Why do nestling birds fledge early in the day?

Diel patterns of predation and fledging at nests of four species of grassland songbirds

“…Radersma et al (2015) found that Great Tits (Parus major) fledging earlier in the day led to higher recruitment to the breeding population the following year. In contrast, Santema et al (2021) found that in Blue Tits While we focused on direct effects of predation, there may be indirect ways that predation affects species differences in fledging time, though studies are lacking on this topic. Some insight may come from studies on the dawn chorus.…”

“…However, a substantial fraction of nestlings fledged outside of the early morning low predation activity period as the nestlings balanced the relative costs and benefits of leaving the nest throughout the day. The idea that energetics plays a role in the timing of fledging has been a common alternative to predation as the driver in studies outside of the grassland system (Chiavacci et al, 2015;Johnson et al, 2004;Santema et al, 2021).…”

“…In the grassland system, an indirect effect of predation may manifest itself in relation to time of fledging by affecting when adults provision their young. Santema et al (2021) argued that the time of day that nestlings fledged was linked to the nestlings being energetically ready. If grassland bird species vary when adults begin their daytime activities (i.e., see their environment and detect predators) due to variation in visual capabilities, then nestlings of the different species will become energetically ready to fledge at different times, leading to a continuum of species' times of first fledging.…”

“…Overall, as more researchers investigate processes that might affect the time of day that nestlings fledge in a variety of bird species (Chiavacci et al, 2015;Johnson et al, 2004;Lemel, 1989;Nilsson, 1990;Ribic, Ng, et al, 2018;Santema et al, 2021;Schlicht et al, 2012), it is becoming clearer that there are no universal answers-no single set of drivers appears to apply to all birds. As our technical capabilities to study this phase of fledging behavior improve, researchers will be able to address the complex interplay of nestling development, energetics, predation, and potentially other factors driving fledging behavior.…”

Food supplementation of parents before hatching of the young prolongs the nestling period in European pied flycatchers (Ficedula hypoleuca)

“…If no parent had been observed after 30 minutes, the nestbox was checked for any remaining nestlings, but nests were empty in all cases. The number of days elapsed between the first observation of a hatchling and the day when no nestling remained in the nest was established as fledging age, including the first day with an empty nest assuming the nestlings had fledged in the early morning of that same day (Chiavacci et al 2015, Radersma et al 2015, Santema et al 2021. Fledging on day x represents a fledging age of x-1.…”