Whether and how nestlings of Japanese tits respond to nest predator risk: Testing active and passive growth responses

Nestling growth is known to be passively constrained by parental feeding and actively mediated by growth-regulating hormones. While, nest predation pressures can vary across geographic regions, few studies have focused on whether and how nestling growth patterns are adjusted in response to differences in predation pressure. We selected two breeding populations of Japanese tits Parus minor in Jilin Province and Liaoning Province as subjects. By experimentally increasing snake predation risk, we tested whether the growth trajectory of nestlings differed between the two populations and whether growth hormone (GH)/insulin-like growth factor-1 (IGF-1) and food intake supplied by parents were involved in this process. When exposed to snake risk, Jilin-nestlings exhibited faster wing growth and earlier fledging, while Liaoning-nestlings exhibited slower weight growth rate, even though nestlings in both populations exhibited higher GH/IGF-1 levels; additionally, food intake supplied by parents increased in Jilin but declined in Liaoning. The daily weight growth rate was positively associated with food intake, while wing growth rate was positively associated with GH/IGF-1 levels. When exposed to high predation risk, nestlings would actively respond by increasing growth-regulating hormonal levels, but their growth remains highly dependent on food intake (passive constraints), leading to intraspecific difference in growth trajectories. Our results indicate that the nestlings exhibit active growth response to nest predation by increasing GH/IGF-1 levels.