Stage-dependent survival in relation to timing of fledging in a migratory passerine, the Northern Wheatear (Oenanthe oenanthe)

Abstract

A better understanding of factors affecting population change is needed to explain declines of longdistance migrants. As juvenile survival is generally an important determinant of population dynamics, assessing whether juvenile survival is primarily affected either during the post-fledging stage on the natal site or during the migration and winter stages (migration-winter) is important for developing conservation strategies. Here, we assess variation in stage-dependent survival of juvenile Northern Wheatears (Oenanthe oenanthe), a threatened passerine in northwestern Europe. We estimate apparent survival in a Dutch coastal breeding population based on frequent resightings during the whole breeding season. We show that post-fledging survival on the natal grounds was not clearly different from survival during migration-winter and that late fledging reduces survival during both post-fledging and migration-winter. It is unknown which factors are causal to lower survival of late-fledged juveniles and this hampers effective conservation. Meanwhile, conservation measures focusing on nest protection should increase average juvenile survival in the remaining small populations in the short term because the number of successfully fledged early juveniles should increase. Flüggewerden die Ü berlebenswahrscheinlichkeit sowohl in der Nachbrutzeit als auch während des Zuges und der Ü berwinterung verringert. Es ist unbekannt, welche Faktoren das geringere Ü berleben der spät ausgeflogenen Jungvögel bewirken, was einen effektiven Schutz erschwert. Mittlerweile führen Schutzmaßnahmen mit Fokus auf den Schutz von Nestern kurzfristig zu einem Anstieg der durchschnittlichen Ü berlebenswahrscheinlich der Jungvögel in den verbliebenen kleinen Populationen, da die Anzahl der erfolgreich ausgeflogenen frühen Jungvögel ansteigt.