The biotic and abiotic drivers of timing of breeding and the consequences of breeding early in a changing world

Abstract

The decision of when to breed is an important determinant of individual fitness. However, despite a multitude of studies investigating the intraspecific relationship between timing of breeding and reproductive performance, less is known about why the strength of this relationship varies between species. Furthermore, environmental change has the potential to alter the relationship between lay date and fitness, but there is still a limited understanding of what mechanisms drive these differential responses to change environmental conditions. We propose that the potential effects of environmental change on the relationship between timing of breeding and fitness are dependent on 2 primary factors: (1) the potential constraints imposed by breeding early and (2) the drivers of higher fitness of early breeders. We first summarize multiple hypotheses proposed to explain why breeding early, either based on absolute date or relative to conspecifics, increases fitness. We then summarize the factors that may constrain when individuals initiate breeding, including limits on the ability to advance their lay date or extend the length of their breeding season under favorable conditions. Understanding constraints on the timing of breeding allows for the identification of obligate (single-brooded species that do not attempt to breed after a specific date) and facultative (predominantly multi-brooding species that have long breeding seasons) early breeding species that are likely differently affected by climate change. Finally, we propose a simple mathematical formula that incorporates the costs and benefits associated with early breeding to quantify how climate change could influence the benefits of early breeding and either mitigate or exacerbate the costs. Our cost-benefit approach provides a clear framework to predict how species may shift the timing of their breeding to maximize fitness in a changing world.

Duration and variability of spring green‐up mediate population consequences of climate change

“…The 'window of resource availability' is defined as time window between 25th and 75th percentiles of spring green-up. later relative to the onset of spring was equally penalized in our model (but see Sutton & Freeman, 2023). Individuals that initiated breeding outside of the window of resource availability failed breeding entirely and thus, their reproductive success was 0.…”

Utilizing artificial nesting platforms as a management tool: enhancing breeding productivity of Western Flycatchers ( Empidonax difficilis occidentalis ) in southwestern Colorado and southern Arizona, USA

“…In Colorado and Arizona, the Western Cordilleran Flycatcher is limited to one complete nesting cycle each breeding season (Kingery 1998, Darrah andvan Riper 2021), as are most Empidonax flycatchers in North America (Mumford 1964, Walkinshaw 1966, Sogge et al 2003). An abbreviated nesting season greatly influences annual productivity and population dynamics of small passerine birds (e.g., Kus et al 2017, Sutton andFreeman 2023), and if a nest fails during late incubation or the nestling stage, it is not possible for the Western Cordilleran Flycatcher to successfully re-nest. Rearing a second successful brood is prohibited by the late initiation of nesting, nesting cycle length (43 days from nest building to fledging), and the species' August departure for the wintering grounds in Mexico.…”

Why do avian responses to change in Arctic green‐up vary?

“…Although phenological responses to climate variability are well‐documented, responsiveness (i.e., the covariation between climate variability and phenology of life history events) varies within and across taxonomic groups, both in magnitude and direction (Ge et al., 2015; Iler et al., 2013; Zografou et al., 2021). Whereas the factors that influence timing of life history events have been well‐studied across multiple taxa and systems (Sutton & Freeman, 2023; Woods et al., 2022), increasingly, phenology is being studied in the specific context of a changing climate. Current efforts are primarily focused on evaluating correlations between phenological responses and species traits and describing geographic trends in relation to environmental cues (Kluen et al., 2017; Song et al., 2020; Woods et al., 2022).…”