Incubation period length varies with temperature, female age, and clutch size in <i>Protonotaria citrea</i> (Prothonotary Warbler)

Abstract The length of the incubation period is determined by the development rate of embryos, which is dependent on the thermal stability provided by environmental conditions and parental incubation behavior. With climate change, many bird species are advancing their breeding earlier in the season. This exposes broods and parents to more variable temperatures and inclement weather conditions. Assessing how the length of the incubation period varies with weather conditions is important for understanding the potential consequences of phenological advancements with climate change, such as higher offspring mortality risk and parental energetic demands. This study examined the length of the incubation period in response to weather conditions (i.e., temperature and precipitation), female age, clutch size, and timing of breeding (early vs. late season) in a population of nest-box breeding Protonotaria citrea (Prothonotary Warblers). Across 1,036 clutches spanning 22 years, we found the incubation period to range between 10 and 16 days. Results indicated that low temperatures, young mothers, and small clutch sizes contributed to longer incubation periods. Precipitation did not affect the incubation period length. Early and late-season clutches did not differ when accounting for temperature, female age, and clutch size. We also found minimum daily temperatures during the incubation period to be highly correlated with the overall variation in temperatures. These results indicate that advancements in breeding expose clutches to colder and more variable temperatures, resulting in longer incubation periods, but this effect is likely mitigated by larger clutches and a higher proportion of older females in the early season.