underlying the trade-offs between current and future reproduction and by demonstrating the different time scales on which it acts, our study represents an important step in understanding a central theory of life-history evolution.
- Cost of reproduction
- Life history
- Carry-over effect
The trade-off between current and future reproduction is central in life-history theory [1, 2] and has been documented for many taxa including insects, fishes, reptiles, birds and mammals [3–5]. This trade-off can have consequences on different time scales, quantified mainly in studies on birds. For example, manipulating this trade-off via reproductive effort can directly affect nestlings and lead to reduced mass gain or increased mortality [6, 7]. However, effects on the manipulated adults might develop more slowly and may become visible only after the breeding season . Increased adult mortality often occurs in the subsequent winter [9–13]. Several physiology systems have been suggested to mediate the cost of current reproduction, especially the immune system may be an important mechanism [14, 15], but unequivocal evidence is still lacking. Understanding not only which mechanisms mediate trade-offs, but also how fast or slow the mechanisms act, is crucial for an improved understanding of life-history evolution.
Despite the evidence that consequences of a shift in the trade-off between reproduction and self-maintenance can occur on different time scales, apparently no single study has investigated the underlying physiological mechanisms at multiple time levels. Likewise, no experimental study of the trade-off between reproduction and self-maintenance has linked changes in immune function to subsequent survival probabilities in both adults and their offspring. Many studies on the trade-off between reproduction and self-maintenance focus only on one time point: current reproduction [16–19]. A few studies include parameters from a second time point, which are typically reproductive parameters of subsequent reproductive attempts [20–22] or adult condition and performance parameters in the following year [23, 24]. Changes in parental effort that affect future survival probabilities [9, 23] may be mediated by changes in immune function. Trade-offs between reproduction and immune function are well established [15, 24–28], and increased parasite infection rates in birds raising enlarged broods have also been described [14, 15, 28].
Studying the costs of reproduction and the underlying mechanisms requires an experimental approach. One way to influence the costs of reproduction involves manipulating the costs of locomotion (e.g. walking and flying) [16, 18]. For example, handicapping birds with extra weight leads to increased locomotion costs [29–31]. Manipulating costs of locomotion might also affect investment in other physiological systems, such as the immune system, which has its own energetic demands . Modulations of immune function by birds during periods of high locomotory costs [33, 34] and of intense locomotory activity [34–37] are well established. Hence, manipulating locomotion costs of breeding birds provides the opportunity to study the balance between reproductive investment and self-maintenance with a consideration of possible immunological mechanisms.
We present a comprehensive immunological and behavioural dataset on skylarks (Alauda arvensis) with the aim of understanding trade-offs between parental investment in reproduction and self-maintenance along a time axis. We manipulated movement costs in free-living birds by handicapping them with extra weight, and we measured a variety of fitness-related parameters over three different time scales: a) the short-term effects within a breeding attempt, b) the medium-term effects on second broods within the same season and c) after removing the extra weight, the carry-over effects on return rates, immune function and reproduction in the subsequent year. We measured multiple immunological indices in the parents to quantify investment into self-maintenance at each of these time points and to correlate these with future return rates. We quantified current reproduction by measuring number and size of offspring. To explore whether nestlings differed beyond size and fledging rate, we also quantified parameters related to nutrition (feeding rates, diet composition), immune function and recruitment. We expected handicapped adults either to reduce investment in immune function, which might impair survival, or to reduce investment in reproduction, which might hinder nestling quality and recruitment. Within control birds, we did not expect a shift in parental investment from first to second broods, because in skylarks there is no clear trend for early- or late-born nestlings having different fitness benefits (Hegemann et al. unpublished data).
The short-term handicap had moderate effects on nest success measures. Control nests had a success rate of 76% (19 out of 25) compared with 47% (8 out of 17) for experimental nests, but this difference was borderline non-significant (χ 2 = 3.69, P = 0.055). As a consequence control pairs produced more fledglings (2.0 fledglings/control nest, 1.2 fledglings/experimental nest; χ 2 = 4.14, n = 39, P = 0.042). Restricting the comparison to successful nests only, we found no difference in fledgling numbers between treatment groups; both produced on average 2.6 fledglings (n = 19 control nests, 8 experimental nests; χ 2 = 0.02, P = 0.89). Control nests produced 0.22 recruits per fledgling (n = 13 recruits) and experimental nests 0.11 (n = 3 recruits) (χ 2 = 0.82, P = 0.37). Feeding rates equalled 9.9 ± 1.34 visits per hour in the control (n = 11 nests) and 11.8 ± 2.20 visits per hour in the experimental group (n = 9 nests; χ 2 = 0.50, P = 0.48). We found no significant differences between treatment groups in size (F1,13 = 0.51, P = 0.49, Figure 1I), number (χ 2 = 0.43, P = 0.57) or diversity (χ 2 = 0.25, P = 0.61) of prey items fed to nestlings, but the proportion of the main prey item (beetles, order Coleoptera) was significantly lower in the diet of experimental nestlings than in the diet of control nestlings (χ 2 -test, χ 2 = 6.9, P = 0.008, Figure 1J).
