Variation in incubation length and hatching asynchrony in Eastern Kingbirds: Weather eclipses female effects

Incubation length and hatching asynchrony are integral elements of the evolved reproductive strategies of birds. We examined intra- and interpopulation variation in both traits for Eastern Kingbird (Tyrannus tyrannus) populations from New York (NY), Kansas (KS), and Oregon (OR) and found that both incubation length and hatching asynchrony were not repeatable among females, after controlling for a repeatable trait, clutch size. Instead, incubation length and clutch size were influenced by ambient temperature and precipitation. Incubation length exhibited the same median (15 days) and range (13–17 days) at all sites. Model selection results indicated that incubation periods for the smallest and largest clutches were longer in NY than KS when rain was frequent throughout incubation, in replacement nests, and likely when ambient temperatures were low during egg-laying. Full hatching usually required 2 days (but up to 3), with synchronous hatching associated with small clutch sizes, short incubation periods, frequent rain during the egg-laying period, and low ambient temperatures during the first half of incubation. Nestling starvation was uncommon (5–9% of nestlings monitored) and not associated with greater hatching asynchrony. These results indicate that while clutch size, a repeatable female trait, contributed to variation in incubation length and hatching asynchrony in Eastern Kingbirds, weather was a greater source of variation, especially for incubation length. LAY SUMMARY The length of time that eggs remain in a nest exposed to causes of mortality has important influences on the reproductive success of birds. We collected data on incubation length, the length of time elapsed between the hatching of the first and last egg (called hatching asynchrony), and probability of nestling starvation in Eastern Kingbirds breeding at 3 locations (Kansas, New York, and Oregon). Incubation length and hatching asynchrony varied considerably within, but also among sites. Incubation length and hatching asynchrony were both greater in nests with more eggs. However, most other variation was likely due to environmental effects. Cool and wet conditions were associated with long incubation periods while greater hatching asynchrony was found during periods when it was warm and dry. Starvation of nestlings was infrequent, and nests with a greater range of nestling ages (caused by high hatching asynchrony) did not experience greater starvation of young. Nestling starvation was t