Abstract
As is the case for most avian species, there is considerable variation in the egg size of Continental Black-tailed Godwits Limosa l. limosa breeding in The Netherlands. It is interesting that egg size has costs and benefits yet varies considerably at the population level. To better understand this variation in egg size, we tested its relationship to a suite of individual and environmental factors. We found that egg size can decrease up to 2.8% throughout a breeding season and that egg size increases with clutch size by 1.4% with each additional egg in the clutch. Female body mass and body size explained 5% of the total variation in egg size observed across the population. Furthermore, females wintering south of the Sahara laid 3% smaller eggs than those wintering north of the Sahara. We also found that egg size increases with age, which may indicate age-related differences in the endogenous and/or exogenous conditions of females. The variation in egg size was, however, mostly the result of consistent differences among individuals across years (repeatability = 0.60). A comparison of daughters with mothers suggested that most of this individual repeatability reflects heritable variation (heritability = 0.64). The actual individual traits that underlie this heritable variation among individuals remain mostly undetermined. Smaller eggs did have a slightly lower chance of hatching, but we found no relationship between egg size and chick survival. Finally, nest and chick survival were strongly correlated with lay date. Thus, in Black-tailed Godwits, lay date may actually reflect a female's endogenous and/or exogenous condition at the moment of egg-laying. This finding may be general across birds, since food supplementation experiments usually result in advanced laying and larger clutch sizes rather than in larger eggs.
Highlights
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We found that egg size increases with age, which may indicate age-related differences in the endogenous and/or exogenous conditions of females
It is possible that the observed variation in egg size is the result of differences within individuals, in response to annual variation in exogenous conditions (e.g. food, precipitation or ARDEA 107(3), 2019 temperature) or changes in endogenous condition
Summary
BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. In Black-tailed Godwits, lay date may reflect a female’s endogenous and/or exogenous condition at the moment of egg-laying This finding may be general across birds, since food supplementation experiments usually result in advanced laying and larger clutch sizes rather than in larger eggs. Egg sizes of birds vary considerably at the population level; the largest eggs are generally at least 50% larger than the smallest eggs within a population (Christians 2002) Such variation could result from consistent differences in egg size among individuals. Larger eggs generally contain higher absolute levels of egg components and produce larger and heavier chicks that survive better (reviewed in Krist 2011 and Williams 2012) These results suggest that egg size has costs and benefits, and that individuals might balance these costs and benefits when allocating resources to egg size. To further examine the considerable variation in the egg sizes of birds, we explore the determinants of egg size in Continental Black-tailed Godwits (hereafter Godwits)
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