The short-term handicap impacted nestling quality. Nestlings raised by experimental parents had higher agglutination titres and higher haptoglobin concentrations than those raised by control parents in 2007, but this pattern was reversed in 2008 (year*treatment interaction χ 2 = 4.84, P = 0.028, Figure 1G and χ 2 1 = 4.05, P = 0.044; n = 62 nestlings, Figure 1H). Nestlings of experimental parents were 7.9% lighter than control nestlings (Figure 1F), but this difference – consistent in 2007 and 2008 – was not significant (χ 2 = 1.66, n = 69, P = 0.19).
The probability of nest success during the second broods differed between treatment groups depending on year. In 2007, 78% of control nests were successful compared with 25% of experimental nests. In 2008, 62% of control and 87% of experimental nests were successful (interaction year*treatment χ2 = 4.52, P = 0.033). Restricted to successful nests, number of fledglings did not differ between control (3.3 fledglings/successful nest, n = 12 nests) and experimental nests (3.4 fledglings/successful nest, n = 8 nests) (χ 2 = 0.11, P = 0.74). The number of recruits per fledgling was 0.10 for control nests and 0.15 for experimental nests, a non-significant difference (χ 2 = 0.41, n = 17, P = 0.52). The droppings of experimental nestlings contained remains of longer animal prey than control groups, a non-significant trend (F1,13 = 4.40, P = 0.056, Figure 2I). The number of animals (F1,13 = 1.81, P = 0.20), the diversity of prey (F1,13 = 2.28, P = 0.13) and the proportion of the main prey item (beetles, order Coleoptera) did not differ between groups (χ 2 -test, χ 2 = 2.5, P = 0.12, Figure 2J).
Body mass (χ 2 = 0.89, n = 52, P = 0.34), agglutination titre (χ 2 = 0.60, n = 53 P = 0.44) and haptoglobin concentration (χ 2 = 0.05, n = 53 P = 0.82) of nestlings did not differ between treatments (Figure 2F-H).
Carry-over effects on body mass, immune parameters and reproductive measures in the year following the experiment
Control mean ± se
Experimental mean ± se
N (Control/ Experimental)
Body mass (g)
32.5 ± 1.3
34.6 ± 1.1
0.45 ± 0.23
1.54 ± 0.43
3.8 ± 0.2
4.5 ± 0.4
0.33 ± 0.04
0.39 ± 0.03
0.27 ± 0.06
0.61 ± 0.23
17.2 ± 2.4
24.6 ± 5.7
67 ± 5.6
50 ± 6.5
4.6 ± 2.2
5.2 ± 0.7
11.2 ± 3.5
13.2 ± 3.2
3.4 ± 0.40
3.75 ± 0.16
Nestling body mass
23.1 ± 0.55
22.4 ± 0.52
Nestling agglutination (titer)
2.0 ± 0.45
1.6 ± 0.36
Nestling haptoglobin (mg/ml)
0.23 ± 0.01
0.24 ± 0.02
Prediction of survival by immune function
Skylarks handicapped by an extra weight modulated the trade-off between parental effort and investment into immune function differently at different time scales. During first broods adults maintained their condition and the costs were paid by the offspring. During the second brood, after birds carried their extra weight for several weeks, the costs were shifted to the adults, affecting their body condition and their return rates to the following breeding season. The costs on reproduction during first broods were expressed by fewer successful breeding attempts of experimental pairs. Furthermore, experimental nestlings showed altered immune parameters which coincided with a different diet they received. These nestlings also had lower recruitment rates, but the difference was not significant and sample sizes were small. During second broods, handicapped adults invested similar into reproduction than control birds. They brought a similar diet to their nestlings and likewise, the immune function of their nestlings did not differ from control nestlings. Instead, adults paid the costs, reflected in changes in their immune function. After removal of the handicap, adult return rate was 12.5% lower, but again this was not significant and sample sizes were modest. However, split up by treatment group, immune parameters measured when handicaps were removed from experimental birds predicted local survival. This suggests that reduced return rates and changes in immune function are linked. In the breeding season one year after the experiment, the returning birds no longer differed by treatment group in terms of immune parameters or reproductive performance.
After attachment of the extra weights, experimental birds faced higher nest failure rates, their fledglings expressed altered immune responses and these fledglings were (statistically insignificant) less likely to be detected as recruits. However, we found no effects on the immune system or body mass in adults over this short term. This indicates that during first broods, skylarks shift the costs of increased work load onto the nestlings. Such a pattern has been described for several species [6, 7, 38, 39] but is generally associated with long-lived rather than short-lived species [2, 7]. However, we cannot exclude the possibility that either restricted sample sizes or the short handicap period also contributed to the lack of effects on adults during within-broods measurements.
By their second brood, handicapped adult skylarks modulated several of their own immune indices, but their parental effort was not different from controls. This result suggests that the costs shifted back to the parents while parental effort and thus nestling condition was maintained. To our knowledge, our study is the first to document a shift in the trade-off between reproduction and self-maintenance from first to second reproductive attempt of a season and reflected by physiological changes. In adult skylarks lysis titres increase in the course of the breeding season , but handicapped birds were apparently not able to raise their complement activity as much as control birds. This suggests that birds reduced their investment in immune function after we experimentally increased their costs of reproduction. The effect of our experimental manipulation on haptoglobin concentration was sex-specific. Across our skylark population, haptoglobin remains constant over the breeding season . Males are highly aggressive against neighbouring males. Carrying an extra weight is expected to decrease manoeuvrability , and consequently handicapped males might be less competitive and may suffer from more injuries than control males. Injuries usually cause an inflammation and haptoglobin levels in skylarks decrease following an inflammatory response . This may explain why experimental males showed decreased haptoglobin concentrations after carrying an extra weight.
