Ecophysiological aspects of rapid population growth in a novel migratory blackcap (Sylvia atricapilla) population: an experimental approach

Abstract

Blackcaps (Sylvia atricapilla) that breed in central Europe have usually migrated to Mediterranean or African wintering grounds. In the past several decades, a portion of this breeding population has started migrating to the British Isles to overwinter and this population has increased dramatically. Several factors, including higher annual survivorship (due to supplemental feeding and reduced migratiry distance), assortative mating, and enhanced reproductive success may be involved in this rapid population growth. As part of an intensive, long-term study of this population, we tested the hypothesis that the differences in photoperiod experienced by British-wintering versus Mediterranean-wintering blackcaps might lead to relatively early vernal (i.e., migratory and/or reproductive) physiological condition in members of the former group. We found that birds exposed to photoperiodic conditions that simulated migration to Britain to overwinter generally initiated vernal migratory activity earlier than birds held under conditions simulating migration to traditional wintering areas in central Spain. This difference, coupled with the shorter migratory distance to the British Isles, leads to significantly earlier estimated arrival dates for blackcaps that winter in Britain compared to central Spain. Bimodality in arrival times suggests that assortative mating on central European breeding grounds might occur between members of the different wintering populations. Males exposed to British-winter photoperiods showed significantly earlier testicular development than males kept under Spanish-winter photoperiods. Early arrival on the breeding grounds, coupled with accelerated reproductive condition, should lead to a relatively early reproductive effort, perhaps increasing average reproductive success. In general, these results support the hypothesis that differences in photoperiod on the wintering grounds may play an important role in the dynamic state of this population.

Phenotypic changes in spring arrival: evolution, phenotypic plasticity, effects of weather and condition

“…As a consequence, they arrive significantly earlier in spring and start breeding earlier (Bearhop et al 2005). But the main reason for their advancement in spring is that their circannual rhythm is accelerated by the photoperiodic conditions in the north (Terrill & Berthold 1990. At the beginning of this process, i.e.…”

Spring arrival along a migratory divide of sympatric blackcaps (Sylvia atricapilla)

“…Using stable isotope analyses as a marker of origin, Bearhop et al (2005) documented that birds migrating along this newly established northwestern route are more likely to mate with each other than with migrants from the traditional south-western route with winter quarters in the Mediterranean region. The lag in mean arrival time from the two distinct wintering grounds may be sufficient to diminish gene flow (Bearhop et al 2005;Berthold et al 1992;Terrill and Berthold 1990). However, the delay in spring arrival has only been estimated indirectly based upon flight distance and average migration rate (Terrill and Berthold 1990) rather than based upon observed arrival dates in spring.…”

“…The lag in mean arrival time from the two distinct wintering grounds may be sufficient to diminish gene flow (Bearhop et al 2005;Berthold et al 1992;Terrill and Berthold 1990). However, the delay in spring arrival has only been estimated indirectly based upon flight distance and average migration rate (Terrill and Berthold 1990) rather than based upon observed arrival dates in spring.…”

Population differentiation of migratory directions in birds: comparison between ringing results and orientation behaviour of hand-raised migrants

“…the NW migration of Central European blackcaps, are largely unknown. Differential arrival times on the breeding grounds (Terrill and Berthold 1990) could lead to assortative mating among birds coming from the same wintering grounds. This hypothesis could be tested by selective breeding (in captivity) of early and late arriving birds and testing their offspring's innate orientational preferences.…”