18 Apr 2013
Causes of population change in a long-distance migratory passerine, the Willow Warbler (Phylloscopus trochilus)

BRANTA — Catriona Morrison

Causes of population change in a long-distance migratory passerine, the Willow Warbler Phylloscopus trochilus

Institution: University of East Anglia, UK
Supervisors: Jenny Gill, Rob Robinson, Jacquie Clark
Details: PhD 2011 (Completed)
Address: BTO, Thetford, UK (Apr 2013) Email
Subject Keywords: Willow Warbler, seasonal interactions, demography, African migrants
Species Keywords: Willow Warbler Phylloscopus trochilus



Over the past 40 years there have been large declines in the abundance of many migratory bird species which breed in Europe and winter in Africa. As potential causes of these declines exist across both the breeding and non-breeding seasons, identifying the key drivers of these population changes is complex. In this thesis, national-scale surveys of bird abundance and demography collected by the British Trust for Ornithology are used to explore the causes of these population changes. Using data from the Breeding Bird Survey, I show that, within Britain, there is strong evidence for geographic variation in the rate of population change of most long-distance migrant species, and particularly in one of Europe’s most abundant summer passerine migrants, the willow warbler, Phylloscopus trochilus. Since the mid-1990s, willow warbler populations have experienced rapid declines in abundance in the south-east of Britain while slightly increasing in the north-west. Using data from the Nest Record Scheme, I show that willow warbler productivity has also declined in the south-east, but remained stable in the north-west and may therefore have contributed to the differing patterns of population change. However, these patterns of population change could also be influenced by changes in environmental conditions during the non-breeding season. Significant variation in the stable isotope ratios of winter-grown feathers of willow warblers from different parts of Britain may indicate that birds from different parts of Britain also use different locations or resources within Africa, and may thus experience different environmental conditions. Using data from the Constant Effort Site (CES) scheme, I show that the survival rates of adult willow warblers are also significantly lower in the south-east than the north-west of Britain, and lower in females than males, but adult survival has not changed significantly in recent decades. However, the sex ratio of willow warblers caught in CES sites has become significantly more male-biased in both regions, suggesting possible increases in female mortality prior to adulthood. Thus, for willow warblers in south-east Britain, productivity has declined, juvenile female survival may have declined and adult survival rates are lower than those in north-west Britain. These findings highlight the likely importance of interactions between breeding and non-breeding season processes for the population dynamics of migratory species, and emphasize the need for a greater understanding of the processes influencing the survival of migrants in their first year of life.


Published Papers

Morrison,C.A., Robinson,R.A., Clark,J.A.& Gill,J.A. (2010) Spatial and temporal variation in population trends in a long-distance migratory bird. Diversity & Distributions 16: 620.

Morrison,C.A., Robinson,R.A., Clark,J.A., Marca, A.D.,, Newton, J. & Gill, J.A. (2013) Using stable isotopes to link breeding population trends to winter ecology in a migratory warbler. Bird Study (In press).

Morrison, C.A, Robinson, R.A., Clark, J.A., Risely, K. & Gill, J.A. 2013. Recent population declines in Afro-Palaearctic migratory birds: the influence of breeding and non-breeding seasons. Diversity and Distributions. doi: 10.1111/ddi.12084 View

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