Our results show that the increased locomotory costs during reproduction and the lowered investment into immune function have carry-over effects that relate to return probabilities for both adults and their offspring. Based on modest sample sizes, we found only insignificant trends towards reduced local survival in adults and reduced local recruitment rates in their fledglings. But in adults, return rates could be predicted by immune parameters measured at the end of the breeding season. Trade-offs between reproduction and immune investment [25–28, 42], and links between immune function and survival [43, 44] are well documented. However, studies linking trade-offs between reproduction and immune function with subsequent survival were missing so far. We show that skylarks modulated immune parameters when costs of reproduction increased and these immune parameters relate to subsequent return rates. This also builds a case that we measured true survival rather than dispersal, especially given that skylarks anyhow show hardly any breeding dispersal [45, 46]. Thus, our findings build on the results of Daan et al. , who demonstrated that kestrels (Falco tinnuculus) show increased mortality during winter rather than emigration after having raised experimentally enlarged broods. Survival in these kestrels was related to energy expenditure during breeding and this result led to the hypothesis that increased work load might cause a “physiological weakening” mediated by reductions in immune function . While their study lacked a mechanistic link, we provide evidence that changes in immune defences may act as a mediator. One year after our manipulations, we did not find any immunological or reproductive effects of the experiment in the surviving birds, while such carry-over effects are known for other species .
Experimental manipulations of parental effort often have no effect on offspring body mass or structural size [16, 19, 48]. These negative results are typically interpreted as maintenance of current parental effort. In our study, feeding rates of nestlings and body mass of fledglings did not differ between treatment groups. Despite this, nestlings did differ in terms of immunological indices, which suggest adjustments in parental effort. One mediator may be diet, since the immune system and its development require energy and specific nutrients . Indeed, we found that handicapped parents brought a modified diet to their nestlings. During first broods, when nestlings had altered immune function, the diet of experimental nestlings contained a significant lower proportion of beetles, their main food item. This suggests that these adults were less selective when collecting food. This change in adult behaviour coincides with a change in nestling immune function. In second broods the proportion of the main food type did not differ anymore and neither did nestling immune function. This strongly suggests that the species composition of nestling diet is an important factor shaping nestling immune function. Changes in foraging behaviour have been described previously for manipulated birds [17, 49–52], but clear links to the physiology of the nestlings have remained elusive. We shed light on these links by showing that dietary differences correlate with immunological effects and lowered recruitment rates.
We demonstrated that skylarks modulate the trade-off between current reproduction and survival differently over short-, medium- and long-time scales. Further we provided evidence that investment into the immune system is one physiological mechanism that mediates survival in adults and recruitment of their offspring. Our study represents an important step in understanding physiological mechanisms underlying the trade-offs between current and future reproduction, and thus adds to our understanding of life-history evolution.
Birds and experimental treatments
We studied skylarks in the Aekingerzand, the Netherlands (N 52°55′; E 6°18′) in 2007, 2008 and 2009 using a colour-ringed study population [34, 45]. The skylark is a temperate zone passerine that breeds on the ground. Each pair starts 2-5 breeding attempts per year between the end of April and the end of July to compensate high nest predation rates [34, 53, 54]. The rate of breeding failure is high; consequently most pairs have only zero, one, or two successful broods per year (three successes are exceptionally rare, Hegemann et al. unpublished). It is only possible to reliable catch both parents of a pair when they are feeding nestlings.
To initiate the experiment, adults were caught when feeding nestlings (mean = 3 days old, range: 1-8 days old) during the first half of the breeding season (21-April-2007- 31-May-2007 and 04-May-2008 – 10-June-2008). We refer to the data collected at this initial capture as baseline values and to the breeding attempt as first broods. We cannot exclude that single pairs initiated an earlier breeding attempt that failed during the egg stage and before we found the nest. However, we are confident, that all pairs had no earlier nest containing nestlings because feeding behaviour is more obvious to detect. As pairs were assigned alternately to control and experimental groups, an earlier failed nesting attempt should not introduce any bias to our experiment. We attached an extra weight to experimental birds with a figure-eight harness made of elastic cotton thread [31, 55] before release. Ranging from 3.0 to 3.9 g (total weight), the extra weight equalled 10% of an individual’s body mass. Experimental birds were handicapped by the combined effects of carrying the extra weight and wearing the harness; control birds remained without harness or extra weight.
We included experimental birds and their nests in this study only when both parents of a nest received a handicap to avoid the possibility that an unhandicapped partner compensated for a handicapped one [18, 56]. For controls we included birds when we caught both parents of a nests (n = 8 nests) and also birds and their nests when only one parent was captured (n = 8 nests). We have no indication that capture, blood sampling and ringing (the only procedures imposed on control birds) had an effect on adult behaviour and nestling provisioning. Nestlings from nests with only one parent captured did not differ from nestlings where both parents were caught (nestling body mass: 21.7 ± 3.5 versus 21.1 ± 4.1 g, P = 0.66; agglutination: 2.5 ± 2.8 versus 2.2 ± 2.1 titres, P = 0.99; haptoglobin: 0.26 ± 0.11 versus 0.29 ± 0.14 mg/ml, P = 0.29). Thus, inclusion of controls for which only one parent was caught should not substantively bias our results, but this inclusion will increase the robustness of our conclusions through an increased sample size.
