Reproductive fitness is associated with female chronotype in a songbird

Abstract

Research on biological rhythms has revealed widespread variation in timing within populations. Repeatable individual chronotypes have been linked to performance in humans but, in free-living species, benefits of chronotype are poorly understood. To address this gap, we investigated fitness correlates of incubation patterns in female songbirds (great tit, Parus major) at urban and forest sites. We confirm repeatable chronotypes (r ≥ 0.31) and show novel links between chronotype and reproductive fitness. In both habitats, females that started activity earlier in the day raised more fledglings. We also observed that forest females started their day at similar times throughout the breeding season, whereas urban females tied their onset of activity closely to sunrise. Our study points to possible mechanisms that underlie chronotype variation and provides sought-after evidence for its relevance to fitness.

Experimental light at night explains differences in activity onset between urban and forest great tits

“…Our experimental manipulation allowed us to show that the previously reported [16,21] difference in timing of behaviour between urban and forest bird populations can be largely explained by light pollution rather than by other co-varying urban factors. ALAN effects as observed in the forest explained part of the advanced morning activity of urban females, which were exposed to environmental ALAN outside of their nests.…”

“…For example, an earlier onset of activity could mean that timing is not optimally synchronized to foraging opportunities, or that birds are more at risk of predation [36]. However, parallel evidence from our system suggests that early onset of activity in great tit females is associated with an increase in the number of nestlings produced [21]; whether this also corresponds to higher recruitment rate is currently unknown.…”

“…We used 12 clutches for each of the forest controls, forest ALAN and urban groups (electronic supplementary material, table S1). Forest controls and urban clutches are also a subset of earlier research [21]. Clutches in the forest were assigned alternatingly to the forest control or forest ALAN groups.…”

“…Thus, females must balance the need to keep the eggs within the suitable range of temperatures for embryonic development, while also obtaining enough food [19,20]. Females may thus display individualized incubation patterns, and in our study system early rising great tits benefit from higher reproductive success [21]. Males might also influence female incubation pattern and timing as they provide food to the females during incubation [17].…”

Environmental constraints can explain clutch size differences between urban and forest blue tits: insights from an egg removal experiment

“…At the forest site, we included 17 control and 16 experimental clutches. When the female stopped laying for two days, and the eggs were warm to touch, we assumed the female had completed the clutch and started incubating (64). Assuming a minimum incubation length of 14 days from clutch completion (65), 13 days after clutch completion, we started nest-box visits every two days to record the exact hatch date.…”