Forskning ved Københavns Universitet - Københavns Universitet

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Songbirds on the move: investigating migratory patterns, winter ecology and conservation of sub-Saharan migrants.

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

  • Mathilde Lerche-Jørgensen
Every year more than 2.1 billion birds migrate between their breeding grounds in Europe and their wintering ground in sub-Saharan Africa, tracking resources between seasons. However, over the recent decades the populations of these migrant species are generally declining throughout Europe. Although the decline has been linked to environmental conditions e.g. rainfall patterns and the type of the wintering habitat as well as to problems in adapting arrival time to climate change, little is known about the true drivers of these declines or how to reverse them. In this thesis I investigated general migratory patterns related to the timing of migration in order to understand possible effects of climate change on the annual cycle of migratory birds. My aim was further to increase knowledge about migration routes, staging sites and winter ecology in order to enable informed decision making about where and how to conserve the sub-Saharan migrants.It is generally assumed that competition for early arrival to the breeding area can cause migrants to arrive before it is optimal for their survival. In Chapter I, I assessed if arrival timing to the breeding area has consequences for adult annual survival in short and long distance migratory passerines. Further, I determined if migrants’ time their arrival to optimize survival. For long distance migrants, I found that those arriving early had low survival compared to the ones arriving at intermediate dates. In contrast, short distance migrants which arrived earliest had the highest survival compared to those that arrived later in spring. These results illustrate that rates of survival differ between short and long distance migrants as a consequence of their arrival date. In long distance migrants, males generally arrived before it was optimal for their survival, while females arrived at times that were most optimal. This suggests that competition for early arrival to the breeding ground can advance arrival date prior to the time that optimises survival in males.Mapping population specific migration routes’, staging sites and connectivity is important to determine conservation priority areas for the sub-Saharan migrants, and to understand at what stage in the annual cycle the drivers of decline are most influential. In Chapter II, I mapped migration routes and staging sites of one of the smallest sub-Saharan migrants, the willow warbler Phylloscopus trochilus, from a breeding site in Denmark to their overwintering sites in sub-Saharan Africa. All individuals migrated south-west through Spain and along the Atlantic coast before they diverged in their route. This divergence likely occurred when reaching Sahel, before wintering in the savanna zone of West Africa, in which more than 3000 km separated the most eastern and western birds, suggesting low connectivity. This high divergence in termination of migration seemed not to be related to timing, current or previous year’s vegetation conditions, nor biometrics, associated with flight performance and dominance hierarchy, and remains unexplained.It is generally assumed that migrants use the same habitat in the winter and breeding seasons, sometimes referred to as ‘niche-tracking’. In Chapter III, I investigated if home range size and habitat use in the breeding area is similar to the home range size and habitat use in the wintering area for three morphologically similar Phylloscopus warbler species, willow warbler, wood warbler P. sibilatrix and chiffchaff P. collybita. We found that while willow warbler’s increased their home range size in the wintering areas, the home range size for wood warblers did not increase. This might reflect that willow warblers are habitat generalists and wood warblers and chiffchaffs are habitat specialists, which could be limiting the availability of suitable habitat. All three species used more open habitat in the winter areas than in the breeding areas. However, while willow warbler and wood warbler maintained some degree of tall tree cover inside their home ranges, chiffchaff changed from a tree cover of more than 80% to tree cover of less than 1% indicating a shift in niche. These findings suggest that the assumption of niche tracking might not apply for all species.Land cover changes in the wintering area are feared to be one of the main causes for the decline in sub-Saharan migrants, yet we lack a general understanding of the winter ecology in migrants. In Chapter IV, I investigated density, home range size, territoriality and tree selection of the three migrant species; willow warbler, pied flycatcher Ficedula hypoleuca and melodious warbler Hippolais polyglotta, on disturbed and undisturbed woodland sites in West Africa. The three species differed in wintering strategies; willow warbler showed non-territorial behaviour and had larger home ranges than the other two. All species had smaller home range size in the undisturbed than at the disturbed site, probably due to lower tree density at the disturbed site. There was a general preference for taller trees and of the tree genus Acacia. The densities of pied flycatcher were lowest at the disturbed site suggesting the disturbed site was less suitable. In contrast, willow warbler densities were higher at the disturbed site, indicating that the disturbed site was not the least suitable for willow warblers. This implies that not all species are equally affected by agricultural disturbance.Studies of wintering ecology are often limited to a few sites which limit the ability to make general conclusions. In Chapter V, I estimated winter home range size and density of two farmland specialist species in an area with a low degree of human disturbance, and compared it to patterns from an area with increased disturbance. I found that density for both species and home range size for whinchat Saxicola rubetra was similar to what is found in more intensively farmed countries in West Africa. Yet, the large and overlapping home ranges found in yellow wagtails Motacilla flava contrast strikingly to the small home ranges and high territoriality found in species present in more intensively farmed West-African country.Overall, the findings in this thesis indicate that the competition for early spring arrival in long-distance migrants have survival consequences, which may influence their potential to adapt to climatic changes. While migratory route can be highly conserved within a species, individuals can show large variation in wintering sites. Generally there seems to be larger variation between species with regards to niche-tracking than previously assumed, which is also true with respect to their response to human induced changes to the landscape, indicating differences in vulnerability to land cover changes. These findings help to fill some of the knowledge gaps in migrant bird research which are important to improve the conservation of long-distance migrants wintering in sub-Saharan Africa.
OriginalsprogEngelsk
ForlagNatural History Museum of Denmark, Faculty of Science, University of Copenhagen
StatusUdgivet - 2017

ID: 181357641