After initiation of the experiment, adult birds were part of up to three different data subsets to measure effects of the handicap on the trade-off between reproduction and self-maintenance over different time scales (within-brood, within-season, carry-over). The first data subset, which was used to evaluate the short-term effects of the handicap, included adults that we sampled before their nestlings were 3 days old and that we resampled when the offspring were 7-11 days old (“within-brood” measurements, n = 6 experimental, n = 8 controls). Nestlings leave the nest when 8 or 9 days old and will be fed by the parents until about 30 days old.
To evaluate the longer-term effects of the handicap, we recaptured and resampled 16 control and 16 experimental birds approximately 5 weeks (control birds: median = 39.5 days, range: 28-73 days; experimental birds: median = 35 days, range 27-52 days) after the first capture and when they were feeding nestlings of their second brood (“within-season” measurements). Upon that capture, we removed the extra weight of experimental birds. The cryptic behaviour of skylarks and their well-hidden nests mean that nests depredated at the egg stage may have been missed. However, we are confident that we found all successful nests of our focal birds, as feeding events are more obvious. We found a second nest for 66% of all birds. The chance to find a second nest did not differ between treatment groups (χ 2 = 0.46, N = 43, P = 0.50) or years (χ 2 = 0.05, N = 42, P = 0.83).
To evaluate carry-over effects of the handicap on survival to the breeding season following the treatment (2008 and 2009, respectively), we examined return rates of adults (to estimate survival) and young (to estimate recruitment) by ring reading. Both natal and breeding dispersal is very limited in skylarks . To further evaluate the returning birds (for all of which we also had within-season measurements), we measured reproductive output (see below), and in those birds that were successfully recaptured (5 control, 6 experimental), we re-measured body mass and immune parameters. Two experimental and two control pairs stayed together from one breeding season to the next; all other birds had a new partner.
Sample and data collection
Adults were sampled upon each capture, and nestlings were sampled around 8 days of age. Blood samples (~100-150 μl from adults, ~ 70-100 μl from nestlings) were collected into heparinised capillary tubes from the brachial vein. Adults were bled directly after capture (median: 5 min; range: 3-15 min) and before any impacts of handling stress on immune parameters are expected [57, 58]. Blood smears for leukocyte enumeration (adults only) were made from a drop of fresh blood. The remaining blood was stored on ice until centrifuged in the lab (10 min, 7000 rpm). Plasma was frozen for future analyses. Structural measurements and body mass were recorded after blood collection, and birds were ringed with metal and colour rings. Adult birds were sexed biometrically, nestlings were sexed molecularly .
We measured three general categories of immune defence. We used plasma to quantify titres of complement-like lytic enzymes (lysis) and non-specific natural antibodies (agglutination) [34, 60]. Blood of 8-day-old nestlings did not show lytic activity (Hegemann et al. unpublished). We used a functional assay to measure haptoglobin-like activity (hereafter haptoglobin in mg/ml) [34, 61]. In skylarks, haptoglobin decreases following an immune challenge . Leukocyte proportions (lymphocytes, heterophils, basophils, monocytes or eosinophils) based on the first 100 white blood cells (WBC) were determined from blood smears by one person (C. Gotteland), who was blind to year and treatment. Leukocyte proportions reflect both innate and acquired components and change in response to immunological stimulation . Analyses of leukocyte profiles include the ratio of heterophils and lymphocytes (H/L ratio) which is related to different types of stressors, including immunological ones . In most blood smears (61%) no basophils were detected, so we did not analyse this cell type. We took biological and methodological factors into consideration when choosing to focus mainly on measures of innate immunity: This sub-system is an important first line of defence , and this importance might translate into consistency over longer time scales, a point that coordinates with our main hypothesis. Additionally, while measures of innate immunity can vary over shorter scales (e.g. reflecting current “health status” or “physiological condition,” , the absence of immunological memory in vertebrate innate sub-systems allows for interpretation of repeated samples without confounding the magnitude of an index and the exposure to a particular disease [66, 67].
Nest success rates (at least one fledged nestling vs. nest failure) and number of fledglings were recorded on day 8. After ringing nestlings leave the nest. We measured feeding rates on first broods in 2008 (n = 14 nests) by observing nests with binoculars for one hour in the morning. Feeding rates (number of feeding events/hour) were measured when nestlings were 4 days (n = 5 control nests, 4 experimental nests) and 6-7 days (n = 6 control nests, 5 experimental nests) old.
Skylark nestlings usually defecate during ringing. To analyse nestling diet, we collected these droppings per nest, preserved them with table salt and froze them until analyses. Droppings of 27 nests (first broods: 9 control, 3 experimental; second broods: 9 control, 6 experimental) were analysed  by H.F., who was blind to brood and treatment. We summarized the dropping analyses in three variables per nest: number of animal prey individuals, average length of animal prey, and number of different prey types. We also compared the proportion of the main food type, beetles (order Coleoptera), between treatment groups. Animal prey length (reflecting biomass) was estimated from prey remains using a reference collection and information from literature [68–71].
