Disentangling the effects of environmental conditions on wintering and breeding grounds on age‐specific survival rates in a trans‐Saharan migratory raptor

Abstract

Migratory species are subject to environmental variability occurring on breeding and wintering grounds. Estimating the relative contribution of environmental factors experienced sequentially during breeding and wintering, and their potential interaction, to the variation of survival is crucial to predict population viability of migratory species. Here we investigated this issue for the Montagu's harrier Circus pygargus, a trans‐Saharan migrant. We analysed capture–recapture data from a 29‐year long monitoring of wing‐tagged offspring and adults at two study sites in France (Rochefort‐RO and Maine‐et‐Loire‐ML). The study period covers a climatic shift occurring in the Sahel with increasing rainfall following a period of droughts (Sahel greening). We found that harriers’ adult survival in RO (between 1988 and 2005) varied over time and was sensitive to the interaction between the amount of rainfall in the Sahel and the annual mean breeding success, two proxies of prey availability. The occurrence of adverse conditions on breeding and wintering grounds in the same year decreased survival from 0.70–0.77 to 0.48 ± 0.05. Juvenile survival in RO was slightly more sensitive to conditions in Europe than in the Sahel. Unexpectedly, lower survival rates were found in years with higher mean breeding success, suggesting compensatory density feedbacks may operate. By contrast, adult survival in ML, monitored between 1999 and 2017, was higher compared to RO (0.76 ± 0.03 versus 0.66 ± 0.02), remained constant and unaffected by any proxy of prey availability. This difference seems consistent with the fact that harriers in ML experienced better and especially less variable environmental conditions during breeding and wintering seasons compared to RO. Overall, we showed that survival of a migratory bird is sensitive to the level of variability in environmental conditions and that adverse conditions on wintering grounds can amplify the negative effects of conditions during the previous breeding season on birds’ survival.

The demography of an increasing insular Eurasian Scops Owl (Otus scops) population in western France

“…As found in other European trans-Saharan migratory species (Kanyamibwa et al 1989, Barbraud et al 1999, Johnston et al 2016, Millon et al 2019, Boano et al 2020, there was a positive effect of Sahel rainfall on survival. For the Scops Owl, the results suggest that September precipitation conditions in the Sahel are important.…”

“…2016, Millon et al . 2019, Boano et al . 2020), there was a positive effect of Sahel rainfall on survival.…”

Multi-species prey dynamics influence local survival in resident and wintering generalist predators

“…This likely increases habitat suitability of the study area for wintering buzzards over the years. For migratory animals, wintering areas are crucial for their survival and population dynamics, since winter is a particularly challenging season with harsher climatic conditions and higher competition for food (Grande et al 2009 ; Hinnebusch et al 2010 ; Wellicome et al 2014 ; Sergio et al 2014b ; Baltag et al 2018 ; Millon et al 2019 ). Furthermore, studying winter survival of species having resident populations and migrant populations, such as buzzards, have great interest to assess the ecological and evolutionary effects of a warming climate and the shift in distribution ranges on wintering strategies (Paprocki et al 2014 ; Martín et al 2014 ; Sanz-Aguilar et al 2015 ).…”

“…For short-lived species, stochastic variability in food availability and predators largely influence their population dynamics (bottom-up and top-down regulation, respectively) (Hanski et al 1993 ; Frederiksen et al 2006 ). For long-lived animals lacking predators, fluctuations in prey availability, in the absence of additive mortality, regulate their population dynamics by density-dependence (Saether and Bakke 2000 ; Millon et al 2019 ). Life-history theory predicts that survival of long-lived organisms tends to be buffered against environmental stochasticity, whereas other traits such as recruitment, skip breeding and mainly fertility are more sensitive to this stochasticity (Lande et al 2003 ; Karell et al 2009 ).…”

Differential survival throughout the full annual cycle of a migratory bird presents a life‐history trade‐off

“…These include studies based on resightings of marked individuals that are able to estimate apparent survival during different annual stages (e.g. Lok et al., 2015; Millon et al., 2019; Rockwell et al., 2017; Rushing et al., 2017; Sillett & Holmes, 2002; Swift et al., 2020), as well as studies that have used global‐scale remote‐tracking systems to track individuals with great precision, thus enabling the determination of the timing and location of mortalities and, thus, estimation of true survival throughout the full annual cycle (e.g. Cheng et al., 2019; Klaassen et al., 2014; Oppel et al., 2015; Senner et al., 2019; Sergio, Tavecchia, et al., 2019).…”