Higher Frequency of Extra-Pair Offspring in Urban Than Forest Broods of Great Tits (Parus major)

Abstract

Urbanization increasingly changes the ecological conditions for wild animal populations, influencing their demography, reproduction, and behavior. While studies on the ecological consequences of urbanization frequently document a reduced number and poorer body condition of offspring in urban than in non-urban bird populations, consequences for other components of reproduction are rarely investigated. Mating with partners outside the social pair-bond is widespread in birds, and although theory predicts that the occurrence of extra-pair fertilizations (EPF) may be sensitive to the altered ecological conditions of cities, the effect of urbanization on EPF is poorly known. Here we used data from two urban and two forest populations collected over 3 years to test whether the frequency of extra-pair offspring (EPO) in great tit broods differed between the habitats. We found that significantly more broods contained EPO in urban habitats (48.9%) than in forests (24.4%). In broods with EPO, the number and proportion of EPO was similar in urban and forest broods. These results suggest that females that live in urban habitats are more likely to engage in EPF than those living in forests. Urban environments may either provide more spatiotemporal opportunities to EPF because of higher breeding density, and/or enhance motivation for EPF to increase fertility in polluted environments. In addition, females with higher propensity to engage in EPF may more likely settle in urban habitats.

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

“…For example, Lessels et al (1996) reported that the proportion of male offspring increased with hatching asynchrony in great tits. Furthermore, Pipoly et al (2019) found in the same populations and breeding seasons as in the present study that the number of extra-pair offspring was higher in urban habitats than in forests, which might influence sex ratio adjustment. The other possible interpretation of our findings is that the observed variance in brood sex ratios is largely random, with no facultative sex ratio adjustment going on (Ewen et al 2004).…”

“…To statistically compare the sex ratios between the two habitat types (urban sites vs. forest sites) we calculated linear contrasts from the full and reduced models. These linear contrasts were pre-planned comparisons between the two urban sites vs. the two forest sites (see also Pipoly et al 2019 and Vincze et al 2019 for the same approach to compare habitat types by pre-planned linear contrasts and for additional details of the method). Each linear contrast was back-transformed from the log-scale to provide the odds ratio (OR, i.e.…”

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

“…For example, Lessells et al [40] reported that the proportion of male offspring increased with hatching asynchrony in great tits. Furthermore, Pipoly et al [48] found in the same populations and breeding seasons as in the present study that the number of extra-pair offspring was higher in urban habitats than in forests, which might influence sex ratio adjustment. The other possible interpretation of our findings is that the observed variance in brood sex ratios is largely random, with no facultative sex ratio adjustment going on.…”

“…To statistically compare the sex ratios between the two habitat types (urban sites vs. forest sites) we calculated linear contrasts from the full and reduced models. These linear contrasts were pre-planned comparisons between the two urban sites vs. the two forest sites (see also Pipoly et al [48] and Vincze et al [68] for the same approach to compare habitat types by pre-planned linear contrasts and for additional details of the method). Each linear contrast was back-transformed from the log-scale to provide the odds ratio (OR, i.e.…”

Differences in feather structure between urban and forest great tits: constraint or adaptation?

“…In the second step, we used these linear models for each response variable to conduct the pre‐planned comparisons by calculating linear contrasts from each model's marginal means (package ‘emmeans') between the two forest and two urban sites (for similar approach see also Pipoly et al 2019, Vincze et al 2019) separately for FY and AFY birds. We compared habitats separately for the two age groups because they likely differed in their moulting conditions.…”