We analysed data using R version 2.14.0 . A detailed description of statistical methods can be found in Additional file 2 in the supporting information. Here, we give a brief summary of all statistical tests. For within-brood and within-season measurements, we used linear models and the differences between the two measurements as the dependent variables. We preferred calculating the difference between time points over using a repeated design in a mixed model, because the latter treats both time points equal, while we are specifically interested in the change of each response variable during the experiment. We included the baseline values of the corresponding response variable as a covariate to account for potentially different starting points among individuals. Including nest as a random effect (to account for possible non-independence of pair members) did not significantly improve the fit of any starting model (always p > 0.54), thus we decided for the simpler and hence more powerful linear models without nest as random effect. Nest success rates, number of fledglings and number of recruits per fledgling were analysed on the nest level with generalized linear models. Body mass and immune parameters of nestlings were analysed with linear mixed models, and feeding rates were analysed with generalized linear mixed models with a Poisson error structure, all including nest as a random effect to account for non-independence of siblings. Return rates of adults were analysed with generalized linear models with binomial error structure. We tested if returning could be predicted by any measurement at the end of the experiment (removal of weights from experimental birds). We did this by sequentially including the interaction between treatment and each response variable.
We included, when applicable, the following variables in each model: treatment, year, sex, baseline value, age of nestlings, number of nestlings and length of experiment (number of days between measurements). We also included two-way-interactions involving treatment. We simplified the starting models using backward elimination based on likelihood-ratio tests and F-statistics (Chisq-statistics for generalized linear models with binomial or poisson error structure and for mixed models) and with P < 0.05 as the selection criterion (“drop1”-function of R) until reaching the final model with only significant terms. Assumptions of all models were checked on the residuals of the final model. We report interactions only when significant. Full statistics of all main effects can be found in Additional file 1: Tables S1-S3. Treatment groups did not differ by chance in brood size, body mass or any immune parameter at the initiation of the experiment (always P > 0.21).
The study was performed under license D4743A and DEC5219C of the Institutional Animal Care and Use Committee of the University of Groningen.
We thank T. Piersma, C. Both and the late R. Drent for discussions about experimental design, R. Voesten for field work, M. van der Velde for molecular sexing, C. Gotteland and E. Gilot for slide counts, K. Meirmans and S. Wallert for feeding observations, M.A. Versteegh, I.R. Pen for advice on statistics and D. Visser for help with the graphic design of figures. S. Verhulst, C. Both, two anonymous reviewers and the editor T. Price provided useful comments on earlier drafts. Staatsbosbeheer Drents-Friese Wold kindly allowed working in their area. Financial support came from BirdLife Netherlands (BIT), a Rosalind Franklin Fellowship (BIT), the Netherlands Organization for Scientific Research (BIT: 863.04.023, KDM: 863.08.026), Schure-Beijerinck-Popping Fonds (AH) and the Deutsche Ornithologen-Gesellschaft (AH).
- Stearns SC: The evolution of life histories. 1992, Oxford: Oxford University PressGoogle Scholar
- Williams GC: Natural selection costs of reproduction and a refinement of lacks principle. Am Nat. 1966, 100: 687-690. 10.1086/282461.View ArticleGoogle Scholar
- Harshman LG, Zera AJ: The cost of reproduction: the devil in the details. Trends Ecol Evol. 2007, 22: 80-86. 10.1016/j.tree.2006.10.008.View ArticlePubMedGoogle Scholar
- Magnhagen C: Predation risk as a cost of reproduction. Trends Ecol Evol. 1991, 6: 183-186. 10.1016/0169-5347(91)90210-O.View ArticlePubMedGoogle Scholar
- Koivula M, Koskela E, Mappes T, Oksanen T: Cost of reproduction in the wild: Manipulation of reproductive effort in the bank vole. Ecology. 2003, 84: 398-405. 10.1890/0012-9658(2003)084[0398:CORITW]2.0.CO;2.View ArticleGoogle Scholar
- Moreno J, Merino S, Potti J, de León A, Rodríguez R: Maternal energy expenditure does not change with flight costs or food availability in the pied flycatcher (Ficedula hypoleuca): costs and benefits for nestlings. Behav Ecol Sociobiol. 1999, 46: 244-251. 10.1007/s002650050616.View ArticleGoogle Scholar
