Abstract
Seasonal declines in avian clutch size are well documented, but seasonal variation in other reproductive parameters has received less attention. For example, the probability of complete brood mortality typically explains much of the variation in reproductive success and often varies seasonally, but we know little about the underlying cause of that variation. This oversight is surprising given that nest predation influences many other life-history traits and varies throughout the breeding season in many songbirds. To determine the underlying causes of observed seasonal decreases in risk of nest predation, we modeled nest predation of Dusky Flycatchers (Empidonax oberholseri) in northern California as a function of foliage phenology, energetic demand, developmental stage, conspecific nest density, food availability for nest predators, and nest predator abundance. Seasonal variation in the risk of nest predation was not associated with seasonal changes in energetic demand, conspecific nest density, or predator abundance. Instead, seasonal variation in the risk of nest predation was associated with foliage density (early, but not late, in the breeding season) and seasonal changes in food available to nest predators. Supplemental food provided to nest predators resulted in a numerical response by nest predators, increasing the risk of nest predation at nests that were near supplemental feeders. Our results suggest that seasonal changes in foliage density and factors associated with changes in food availability for nest predators are important drivers of temporal patterns in risk of avian nest predation.
Highlights
The reproductive period is a critical time for many organisms; individuals must resolve when, where, and how many offspring to produce
Numerous authors have reported that the risk of nest predation changes during the breeding season [22,23,24,25], but surprisingly few empirical studies have examined how and why nest predation changes with breeding date. We examined these questions in Dusky Flycatchers (Empidonax oberholseri) nesting in montane meadows in northern California, where nests were at greater risk of predation early in the breeding season [26]
Previous studies have suggested that the seasonal decline in the risk of nest predation was caused by seasonal changes in foliage density [71,72] or changes in activity or behavior of predators [23,24,73,74]
Summary
The reproductive period is a critical time for many organisms; individuals must resolve when, where, and how many offspring to produce. Individuals that breed later in the breeding season typically produce smaller broods or have offspring of lower quality [1,4,7]. Despite many studies seeking to identify the mechanism(s) underlying this common pattern, the cause(s) of the seasonal decline in reproductive success are still debated [10,11,12], in part because experiments cannot manipulate timing of breeding without affecting individual quality. Past studies have rarely tested more than one mechanistic hypothesis to explain the underlying cause of the seasonal decline in avian reproductive success (but see [13,14])
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