Does offspring sex ratio differ between urban and forest populations of great tits (Parus major)?

Abstract

SummarySince male and female offspring may have different costs and benefits, parents may use sex ratio adjustment to increase their fitness under different environmental conditions. Urban habitats provide poorer conditions for nestling development in many birds. Therefore, we investigated whether great tits (Parus major) produce different brood sex ratios in urban and natural habitats. We determined the sex of nestlings of 126 broods in two urban and two forest habitats between 2012 and 2014 by molecular sexing. We found that brood sex ratio did not differ significantly between urban and forest habitats either at egg-laying or near fledging. Male offspring were larger than females in both habitats. This latter result suggests that male offspring may be more costly to raise than females, yet our findings suggest that urban great tits do not produce more daughters despite the unfavourable breeding conditions. This raises the possibility that other aspects of urban life, such as better post-fledging survival, might favour males and thereby compensate for the extra energetic costs of producing male offspring.

Feather, But Not Plasma, Glucocorticoid Response to Artificial Light at Night Differs between Urban and Forest Blue Tit Nestlings

“…Second, the sex of the nestlings used in our experiment was not determined. In one study on great tits, offspring sex ratios did not vary between urban and rural populations ( Ágh et al 2020 ). However, as glucocorticoid responses might vary between males and females ( Goymann 2005 ; Bonier et al 2007 ; Edwards et al 2013 ), it will be necessary to also consider sex as an important variable in future studies.…”

Sex ratios and the city: Secondary offspring sex ratios, parental corticosterone, and parental body condition in an urban-adapted bird

“…A recent meta-analysis suggests that the relationship between CORT and urbanization in birds and reptiles is inconsistent and may depend on life-history stage and how tolerant species are to urban conditions (Injaian et al, 2020). As a result, the links between urbanization, CORT, parental body condition, and offspring sex ratios in birds remain unclear (Rejt et al, 2005;Bókony et al, 2012;Bonier, 2012;Foltz et al, 2015;Bonderud et al, 2017;Ágh et al, 2020;Iglesias-Carrasco et al, 2020;Strubbe et al, 2020).…”