- Mauck RA, Grubb TC: Petrel parents shunt all experimentally increased reproductive costs to their offspring. Anim Behav. 1995, 49: 999-1008. 10.1006/anbe.1995.0129.View ArticleGoogle Scholar
- Nilsson JA, Svensson E: The cost of reproduction: A new link between current reproductive effort and future reproductive success. Proc R Soc B. 1996, 263: 711-714. 10.1098/rspb.1996.0106.View ArticleGoogle Scholar
- Daan S, Deerenberg C, Dijkstra C: Increased daily work precipitates natural death in the kestrel. J Anim Ecol. 1996, 65: 539-544. 10.2307/5734.View ArticleGoogle Scholar
- Verhulst S: Multiple breeding in the Great Tit, II. The costs of rearing a second clutch. Funct Ecol. 1998, 12: 132-140. 10.1046/j.1365-2435.1998.00165.x.View ArticleGoogle Scholar
- Dijkstra C, Bult A, Bijlsma S, Daan S, Meijer T, Zijlstra M: Brood size manipulations in the kestrel (falco tinnunculus) - effects on offspring and parent survival. J Anim Ecol. 1990, 59: 269-285. 10.2307/5172.View ArticleGoogle Scholar
- Golet G, Irons D, Estes J: Survival costs of chick rearing in black-legged kittiwakes. J Anim Ecol. 1998, 67: 827-841. 10.1046/j.1365-2656.1998.00233.x.View ArticleGoogle Scholar
- Stjernman M, Raberg L, Nilsson JA: Survival costs of reproduction in the blue tit (Parus caeruleus): a role for blood parasites?. Proc R Soc B. 2004, 271: 2387-2394. 10.1098/rspb.2004.2883.PubMed CentralView ArticlePubMedGoogle Scholar
- Gustafsson L, Nordling D, Andersson M, Sheldon B, Qvarnstroma A: Infectious diseases, reproductive effort and the cost of reproduction in birds. Philos Trans R Soc Lond B. 1994, 346: 323-331. 10.1098/rstb.1994.0149.View ArticleGoogle Scholar
- Norris K, Anwar M, Read AF: Reproductive effort influences the prevalence of haematozoan parasites in great tits. J Anim Ecol. 1994, 63: 601-610. 10.2307/5226.View ArticleGoogle Scholar
- Tieleman BI, Dijkstra TH, Klasing KC, Visser GH, Williams JB: Effects of experimentally increased costs of activity during reproduction on parental investment and self-maintenance in tropical house wrens. Behav Ecol. 2008, 19: 949-959. 10.1093/beheco/arn051.View ArticleGoogle Scholar
- Beaulieu M, Spee M, Lazin D, Ropert-Coudert Y, le Maho Y, Ancel A, Raclot T: Ecophysiological response of Adelie penguins facing an experimental increase in breeding constraints. J Exp Biol. 2010, 213: 33-39. 10.1242/jeb.035378.View ArticlePubMedGoogle Scholar
- Harding AMA, Kitaysky AS, Hamer KC, Hall ME, Welcker J, Talbot SL, Karnovsky NJ, Gabrielsen GW, Gremillet D: Impacts of experimentally increased foraging effort on the family: offspring sex matters. Anim Behav. 2009, 78: 321-328. 10.1016/j.anbehav.2009.05.009.View ArticleGoogle Scholar
- Leclaire S, Helfenstein F, Degeorges A, Wagner RH, Danchin E: Family size and sex-specific parental effort in black-legged kittiwakes. Behaviour. 2010, 147: 1841-1862. 10.1163/000579510X538872.View ArticleGoogle Scholar
- Tinbergen JM: Costs of reproduction in the great Tit - intraseasonal costs associated with brood size. Ardea. 1987, 75: 111-122.Google Scholar
- Lindén M: Reproductive trade-Off between 1st and 2nd clutches in the great Tit parus major - an experimental-study. Oikos. 1988, 51: 285-290. 10.2307/3565309.View ArticleGoogle Scholar
- Smith H, Kallander H, Nilsson J: Effect of experimentally altered brood size on frequency and timing of 2nd-clutches in the great Tit. Auk. 1987, 104: 700-706.View ArticleGoogle Scholar
- Wernham CV, Bryant DM: An experimental study of reduced parental effort and future reproductive success in the puffin, Fratercula arctica. J Anim Ecol. 1998, 67: 25-40. 10.1046/j.1365-2656.1998.00166.x.View ArticleGoogle Scholar
- Hanssen SA, Hasselquist D, Folstad I, Erikstad KE: Cost of reproduction in a long-lived bird: incubation effort reduces immune function and future reproduction. Proc R Soc B. 2005, 272: 1039-1046. 10.1098/rspb.2005.3057.PubMed CentralView ArticlePubMedGoogle Scholar
- Deerenberg C, Arpanius V, Daan S, Bos N: Reproductive effort decreases antibody responsiveness. Proc R Soc B. 1997, 264: 1021-1029. 10.1098/rspb.1997.0141.PubMed CentralView ArticleGoogle Scholar
- Nordling D, Andersson M, Zohari S, Gustafsson L: Reproductive effort reduces specific immune response and parasite resistance. Proc R Soc B. 1998, 265: 1291-1298. 10.1098/rspb.1998.0432.PubMed CentralView ArticleGoogle Scholar
- Hasselquist D, Wasson MF, Winkler DW: Humoral immunocompetence correlates with date of egg-laying and reflects work load in female tree swallows. Behav Ecol. 2001, 12: 93-97. 10.1093/oxfordjournals.beheco.a000384.View ArticleGoogle Scholar
