A large number of studies have focused on variation in clutch initiation date (e.g., Visser et al., 1998 Charmantier et al., 2008 Schaper et al., 2012), but other mechanisms for altering the timing of hatching exist. For many bird species living in seasonal environments, it is critical for reproduction to be timed to match temporally ephemeral food resources, with hatching expected to coincide with the peak of food abundance ( Perrins, 1970 Monros et al., 1998 Naef-Daenzer et al., 2004 Simmonds et al., 2017). Reproductive success of birds is often dependent upon timing of breeding ( Daan et al., 1997 Houston and McNamara, 1999), with individuals that breed earlier in the season typically having higher fitness ( Perrins, 1970 Both, 2010). We suggest that long-tailed tits’ nests and parental behavior protect eggs from mortality arising directly from adverse weather conditions. In contrast, we found only marginal effects of environmental conditions or incubation duration on hatching success, implying that wet conditions cause slower embryo growth and hence longer incubation periods, rather than causing embryo fatality. We show that longer incubation periods are likely to be costly due to increased exposure to nest predators.

Secondly, we assessed the consequences of variation in the length of incubation periods for the risk of nest predation and the hatching success of surviving clutches. Surprisingly, warmer ambient conditions were associated with longer incubation periods. Larger clutches were also more resilient to increases in incubation duration associated with wet conditions. We found support for our predictions that drier conditions, later breeding attempts and larger clutches were associated with shorter incubation periods. The primary aim of this study was to use a 24-year dataset to investigate the abiotic and biotic factors influencing variation in incubation duration in long-tailed tits ( Aegithalos caudatus), a species in which incubation duration varies substantially (range: 12–26 days). Thus, in addition to lay date, phenological studies may also have to take account of variation in the length of the incubation period, which is likely to depend on both environmental conditions and parental traits. Timing is typically investigated with reference to lay dates, but it is the time of hatching that determines the ambient conditions and food availability that nestlings experience. The timing of breeding often has a profound influence on the reproductive success of birds living in seasonal environments with rapidly changing nestling food availability.

Department of Animal and Plant Sciences, University of Sheffield, Sheffield, United Kingdom.

Bird incubator