Introduction | Publications

Introduction

A Great Reed Warbler in Zambia, after it’s journey across the Sahara and a stop-over period in north-central Africa.

Many songbirds that breed in Europe migrate across the Sahara to spend the northern winter in tropical Africa, and many are known to be declining greatly in numbers globally. However, our understanding of their population growth and decline is limited by very sparse information on individual success during the non-breeding season in Africa. Dr Marjorie Sorensen studied the little-known African lives of Great Reed Warblers (Acrocephalus arundinaceus) and Willow Warblers (Phylloscopus trochilus) in Zambia during her PhD research. Here are some of the questions she addressed:

How do conditions at migratory staging sites affect winter success? Recently, miniaturised-light level geolocators have allowed small bodied migratory birds to be tracked for the first time, and migratory patterns to and from Africa to be illuminated. Surprisingly, rather than direct flights to single winter sites the use of multiple long-term staging sites is common. Staged migration complicates our understanding of population dynamics and raises questions about whether conditions experienced during staging may carry over to affect long term success.

Marjorie worked with Dr Jason Newton (Scottish Universities Environmental Research Centre), Dr Susanne Jenni-Eiermann (Swiss Ornithological Institute) and Dr Graham Fairhurst (University of Saskatchewan) to apply lab techniques in stable isotope, plasma metabolite, and stress hormone analysis to assess conditions experienced during staging and their possible consequences for winter success. Marjorie and team’s paper in BMC Ecology shows how environmental conditions at the Great Reed Warbler’s staging sites in the Horn of Africa appear to carry over to affect their condition at their final wintering site in Zambia.

A Willow Warbler radio-tagged by Marjorie Sorensen to establish whether despite its extravagant song, this species really is territorial on the non-breeding grounds in Africa. Read the answer here.

Why do migratory birds sing in winter?

Why do migratory birds sing in winter? High investment in expensive winter song is a puzzling behaviour for Palearctic-African migrants. Sixty-two percent of migratory songbird species sing on their wintering grounds in Africa, outside the context of breeding and mate attraction. While song quality during breeding is known to be important for overall fitness, the function of winter song has never been tested. Marjorie tested three competing hypothesis for winter song function: 1) the long-held territory defence hypothesis, that individuals sing for defence of winter feeding territories, 2) the testosterone hypothesis, that winter singing is a non-adaptive carry-over from elevated testosterone levels during breeding, and 3) the song improvement hypothesis, that winter singing functions to improve individual song quality and future reproductive success. Please see Marjorie’s papers on Willow Warblers (in Behavioural Ecology) and Great Reed Warblers (in The American Naturalist) singing in Africa, and these articles featuring her research in New Scientist, The Atlantic, Science News and The Conversation; and listen to Marjorie discussing her work on the Canadian Broadcasting Corporation’s science programme, Quirks & Quarks.

Does avian malaria have negative consequences for wintering birds?

Avian malaria effects on host population dynamics in wild birds are poorly understood. This is due, in part, to the majority of studies taking place on European breeding grounds where birds have already entered the chronic stage of infection. While some slight negative effects of chronic infection have been documented for host fitness in Europe, understanding the consequences of avian malaria requires sampling individuals in areas of active parasite transmission. Some species-specific strains are transmitted in Africa and Marjorie worked with Dr Staffan Bensch (Lund University) to determine the type and intensity of these infections. Wintering birds may be in the acute stage of infection; therefore, sampling individuals in Zambia can provide valuable information on the effects of malaria infection for host winter success, survival, and population dynamics. See Majorie’s papers in Journal of Avian Biology and Ecology and Evolution for her answers so far.