- Knowles SCL, Nakagawa S, Sheldon BC: Elevated reproductive effort increases blood parasitaemia and decreases immune function in birds: a meta-regression approach. Funct Ecol. 2009, 23: 405-415. 10.1111/j.1365-2435.2008.01507.x.View ArticleGoogle Scholar
- Kvist A, Lindstrom A, Green M, Piersma T, Visser GH: Carrying large fuel loads during sustained bird flight is cheaper than expected. Nature. 2001, 413: 730-732. 10.1038/35099556.View ArticlePubMedGoogle Scholar
- Engel S, Biebach H, Visser GH: Metabolic costs of avian flight in relation to flight velocity: a study in Rose Coloured Starlings (Sturnus roseus). J Comp Physiol B. 2006, 176: 415-427. 10.1007/s00360-006-0063-1.View ArticlePubMedGoogle Scholar
- Schmidt-Wellenburg CA, Engel S, Visser GH: Energy expenditure during flight in relation to body mass: effects of natural increases in mass and artificial load in Rose Coloured Starlings. J Comp Physiol B. 2008, 178: 767-777. 10.1007/s00360-008-0267-7.PubMed CentralView ArticlePubMedGoogle Scholar
- Klasing KC: The costs of immunity. Acta Zool Sinica. 2004, 50: 961-969.Google Scholar
- Moreno-Rueda G: Experimental test of a trade-off between moult and immune response in house sparrows Passer domesticus. J Evol Biol. 2010, 23: 2229-2237. 10.1111/j.1420-9101.2010.02090.x.View ArticlePubMedGoogle Scholar
- Hegemann A, Matson KD, Both C, Tieleman BI: Immune function in a free-living bird varies over the annual cycle, but seasonal patterns differ between years. Oecologia. 2012, 170: 605-618. 10.1007/s00442-012-2339-3.PubMed CentralView ArticlePubMedGoogle Scholar
- Owen JC, Moore FR: Swainson’s thrushes in migratory disposition exhibit reduced immune function. J Ethol. 2008, 26: 383-388. 10.1007/s10164-008-0092-1.View ArticleGoogle Scholar
- Owen JC, Moore FR: Relationship between energetic condition and indicators of immune function in thrushes during spring migration. Can J Zool. 2008, 86: 638-647. 10.1139/Z08-038.View ArticleGoogle Scholar
- Matson KD, Horrocks NPC, Tieleman BI, Haase E: Intense flight and endotoxin injection elicit similar effects on leukocyte distributions but dissimilar effects on plasma-based immunological indices in pigeons. J Exp Biol. 2012, 215: 3734-3741. 10.1242/jeb.072264.View ArticlePubMedGoogle Scholar
- Navarro J, González-Solís J: Experimental increase of flying costs in a pelagic seabird: effects on foraging strategies, nutritional state and chick condition. Oecologia. 2007, 151: 150-160. 10.1007/s00442-006-0559-0.View ArticlePubMedGoogle Scholar
- Alonso-Alvarez C, Tella JL: Effects of experimental food restriction and body-mass changes on the avian T-cell-mediated immune response. Can J Zool. 2001, 79: 101-105. 10.1139/z00-190.View ArticleGoogle Scholar
- Dietz MW, Piersma T, Hedenstrom A, Brugge M: Intraspecific variation in avian pectoral muscle mass: constraints on maintaining manoeuvrability with increasing body mass. Funct Ecol. 2007, 21: 317-326. 10.1111/j.1365-2435.2006.01234.x.View ArticleGoogle Scholar
- Hegemann A, Matson KD, Versteegh MA, Villegas A, Tieleman BI: Immune response to an endotoxin challenge involves multiple immune parameters and is consistent among the annual-cycle stages of a free-living temperate zone bird. J Exp Biol. 2013, 216: 2573-2580. 10.1242/jeb.083147.View ArticlePubMedGoogle Scholar
- Ardia DR, Schat KA, Winkler DW: Reproductive effort reduces long-term immune function in breeding tree swallows (Tachycineta bicolor). Proc R Soc B. 2003, 270: 1679-1683. 10.1098/rspb.2003.2424.PubMed CentralView ArticlePubMedGoogle Scholar
- Hanssen S, Hasselquist D, Folstad I, Erikstad K: Costs of immunity: immune responsiveness reduces survival in a vertebrate. Proc R Soc B. 2004, 271: 925-930. 10.1098/rspb.2004.2678.PubMed CentralView ArticlePubMedGoogle Scholar
- Møller AP, Saino N: Immune response and survival. Oikos. 2004, 104: 299-304. 10.1111/j.0030-1299.2004.12844.x.View ArticleGoogle Scholar
- Hegemann A, van der Jeugd HP, de Graaf M, Oostebrink LL, Tieleman BI: Are Dutch Skylarks partial migrants? Ring recovery data and radio-telemetry suggest local coexistence of contrasting migration strategies. Ardea. 2010, 98: 135-143. 10.5253/078.098.0202.View ArticleGoogle Scholar
- Duncan R: Breeding skylarks in Aberdeen. Grampian Ringing Group. 1987, 5: 49-55.Google Scholar
- Deerenberg C, Pen I, Dijkstra C, Arkies BJ, Visser GH, Daan S: Parental energy-expenditure in relation to manipulated brood size in the European kestrel falco tinnunculus. Zool-Anal Complex Sy. 1995, 99: 39-48.Google Scholar
- Sanz JJ, Kranenbarg S, Tinbergen JM: Differential response by males and females to manipulation of partner contribution in the great tit (Parus major). J Anim Ecol. 2000, 69: 74-84. 10.1046/j.1365-2656.2000.00373.x.View ArticleGoogle Scholar
