Advancing breeding phenology does not affect incubation schedules in chestnut-crowned babblers: Opposing effects of...

Projecting population responses to climate change requires an understanding of climatic impacts on key components of reproduction. Here, we investigate the associations among breeding phenology, climate and incubation schedules in the chestnut-crowned babbler (Pomatostomus ruficeps), a 50 g passerine with female-only, intermittent incubation that typically breeds from late winter (July) to early summer (November). During daylight hours, breeding females spent an average of 33 min on the nest incubating (hereafter on-bouts) followed by 24-min foraging (hereafter off-bouts), leading to an average daytime nest attentiveness of 60%. Nest attentiveness was 25% shorter than expected from allometric calculations, largely because off-bout durations were double the expected value for a species with 16 g clutches (4 eggs × 4 g/egg). On-bout durations and daily attentiveness were both negatively related to ambient temperature, presumably because increasing temperatures allowed more time to be allocated to foraging with reduced detriment to egg cooling. By contrast, on-bout durations were positively associated with wind speed, in this case because increasing wind speed exacerbated egg cooling during off-bouts. Despite an average temperature change of 12°C across the breeding season, breeding phenology had no effect on incubation schedules. This surprising result arose because of a positive relationship between temperature and wind speed across the breeding season: Any benefit of increasing temperatures was canceled by apparently detrimental consequences of increasing wind speed on egg cooling. Our results indicate that a greater appreciation for the associations among climatic variables and their independent effects on reproductive investment are necessary to understand the effects of changing climates on breeding phenology.