Survival and development of predator avoidance in the post-fledging period of the Whinchat (Saxicola rubetra): consequences for conservation measures

Abstract

Farmland bird populations in Europe are shrinking, largely due to modern agriculture practices. In grasslands, the shift to early mowing is believed to be particularly responsible for this decline because it is interwoven with breeding time—a change that birds in general have not adapted to. We studied the post-fledging survival of the Whinchat, an altricial grassland passerine. Based on a sample of 74 radiotagged young Whinchats, we confirmed that they fledge at 13–15 days. Twenty-four fledged birds died, while 18 of them were depredated. The survival probability of young Whinchats, calculated using Kaplan–Meier estimates, was lowest during the first day after fledging, and slowly leveled off later. In total, the probability that a juvenile survived the first month after fledging was 0.52. During the first days after fledging, all chicks were found hiding on the ground among the grass stubble. The generalized linear mixed model predicts that half of the fledglings at 20 days old and 20% at 22 days old (8–10 days after fledging) will not escape but will instead remain still when approached by a threat. Sixteen depredations occurred during this period, all by mammal predators. Afterwards, the fledglings changed their strategy, escaping through flight. After 12 days, more and more of the chicks were found exposed on top of the grass or on the bush instead of hiding on the ground. The transition from sit and hide to escape behavior reduced the predation rate considerably. Only two chicks were depredated later, both by an avian predator. Due to the initial predator avoidance strategy, the current conservation measure (to postpone mowing until the chicks in 80% of all nests have fledged) is not sufficient to halt population decline on improved meadows. We suggest that at least 10 days more, and possibly even 14 days more, are necessary to maintain viable Whinchat populations.