Nesting Synchrony and Clutch Size in Migratory Birds: Capital versus Income Breeding Determines Responses to Variable Spring Onset

Abstract

Synchronous reproduction of birds has often been explained by benefits from nesting together, but this concept fails to explain observed intraspecific variation and climate-mediated changes of breeding synchrony. Here, we present a theoretical model of birds that store resources for reproduction (capital breeders) to show how breeding synchrony, clutch size and offspring recruitment respond to changes in timing of first possible breeding date. Our approach is based on individual fitness maximization when both pre-breeding foraging and offspring development are time-constrained. The model predicts less synchronous breeding, smaller clutch size, and higher chances for offspring recruitment in capital breeding birds that advance their nesting. For contrast, we also show that birds that need to acquire resources during egg laying (income breeders) do not change nesting synchrony but increase clutch size along with earlier breeding. The prediction of stronger nesting synchronization of capital breeders in years with late nesting onset is confirmed by empirical data on breeding synchrony of a high-latitude capital breeding sea duck, the common eider (Somateria mollissima). We predict that in warming highlatitude ecosystems, bird species that depend on stored reserves for reproduction are expected to desynchronize their nesting.

Top–down effects override climate forcing on reproductive success in a declining sea duck

“…In contrast, the staple food of eiders, the blue mussel Mytilus trossulus , is abundant and available all year round during the ice‐free season (Westerbom et al 2019). This notwithstanding, one could envision also global warming‐induced breeding desynchronization, particularly in capital breeders relying on stored reserves (Ejsmond et al 2021). As formalized in a life‐history model by Ejsmond et al (2021), late‐arriving and/or poor‐condition individuals may be unable to accumulate sufficient body reserves in time to allow prompt breeding initiation during early spring conditions, translating into increased population‐level breeding asynchrony.…”

“…This notwithstanding, one could envision also global warming‐induced breeding desynchronization, particularly in capital breeders relying on stored reserves (Ejsmond et al 2021). As formalized in a life‐history model by Ejsmond et al (2021), late‐arriving and/or poor‐condition individuals may be unable to accumulate sufficient body reserves in time to allow prompt breeding initiation during early spring conditions, translating into increased population‐level breeding asynchrony. Conversely, late annual breeding phenology in capital breeders is expected to increase breeding synchrony (Ejsmond et al 2021).…”

“…As formalized in a life‐history model by Ejsmond et al (2021), late‐arriving and/or poor‐condition individuals may be unable to accumulate sufficient body reserves in time to allow prompt breeding initiation during early spring conditions, translating into increased population‐level breeding asynchrony. Conversely, late annual breeding phenology in capital breeders is expected to increase breeding synchrony (Ejsmond et al 2021). Clearly, these predictions do not match well with our findings, as median laying occurred later concurrent with decreased breeding synchrony over time.…”

“…We also examined the drivers of intra‐annual variability in breeding phenology and body condition, allowing us to evaluate the relative importance of climate forcing and predation‐driven control on breeding decisions and performance. Current literature is divided as to whether reproductive synchrony is, first and foremost, an antipredator strategy (Ims 1990, Abbey‐Lee and Dingemanse 2019, Descamps 2019), or linked to climate forcing (Lehikoinen et al 2006, Halupka and Halupka 2017, Hällfors et al 2020, Ejsmond et al 2021). The severity of predation or climate effects is also intricately linked to the distribution of individual body conditions within the population.…”

Migratory vertebrates shift migration timing and distributions in a warming Arctic

“…First, Arctic regions differ in the rate of climate warming and local response in for example advancement of resource abundance [205], date of snowmelt [135] or sea ice dynamics [159], which can drive differentiation in responses in migration timing. Second, flexibility in migration timing may depend on reproduction strategies, depending on whether animals reproduce in the Arctic or southern wintering grounds [159,160], or whether animals rely more on internal energy reserves or on local resources for successful reproduction [206][207][208]. Third, Arctic migrants may also be constrained to make advancements in timing by, for example, the time needed for fuel deposition [19,209], little potential to increase travel speed [17], a lack of relevant cues to time their migration [30], or physical barriers during migration, such as earlier ice break-up in rivers [152,210] or available light [119].…”