A consistent long-lasting pattern of spatial variation in egg size and shape in blue tits (Cyanistes caeruleus)

Abstract

BackgroundInterspecies variation in avian egg shape and size is understandable in terms of adaptation, allometry and phylogeny. Within-species variation in egg properties influences offspring fitness and can be explained by differences in allocation of resources into reproductive components of life history in mulidimensionally variable environments. Egg size is inherently traded-off with clutch size, which may also be true of egg shape in some cases. We investigated long-term variation in egg shape and size between two geographically close populations of blue tits Cyanistes caeruleus in relation to clutch size and habitat differences.ResultsThe main finding is that there exists a persistent long-lasting pattern of spatial variation of egg size and shape between the two study populations of blue tits, 10 km apart, controlling for clutch size. Eggs in the urban park site were on average larger in volume and less spherical in shape than eggs in the forest site over 12 years of this study. Egg sizes were negatively associated with clutch sizes. Egg shape was not correlated with clutch size.ConclusionsOur findings suggest that the pattern of variation in egg size and shape results from different trophic richness of the breeding habitats of the study populations, demanding different allocation of resources and, especially, from the contrasting difference in the availability of calcium.

No evidence for trade-off between egg dimensions and clutch size in first, second and third clutches of the Tree Sparrow Passer montanus

“…According to Hõrak et al (1995), "being hatched from a large egg could potentially be beneficial of the laying female, however, high investment into egg quality (size) might conflict with her own energetic demands and willingness to produce more offspring". Studies have found negative (Martin et al 2006, Bańbura et al 2018, positive (Aslan & Yavuz 2010, Dolenec 2016) and no significant correlations (Dolenec 2011, Bao et al 2020 between egg size and clutch size. Negative relationships between egg volume (or egg mass) and clutch size is consequence of compromise between egg size and clutch size with females laying numerous small or few large eggs (e.g.…”

Within-individual consistency and between-individual variation in the shapes of eggs laid by tree swallows (Tachycineta bicolor)

“…Recently, advances in the ease of making accurate measurements of egg shape and the ability to conduct large-scale comparative analyses have led to a renewed effort to understand what factors drive variation in egg shape between species (Duursma et al, 2018; Montgomerie et al, 2021; Nagy et al, 2019; Shatkovska et al, 2018; Stoddard et al, 2017). Despite the number of studies of egg shape, relatively few have examined variation in shape between populations, between individuals, or within individuals of the same species (but see Adamou et al, 2018; Bańbura et al, 2018).…”

“…A complete understanding of egg shape variation will require a scale-dependent framework (Agrawal, 2020) that makes predictions that explicitly consider the scale, based on an understanding of the mechanism(s) involved. Analyses at these different levels are complementary, rather than contradictory (Stoddard et al, 2019), but at present, very few studies provide even descriptive data on correlates of variation in egg shape within species or individuals (but see Adamou et al, 2018; Bańbura et al, 2018).…”

“…While between species variation has received considerable recent research attention, there is also widespread recognition that egg shape varies within species (Adamou et al, 2018; Bańbura et al, 2018). The causes of this variation are less clear, both because few studies measure the shape of many eggs from known individuals over multiple breeding attempts and because it is often unclear how or if explanations for egg shape variation based on between-species life history differences should apply within species or between populations.…”