Parental benefits and offspring costs reflect parent–offspring conflict over the age of fledging among songbirds

Abstract

Parent–offspring conflict has explained a variety of ecological phenomena across animal taxa, but its role in mediating when songbirds fledge remains controversial. Specifically, ecologists have long debated the influence of songbird parents on the age of fledging: Do parents manipulate offspring into fledging to optimize their own fitness or do offspring choose when to leave? To provide greater insight into parent–offspring conflict over fledging age in songbirds, we compared nesting and postfledging survival rates across 18 species from eight studies in the continental United States. For 12 species (67%), we found that fledging transitions offspring from comparatively safe nesting environments to more dangerous postfledging ones, resulting in a postfledging bottleneck. This raises an important question: as past research shows that offspring would benefit—improve postfledging survival—by staying in the nest longer: Why then do they fledge so early? Our findings suggest that parents manipulate offspring into fledging early for their own benefit, but at the cost of survival for each individual offspring, reflecting parent–offspring conflict. Early fledging incurred, on average, a 13.6% postfledging survival cost for each individual offspring, but parents benefitted through a 14.0% increase in the likelihood of raising at least one offspring to independence. These parental benefits were uneven across species—driven by an interaction between nest mortality risk and brood size—and predicted the age of fledging among species. Collectively, our results suggest that parent–offspring conflict and associated parental benefits explain variation in fledging age among songbird species and why postfledging bottlenecks occur.

Breeding seasonality generates reproductive trade-offs in a long-lived mammal

“…Trade-offs over birth timing may be widespread in long-lived species with slow life histories, for which development from birth to independence spans several months, therefore exceeding the length of the most productive season. In such cases, different stage(s) of the reproductive cycle may be synchronized with one or more seasonal food peaks, with the specific pattern dependent on the trade-offs females make among different fitness components (34). Such variation could account for empirical cases where the observed birth peak fails to coincide with the birth timing expected on the basis of a single fitness measure.…”

Cross-scale habitat selection reveals within-stand structural requirements for fledgling Golden-winged Warblers

“…While the post-fledging period is a relatively short component of the full-annual-cycle, lasting up to two months between fledging and juvenile independence, it has been identified as a critical time period wherein young learn to forage and often disperse before becoming independent (Pagen et al 2000, Chandler et al 2012. Additionally, this period has been identified as a potential "population bottleneck" in that it can limit reproductive output and alter demographics (Robinson et al 2004, Sillet and Holmes 2002, Naef-daenzer and Gruebler 2016, Jones et al 2020. The availability of high-quality habitat during the post-fledging period has the potential to mitigate challenges faced by fledgling songbirds by improving food availability and providing protective cover (Yackel Adams et al 2006, Fisher andDavis 2011).…”

“…Post-fledging songbird studies frequently examine habitat selection, with the assumption that preferred (i.e., selected) habitats are of higher quality (Jones 2001, Johnson 2007. Such studies have laid the groundwork for much of what we know about post-fledging habitat associations for passerine species (Anders et al 1998, Goguen 2019, Fiss et al 2020). Habitat selection is also understood to be hierarchical and scale dependent (Johnson et al 1980), and the choices made at one scale can influence those made at other scales (Francis et al 2017).…”

“…Therefore, it is important to consider how selection patterns interact across spatial scales (e.g., how microhabitat selection is influenced by stand-level selection) because this could dictate management actions at one or both scales. For species known to make stand-level habitat shifts during post-fledging (e.g., Wood Thrush, Anders et al 1998; Ovenbird [Seiurus aurocapilla], Streby and Andersen 2013; Golden-winged Warbler [Vermivora chrysoptera], Fiss et al 2020), it would benefit land managers to understand what structural features fledglings use or select and if these preferences are specific to the developmental stage of the stand.…”

“…Mean clutch size for the species is 5 (Confer et al 2011), though a range of 3 to 6 is not uncommon (C. Fiss personal observation). Broodsplitting occurs after fledgling (Peterson et al 2016) and fledglings may travel up to 2 km from nest sites before independence (approximately 28 days post-fledging; Fiss et al 2020). The species has undergone at least 50 yrs of population declines at rates of -8.56%/yr (Appalachians) and -0.89%/yr (Great Lakes; Sauer et al 2017).…”