Effect of nestling status and brood size on concentration of corticosterone of free-living kittiwake chicks
Introduction
Vertebrates respond to perceived stressors through increased circulation of glucocorticoids—steroid hormones released by the hypothalamic pituitary adrenal (HPA) axis (reviewed by Sapolsky et al., 2000). Over the last 15 years, measurement of glucocorticoid levels of individuals has been increasingly promoted as a method to infer the status of populations in the wild (Reeder and Kramer, 2005, Walker et al., 2005a, Walker et al., 2005b, Wikelski and Cooke, 2006, Williams et al., 2008, Wingfield et al., 1997). However, considerable disparity exists within and across species in the circumstances and degree to which glucocorticoid levels are elevated, especially in the context of social interactions within and among groups (e.g., Sapolsky, 1992, Creel, 2005, Lynch et al., 2002). Consequently, there is a need for baseline data on the adrenocortical function of a wide variety of free-living vertebrate species to assess the ‘normal’ hormonal profiles of individuals under different contexts and provide bases of comparison for future studies (Walker et al., 2005a, Wikelski and Cooke, 2006, Wingfield, 2005, Wingfield et al., 1997).
In avian species, wherein corticosterone is the primary glucocorticoid (Sturkie, 1986), a wide array of intra- and inter-nest variation exists in the levels of circulating corticosterone of asynchronously hatched chicks. Nunez-de la Mora et al. (1996) found that subordinate beta chicks of blue-footed boobies (Sula nebouxii) had more than twice the plasma concentration of baseline corticosterone than either dominant alpha chicks or singleton chicks. Conversely, earlier hatched (alpha and beta) chicks had higher levels of baseline corticosterone than later hatched chicks in canaries (Serinus canaria: Schwabl, 1999) and American kestrels (Falco sparverius: Love et al., 2003). Ramos-Fernandez et al. (2000) observed no differences in baseline corticosterone levels in blue-footed booby alpha, beta or singleton chicks, and Blas et al. (2005) found European white stork chicks (Ciconia ciconia Linnaeus) had no intra-nest variation (alphas, betas and gammas equal) but inter-nest comparisons indicated that singleton chicks had higher levels of baseline corticosterone than alpha, beta and gamma chicks.
This variation may be attributed to both the developmental strategies of the individual species (e.g., Schwabl, 1999) and a suite of non-mutually exclusive post- and pre-natal factors affecting the respective chicks. Within broods, the classic model of sibling rivalry posits that the size advantage of an older chick allows it to assert its dominance through aggressive behavior and/or by positioning itself in the nest so as to obtain a disproportionate share of food from the parents (Mock and Parker, 1997, Cotton et al., 1999). The outcome often manifests as significantly elevated levels of circulating corticosterone in the younger chick. This increase in corticosterone can either be due to social stress, i.e., a response to the agonistic behavior itself, wherein the increase is thought to facilitate subordination in subsequent interactions with the older chick (Nunez-de la Mora et al., 1996; although see Vallarino et al., 2006), or simply due to reduced food intake and subsequent deterioration in body condition (e.g., Kitaysky et al., 1999a, Kitaysky et al., 2001). Yet, there are a number of ways by which the effects of hatching asynchrony may be mitigated. For example, in developing eggs, greater albumen mass and higher concentrations of maternally derived resources such as androgens, carotenoids, and immunoglobulins are known to enhance rates of embryological development, post-natal growth and begging in chicks (Schwabl, 1996, Eising et al., 2006, Ferrari et al., 2006, Groothuis et al., 2006). Increased exposure to these resources during early development may improve the ability of the respective chicks to compete with their siblings (Schwabl, 1996, Eising et al., 2006, Ferrari et al., 2006, Groothuis et al., 2006) and thereby allow them to maintain similar hormonal profiles as well.
