Seasonal variation in density dependence in age‐specific survival of a long‐distance migrant

Abstract

Density dependence in vital rates is key to population regulation. Rather than being constant, the strength of density dependence may vary throughout the year, but empirical evidence is limited. Based on 22 years of data of color-banded birds from a recovering population of Eurasian Spoonbills Platalea leucorodia leucorodia, we show, for the first time, seasonal variation in density dependence in survival of a long-distance migrating bird. Combining resightings and dead recoveries at breeding, stopover, and nonbreeding areas enabled us to (1) separate true survival from permanent emigration from the breeding area, and (2) estimate survival in three seasons: summer, early winter (including autumn migration), and late winter (including spring migration). Accompanying the rapid population growth, juvenile annual survival initially increased, manifested in early winter, but thereafter, at high population sizes, it strongly decreased through a combination of decreasing survival in all seasons. Annual survival of subadult (second- and third-year) and adult birds decreased more gradually with increasing population size, with density dependence occurring in early winter for subadults and late winter for adults. Thus, the shape and strength of density dependence in survival varied with age and season. Understanding the seasonal timing of density dependence, especially with reference to underlying mechanisms, is important for the design of effective conservation strategies.

Weather conditions and conspecific density influence survival of overwintering DunlinCalidris alpinain North Wales

“…Dunlin are highly gregarious, particularly during winter, but this appears to come at some cost since juvenile survival decreased with increasing population size. (Saether et al 2000) and Eurasian Spoonbills Platalea leucorodia leucorodia (Lok et al 2013), indicating that competition for prey resources can be an important factor in regulating survival. The negative effects of increased conspecific density seen in this study imply that reduction in appropriate habitat and roost sites through disturbance and coastal squeeze could decrease survival of juvenile birds.…”

“…Survival in other groups of species, including passerines and waterbirds, has also been observed to vary with population density on both the breeding and wintering grounds (Frederiksen and Bregnballe 2000, Lok et al 2013, Norman & Peach 2013. Competition for resources is likely to be the most important factor behind this density-dependent variation, particularly between less experienced foraging juveniles and more efficient adults; interaction with weather and predation have also been observed (Durell et al 2000, Frederiksen & Bregnballe 2000, Whitfield 2003, Tavecchia et al 2007.…”

Dispersal and migration of a specialist waterbird: where do Eurasian Spoonbills (Platalea leucorodia) come to Hungary from?

“…Most of the Spoonbills observed in Hungary were 1cy old, and very few were 2cy old. Spoonbills generally suffer large mortality during their first year of life (Pigniczki 2010, Lok 2013b and only a few immature Spoonbills return to their natal area, because most of them stay in the wintering ground or in proper wetlands between the wintering area and the natal area (Pigniczki 2015, Pigniczki & Végvári 2015.…”

“…Spoonbills from the Wadden Sea area mainly follow the East Atlantic Flyway, and move to the Atlantic coast of France, Iberia or sub-Saharan Africa (Lok et al 2011(Lok et al , 2013a(Lok et al , 2013b, however, a few individuals were also observed in Tunisia (Smart et al 2007). A few Spoonbills of East Atlantic origin also reached continental Central Europe.…”

“…Therefore, several authors studied them in detail, including their dispersal, migration, survival, habitat use, ecology and conservation (Triplet et al 2008, Lok et al 2011, 2013a, 2013b, Kralj et al 2012, Overdijk & Navedo 2012, Pigniczki & Végvári 2015, Pigniczmarked abroad and observed in Hungary. Spoonbills marked abroad and observed in Hungary were studied previously only marginally (Pigniczki 2009, 2010, Pigniczki & Karcza 2013.…”