- Tinbergen JM: Foraging decisions in starlings (sturnus vulgaris). Ardea. 1981, 69: 1-67.Google Scholar
- Wright J, Both C, Cotton P, Bryant D: Quality vs. quantity: energetic and nutritional trade-offs in parental provisioning strategies. J Anim Ecol. 1998, 67: 620-634. 10.1046/j.1365-2656.1998.00221.x.View ArticleGoogle Scholar
- Wright J, Cuthill I: Manipulation of Sex-differences in parental care - the effect of brood size. Anim Behav. 1990, 40: 462-471. 10.1016/S0003-3472(05)80526-3.View ArticleGoogle Scholar
- Wright J, Cuthill I: Biparental care: short-term manipulation of partner contribution and brood size in the starling, Sturnus vulgaris. Behav Ecol. 1990, 1: 116-124. 10.1093/beheco/1.2.116.View ArticleGoogle Scholar
- Donald PF: The skylark. 2004, London: T & A D PoyserGoogle Scholar
- von Blotzheim UN G, Bauer K: Handbuch der Vögel Mitteleuropas. Vol 10 (CD-ROM Version 2001). 1985, Wiesbaden: AULA-VerlagGoogle Scholar
- Rappole JH, Tipton AR: New harness design for attachment of radio transmitters to small passerines. J Field Ornithol. 1991, 62: 335-337.Google Scholar
- Wiebe KL: Negotiation of parental care when the stakes are high: experimental handicapping of one partner during incubation leads to short-term generosity. J Anim Ecol. 2010, 79: 63-70. 10.1111/j.1365-2656.2009.01614.x.View ArticlePubMedGoogle Scholar
- Buehler DM, Bhola N, Barjaktarov D, Goymann W, Schwabl I, Tieleman BI, Piersma T: Constitutive immune function responds more slowly to handling stress than corticosterone in a shorebird. Physiol Biochem Zool. 2008, 81: 673-681. 10.1086/588591.View ArticlePubMedGoogle Scholar
- Matson KD, Tieleman BI, Klasing KC: Capture stress and the bactericidal competence of blood and plasma in five species of tropical birds. Physiol Biochem Zool. 2006, 79: 556-564. 10.1086/501057.View ArticlePubMedGoogle Scholar
- Hegemann A, Voesten R, van Eerde K, van der Velde M, Tieleman BI: The use of tongue spots for aging and wing length for sexing Skylarks Alauda arvensis - A critical evaluation. Ring Migrat. 2012, 27: 7-12. 10.1080/03078698.2012.691000.View ArticleGoogle Scholar
- Matson KD, Ricklefs RE, Klasing KC: A hemolysis-hemagglutination assay for characterizing constitutive innate humoral immunity in wild and domestic birds. Dev Comp Immunol. 2005, 29: 275-286. 10.1016/j.dci.2004.07.006.View ArticlePubMedGoogle Scholar
- Matson KD, Horrocks NPC, Versteegh MA, Tieleman BI: Baseline haptoglobin concentrations are repeatable and predictive of certain aspects of a subsequent experimentally-induced inflammatory response. Comp Biochem Phys A. 2012, 162: 7-15. 10.1016/j.cbpa.2012.01.010.View ArticleGoogle Scholar
- Feldman BF, Zinkl JG, Jain NC: Schalm’s veterinary hematology. 2000, Philadelphia, London: Lippincott Williams & Wilkins, 5Google Scholar
- Davis AK, Maney DL, Maerz JC: The use of leukocyte profiles to measure stress in vertebrates: a review for ecologists. Funct Ecol. 2008, 22: 760-772. 10.1111/j.1365-2435.2008.01467.x.View ArticleGoogle Scholar
- Janeway CA, Travers P, Walport M, Shlomchik MJ: Immunobiology: the immune system in health and disease. 2005, New York, Edinburgh: Garland Science; Churchill Livingstone, 6Google Scholar
- van de Crommenacker J, Horrocks NPC, Versteegh MA, Tieleman BI, Komdeur J, Matson KD: Effects of immune supplementation and immune challenge on oxidative status and physiology in a model bird: implications for ecologists. J Exp Biol. 2010, 213: 3527-3535. 10.1242/jeb.045591.View ArticlePubMedGoogle Scholar
- Janeway CA, Medzhitov R: Innate immune recognition. Annu Rev Immunol. 2002, 20: 197-216. 10.1146/annurev.immunol.20.083001.084359.View ArticlePubMedGoogle Scholar
- Kurtz J: Memory in the innate and adaptive immune systems. Microb Infect. 2004, 6: 1410-1417. 10.1016/j.micinf.2004.10.002.View ArticleGoogle Scholar
- Flinks H, Pfeifer F: Nahrung adulter und nestjunger Schwarzkehlchen (Saxicola torquata rubicola) einer westfälischen Brutpopulation. Vogelwelt. 1987, 108: 41-57.Google Scholar
- Calver MC, Wooller RD: A technique assessing the taxa, length, dry weight and energy content of the arthropod prey of birds. Aust Wildlife Res. 1982, 9: 293-301. 10.1071/WR9820293.View ArticleGoogle Scholar
- Jenni L, Reutimann P, Jenni-Eiermann S: Recognizability of different food types in feces and in alimentary flushes of Sylvia warblers. Ibis. 1990, 132: 445-453.View ArticleGoogle Scholar
- Ralph CP, Nagata SE, Ralph CJ: Analysis of droppings to describe diets of small birds. J Field Ornithol. 1985, 56: 165-174.Google Scholar
- R Development Core Team: R: A language and environment for statistical computing. 2011, Vienna, Austria: R Foundation for Statistical Computing,http://www.r-project.org,Google Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.