Birds advancing lay dates with warming springs face greater risk of chick mortality

Abstract

In response to a warming planet with earlier springs, migratory animals are adjusting the timing of essential life stages. Although these adjustments may be essential for keeping pace with resource phenology, they may prove insufficient, as evidenced by population declines in many species. However, even when species can match the tempo of climate change, other consequences may emerge when exposed to novel conditions earlier in the year. Here, using three long-term datasets on bird reproduction, daily insect availability, and weather, we investigated the complex mechanisms affecting reproductive success in an aerial insectivore, the tree swallow (Tachycineta bicolor). By examining breeding records over nearly half a century, we discovered that tree swallows have continuously advanced their egg laying by ∼3 d per decade. However, earlier-hatching offspring are now exposed to inclement weather events twice as often as they were in the 1970s. Our long-term daily insect biomass dataset shows no long-term trends over 25 y but precipitous drops in flying insect numbers on days with low ambient temperatures. Insect availability has a considerable impact on chick survival: Even a single inclement weather event can reduce offspring survival by >50%. Our results highlight the multifaceted threats that climate change poses on migrating species. The decoupling between cold snap occurrence and generally warming spring temperatures can affect reproductive success and threaten long-term persistence of populations. Understanding the exact mechanisms that endanger aerial insectivores is especially timely because this guild is experiencing the steepest and most widespread declines across North America and Europe.

Warmer April Temperatures on Breeding Grounds Promote Earlier Nesting in a Long-Distance Migratory Bird, the Prothonotary Warbler

“…Beyond the potential for there to be a trophic mismatch, migratory birds nesting earlier on temperate breeding grounds may experience local weather conditions (e.g., temperature and precipitation) that affect when and how successfully they reproduce (Dunn and Winkler, 2010). Unusual weather events, in general, tend to negatively impact breeding birds (Shipley et al, 2020). For example, some migratory species may start breeding earlier because of warm weather on breeding grounds, only to have subsequent colder and/or wetter weather cause adult mortality, or reduce reproductive success by decreasing incubation efficiency and nestling provisioning (Brown and Brown, 2000;Conway and Martin, 2000;Coe et al, 2015;Cox et al, 2019;Shipley et al, 2020).…”

“…Unusual weather events, in general, tend to negatively impact breeding birds (Shipley et al, 2020). For example, some migratory species may start breeding earlier because of warm weather on breeding grounds, only to have subsequent colder and/or wetter weather cause adult mortality, or reduce reproductive success by decreasing incubation efficiency and nestling provisioning (Brown and Brown, 2000;Conway and Martin, 2000;Coe et al, 2015;Cox et al, 2019;Shipley et al, 2020). This is particularly true for aerial insectivores such as barn swallows Hirundo rustica and tree swallows Tachycineta bicolor where cold temperatures and precipitation can cause short-term food (insect) shortages linked to inadequate incubation and poor nestling growth (Coe et al, 2015;Cox et al, 2019), and for barn and cliff swallows Petrochelidon pyrrhonota where adults can starve when activity of flying insects is reduced by periods of cold and/or wet weather lasting four or more days (Brown and Brown, 2000).…”

Reduced Breeding Success Suggests Trophic Mismatch Despite Timely Arrival in an Alpine Songbird

“…Potentially colder days and even cold-snaps including snow and frost during the early season are more likely to occur in earlier years. This in general could negatively affect breeding success, via lower adult survival 39 and reduced reproductive success [40][41][42][43] , even if migrants match the advanced spring conditions. Harsh environmental conditions during the incubation period may also delay hatching, exposing the nest to predators for a longer period at the same time 44 .…”

Is earlier reproduction associated with higher or lower survival? Antagonistic results between individual and population scales in the blue tit

“…For example, in birds, while early reproduction might be beneficial in terms of reproductive success (Harriman et al, 2017; in the focal population: Marrot et al, 2018), it is also potentially associated with subsequent survival cost for the breeders (e.g. Brinkhof et al, 2002; Nilsson, 1994) or the early hatched nestlings (Shipley et al, 2020). The presence of these trade-offs could partly explain phenological mismatches ( i.e .…”

“…The phenological response of organisms to variation in their local environment is a major issue in evolutionary ecology and conservation research, both because the diversity of this response reflects differences in evolutionary potential and strategies between and within species (Thackeray et al, 2016), and because advances in reproductive phenology is seen as one of the universal ecological responses to global warming (Durant et al, 2007;Parmesan, 2007;Radchuk et al, 2019). Although it has been shown that phenology can respond to meteorological variations (noise and cycles) as well as climatic trends in many plant (Bjorkman et al, 2015;Diekmann, 1996) and animal (Duursma et al, 2018;Samplonius et al, 2018;Sheriff et al, 2015;Haest et al, 2018) species, little is known about the mechanisms of these responses and their effects on demography and population dynamics (Forrest and Miller-Rushing, 2010, but see Shipley et al, 2020;Simmonds et al, 2020).…”