Impacts of ambient temperature and clutch size on incubation behaviour onset in a female-only incubator songbird

Ambient temperature is assumed to be the major cue used by passerines to synchronize their laying and hatching dates to the expected peak of prey availability. While laying eggs, females are still able to fine-tune their hatching date following increasing or decreasing patterns of ambient temperature, mostly via changes in incubation onse t. The onset of incubation behaviour in relation to the laying sequence could have later conse- quences for the duration of the incubation period and the extent of hatching asynchrony. Clutch size is also known to affect incubation patterns and might therefore condition potential responses to changing temperatures. In this study we assessed the association of ambient temperature and clutch size with the onset of four different incubation behaviours: partial and full nocturnal incubation, and partial and full diurnal incubation. We also evaluated how the onset and duration of each incubation behaviour might predict the duration of diurnal full incubation and the extent of hatching asynchrony. To achieve our aims, we monitored incubation behaviour using temperature data loggers during the egg-laying period in three Mediterranean Great Tit Parus major populations in three consecutive years. Our results showed that increasing temperatures were related to an advance of diurnal partial incubation, but not its duration or the onset of full incubation behaviour. We did not nd any effect of ambient temperatures on nocturnal incubation. However, females lengthened nocturnal partial incubation and delayed the onset of nocturnal full and diurnal partial incubation when laying larger clutches. Longer diurnal incubation before clutch completion was associated with greater hatching asynchrony. Moreover, longer diurnal partial incubation shortened the duration of the full incubation period. In conclusion, increasing ambient temperatures during the egg-laying period advanced diurnal partial incubation, indirectly shortening the full incubation period and increasing hatching asynchrony.This study was supported by projects CGL2013-48001-C2-1-P and CGL2016-79568-C3-1-P (Spanish Ministry of Economy and Competitiveness, MINECO). D.D.-M. benefitted from an FPI fellowship (BES-2014-069191) provided by the MINECO.Peer reviewe