Abstract
The timing of breeding often has a profound influence on reproductive success in birds living in seasonal environments in which nestling food availability changes rapidly. Timing is typically investigated with reference to lay dates, but the timing of hatching is also influenced by the length of the incubation period, which is likely to depend on both environmental conditions and parental behaviour. The 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 exhibiting substantial variation in incubation duration. We also assess the consequences of variation in the length of incubation periods for hatching success and the risk of nest predation. We found support for our predictions that later breeding attempts, drier conditions and larger clutches were associated with shorter incubation periods. Larger clutches were also more resilient to increases in incubation duration associated with rainy conditions. Surprisingly, warmer ambient conditions were associated with longer incubation periods. Weather conditions did not influence hatching success suggesting that adverse conditions are buffered by nests or by parental behaviour. We also show that longer incubation periods are likely to be costly, however, due to increased exposure to nest predators, and because longer incubations are associated (albeit weakly) with increased risk of hatching failure.
Highlights
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)
Larger clutch sizes were associated with shorter incubation periods (Table 1 and Figure 1D)
All of the top models contained an interaction term between clutch size and proportion of rainy days, which indicated that the effect of increased rainfall on incubation period was lessened by having a larger clutch (Figure 1C)
Summary
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). Females may adjust the length of the egg-laying period by altering clutch size, or by increasing the intervals between laying (Haftorn, 1981; Nilsson and Svensson, 1993; Simmonds et al, 2017). The length of the incubation period could be adjusted by starting incubation prior to clutch completion or by delaying the start of incubation (Haftorn, 1981; García-Navas and Sanz, 2011; Nord and Nilsson, 2011; Álvarez and Barba, 2014; Simmonds et al, 2017), and by changing nest attentiveness once incubation has started (Martin et al, 2007; MacDonald et al, 2013; Coe et al, 2015). Incubation can reduce parental body condition (Tombre and Erikstad, 1996; Hanssen et al, 2005) and immune function (Knowles et al, 2009), thereby lowering fitness by reducing adult survival (Visser and Lessells, 2001) and future reproductive success (Reid et al, 2000a; Hanssen et al, 2005)
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