Experimentally impaired female condition does not affect biliverdin‐based egg colour

Abstract

It has been proposed that blue-green egg coloration is a condition-dependent female sexual trait that may modify paternal care in a post-mating sexual selection scenario. This pattern may arise because the pigment responsible for eggshell colouration (biliverdin) may be a costly and limited resource, whose availability is linked to female health state. Thus, it can be predicted that females whose condition is compromised should be constrained in their capacity to deposit biliverdin in the eggshell, thus producing paler clutches. To test this hypothesis, we performed a handicapping experiment by clipping some feathers of female spotless starlings before egg laying and measuring the colour of their clutches. We expected the handicapping treatment to increase flying costs, impairing female overall condition and resulting in paler clutches. Our experiment was successful in lowering the weight gain of handicapped with respect to control females. However, in contrast to our expectations, we found no effect of the treatment on eggshell colouration. Eggshell colour varied along the laying order, with initial eggs of the laying sequence being relatively paler than the rest of the clutch, but this pattern was not different between experimental groups. Despite a very similar methodology, our results differ from a previous study on the same species and offer no support to the post mating sexual selected hypothesis, questioning the general applicability of the sexual selection role of eggshell coloration.

Plumage and eggshell colouration covary with the level of sex-specific parental contributions to nest building in birds

“…Alternatively, the detected pattern may have also occurred if our original assumption was incorrect and pigmented eggs were not more expensive to produce than unpigmented white eggs (e.g. Hanley and Doucet 2009 ; Honza et al 2011 ; Krištofík et al 2013 ; D’Arpa et al 2022 ). Such possibilities are worthy of further research because, despite the correlative, comparative relationships between eggshell colouration and reproductive investment revealed here, their causal determinants remain largely unclear.…”

Male starling floaters preferentially visit nests of males with reduced resource holding potential

“…Our knowledge of territorial dynamics in bird species is biased towards the behaviour of owners, whereas floaters, which are key actors in territory conflicts, are typically neglected. By experimentally reducing the flight performance of breeders of both sexes [ 19 ], we tested if male and female floaters preferentially prospected nests owned by manipulated breeders. We found that this was the case for male birds, since nests in which the male owner had been wing-clipped attracted a larger number of male floaters than control nests.…”

“…= 9.6) before egg laying. We previously reported that, after controlling for pre-treatment body weight, wing-clipped females were 3.1% lighter than controls while brooding (males could not be caught at this time, but we assume that a similar effect would occur) [ 19 ]. The initial sample size was 24 control nests, 19 nests with wing-clipped females and 25 with wing-clipped males.…”

“…To test this hypothesis, we conducted a wing-clipping manipulation (as in [ 18 ]) in male and female breeders. This manipulation increases wing-load, and has been shown to result in a decrease in female body condition [ 18 , 19 ]. We assume that it has a similar effect in males, decreasing the RHP of both sexes.…”