Among broods, differing abilities of breeding pairs may contribute to variation in stress levels of chicks. Coulson and Porter (1985) found that the chicks of inexperienced black-legged kittiwake (Rissa tridactyla) parents typically exhibit slower growth rates and lower fledging weights compared to those of more experienced, higher-quality parents. Similarly, among common guillemots (Uria aalge), the chicks of higher-quality breeding pairs, defined as pairs with high lifetime reproductive success, are fed more frequently than chicks of lower-quality breeding pairs (Lewis et al., 2006). More simply, differences in brood sizes among nests may lead to variation in levels of corticosterone among chicks. In barn swallows (Hirundo rustica) chicks from smaller sized broods experience lower levels of circulating corticosterone, ostensibly due to the reduced amount of competition for food within the nest (e.g., Saino et al., 2003).
In light of the variation seen among avian species with respect to the adrenal responsiveness of chicks, we set out to determine if, and to what degree, nestling status (i.e., alpha, beta, singleton) and brood size affect baseline and acute stress-induced corticosterone levels of free-living black-legged kittiwake chicks. Black-legged kittiwakes (hereafter: kittiwakes) are colonial, cliff-nesting seabirds that raise semi-precocial, asynchronously hatched broods of 1–3 chicks (Baird, 1994). Chicks are nest-bound and therefore rely completely on their parents for food until fledging (Baird, 1994). They have a fully functioning HPA axis as early as 12 days post-hatching (Brewer et al., 2008) and respond to food shortages with elevated corticosterone levels which lead to increased begging and aggressive behavior (Kitaysky et al., 1999a, Kitaysky et al., 2003). Furthermore, sibling aggression and brood reduction are common (Braun and Hunt, 1983, Roberts and Hatch, 1993). While a number of studies in the last decade have explored the adrenocortical function of kittiwake adults and chicks (e.g., Buck et al., 2007, Fridinger et al., 2007, Kitaysky et al., 1999b, Kitaysky et al., 2003), to our knowledge no study has examined the dynamics of corticosterone levels of chicks within or among nests in a free-living kittiwake colony.
Section snippets
Study location
We conducted this study during four consecutive breeding seasons (2002–2005) at three kittiwake colonies (Gibson Cove, Gull Island, Mary Island) in Chiniak Bay, Kodiak Island, Alaska (N 57°42.65′, W 152°20.29′). Chiniak Bay is located on the northeast coast of Kodiak Island and approximately 20 km × 20 km in area. Approximately 10,000 adult pairs nest among 21 colonies located on the cliff faces of multiple small islands and rock stacks throughout the bay (Kildaw et al., 2005). Gibson Cove, Gull
Results
We sampled 53 alpha, 53 beta, and 58 singleton kittiwake chicks across 4 years (2002–2005; Table 1) from three colonies (Gibson Cove, Gull Island and Mary Island). All alpha and beta chicks were sampled as pairs from intact two-chick broods. Interestingly, Gull Island produced 2 three-chick broods in 2002. We have included the corticosterone data from these broods as an inset in Fig. 2 as reference due to the relative rarity of such occurrences in Chiniak Bay. These data were not included in
Discussion
We sampled free-living 12- to 15-day-old kittiwake chicks across four breeding seasons (2002–2005) to determine whether circulating corticosterone values vary in relation to nestling status and brood size. Despite apparent variability in breeding conditions and differences across years in overall corticosterone levels (Buck et al., 2007, Brewer et al., 2008), we found no effects of status or brood size on the baseline or stress-induced corticosterone concentrations of kittiwake chicks. While
Acknowledgments
Funding for this study was supplied by a grant to C.L.B. by NOAA/NMFS #NA04NMF4390158. We thank T. Cooper, R. Fridinger, S.D. Kildaw, A. Parker, M. Vansooy, S. Vansooy and C.T. Williams for assistance in the field and laboratory. We further thank M.A. Castellini and A.S. Kitaysky and two anonymous reviewers for critical evaluation of this manuscript. All methods in this study were approved by the University of Alaska Fairbanks Institutional Animal Care and Use Committee (assurance #05-43) and
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