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

Abstract

Although it is common for nestlings to exhibit a strong bias for fledging in the morning, the mechanisms underlying this behavior are not well understood. Avoiding predation risk has been proposed as a likely mechanism by a number of researchers. We used video surveillance records from studies of grassland birds nesting in North Dakota, Minnesota, and Wisconsin to determine the diel pattern of nest predation and fledging patterns of four ground‐nesting obligate grassland passerines (Grasshopper Sparrow ( Ammodramus savannarum ), Savannah Sparrow ( Passerculus sandwichensis ), Bobolink ( Dolichonyx oryzivorus ), and Eastern Meadowlark ( Sturnella magna )). We used the nest predation pattern as a surrogate for predation activity to test whether nestlings minimized predation risk by avoiding fledging when predation activity was high and preferentially fledging when predation risk was low. Predation activity was significantly lower starting 3 hr before sunrise and ending 3 hr after sunrise, followed by a transition to a period of significantly higher activity lasting for 4 hr, before declining to an average activity level for the rest of the diel period. There was little evidence that the four grassland bird species avoided fledging during the high‐risk period and Savannah Sparrow fledged at higher rates during that period. All four species had hours during the low‐risk period where they fledged at higher rates, but only Grasshopper Sparrow fledged preferentially during that period. Bobolink and Eastern Meadowlark had multiple hours with high fledging rates throughout the daytime period, resulting in no relationship between probability of fledging and predation risk. Given the species variability in fledging pattern seen in our study, it is unlikely that there is a universal response to any driver that affects time of fledging. Further study is needed to understand the complex interplay between species ecology and drivers such as physiology, energetics, and predation in affecting grassland bird fledging behavior.

Nesting and post‐fledging predation risk influence diel patterns of songbird fledging

“…In particular, the mortality of dependent young appears to be a ubiquitous and important factor shaping life history traits across avian species (summarized in Martin & Briskie 2009) and is therefore predicted to play a critical role in shaping fledging times across songbird species (Chiavacci et al 2015). However, there is still a paucity of studies on variation in diel patterns and duration (how rapidly nestlings in a brood leave the nest) of fledging across songbird species (Ribic et al 2021); as such, we lack even basic information about fledging for most species (Johnson et al 2013). This gap in our knowledge is largely a result of technological and logistical limitations of observing fledging behaviour at larger scales and across species (Johnson et al 2004(Johnson et al , 2013; however, technological advancements have allowed recent studies on the timing and duration of fledging in songbirds (Chiavacci et al 2015, Radersma et al 2015, Ribic et al 2018, Santema et al 2021.…”

“…Despite the limited literature on the fledging of birds, a common finding from past studies is the tendency of songbirds to fledge in the morning (usually within 6 h after sunrise or before midday (astronomical noon)) and for all brood mates to fledge rapidly, usually within a couple hours of the first nestling to leave (Lemel 1989, Johnson et al 2004, Schlicht et al 2012, Chiavacci et al 2015, Ribic et al 2018, Santema et al 2021. Additionally, the 'survival hypothesis' predicts that the predation of dependent offspring in the nest drives songbirds to fledge earlier in the day (Chiavacci et al 2015, Ribic et al 2021. Although evidence for the hypothesis is mixed, past research has shown links between overall rates of nest predation and variation in fledging times across species (Chiavacci et al 2015).…”

“…Although evidence for the hypothesis is mixed, past research has shown links between overall rates of nest predation and variation in fledging times across species (Chiavacci et al 2015). Also, studies have provided evidence that offspring may fledge earlier in the day to avoid peaks in nest predation, which are often higher later in the day (Gill et al 2016, Ribic et al 2021.…”

“…For instance, findings may suggest that birds fledge earlier in the morning to avoid high nest mortality rates or peak times of nest predation during the day (e.g. Chiavacci et al 2015, Ribic et al 2021. However, such results would make less sense if nestlings also fledge at peak times of post-fledging predation, or are fledging into a post-fledging environment in which mortality rates are higher than at any point in the nestling period .…”

“…This important phenomenon, known as force fledging, is an under-studied aspect of avian ecology (Streby et al 2013) and may provide important insights into additional hypotheses regarding diel patterns of fledging. For example, the 'threshold size hypothesis' predicts that fledging begins when nestlings reach a specific developmental state (sensu Johnson et al 2004), whereas the 'energetics hypothesis' predicts that fledging begins after reaching an energetic threshold (Ribic et al 2018) or when nestlings are energetically ready (Ribic et al 2021, Santema et al 2021). However, if nestlings are both developmentally and energetically capable of fledging before such thresholds, as indicated by force fledging, then such findings should provide new insights into those hypotheses.…”