Van de Casteele, T., Adriaensen, F. & Matthysen, E. 1998. Sex-specific relations between fitness components and natal dispersal in the Great Tit Parus major. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 223.

The evolution of natal dispersal has been a major subject of theoretical research for a long time, however, ecological processes causing selection pressures impinging on natal dispersal are not well known. Here, we report upon the results of a nestbox-breeding passerine, the Great Tit Parus major, in a patchy population in Antwerp, Belgium. More specifically, we have tested whether variation in natal dispersal is associated with differences in reproductive success (number of fledglings, fledging weight and size, fledging date) and adult survival, within the sexes. Moreover, we have searched for possible mechanisms explaining variation in reproductive success, namely variation in territory quality, mate quality and/or individual quality. Individuals were classified into three groups: (1) Residents = individuals ringed as pulli in the study area and subsequently breeding in their birth patch. (2) Dispersers = individuals ringed as pulli in the study area and subsequently breeding in a patch different from their birth patch. (3) Immigrants = individuals not ringed as pulli in the study area. For males, immigrants and dispersers had a lower reproductive success in terms of fledging weight and size, and recruited fewer offspring. Resident males were mated with females of higher phenotypic quality (tarsus length) than disperser and immigrant males. Male adult survival was not associated with dispersal status. For females, there were no differences in reproductive success, but immigrant females had a higher fledging success than resident and disperser females. Differences in fledging success were due to an interaction of male's phenotypic quality (tarsus length) and male's phenotypic quality (breast stripe width). Recruitment rate did not, and adult survival did not, consistently differ between females of different dispersal status. Our data support the hypothesis that natal dispersal entails a cost in terms of mate quality for males, and that there is no cost or even a benefit to dispersing for females.

Key words: dispersal, reproductive success, mate quality, survival

Tebbich, S., Fessl, B. & Dvorak, M. 1998. Adaptations to variable environments: Tool-use and feeding behaviour in three species of Darwin finches. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 223 - 224.

Darwin finches live in a highly unpredictable and variable environment. In the Galapagos Islands plant communities vary strongly along an altitudinal gradient. In addition variation in annual rainfall is extreme and irregular. Darwin finches show very special feeding techniques like tool-use (Woodpecker Finch), egg breaking and blood sucking (Sharp-beaked Ground-Finch) which might be adaptations to difficult and unpredictable conditions. The study focuses on an interspecific and intraspecific comparison of the feeding ecology of three closely related species of tree finches (Woodpecker Finch, Large Tree-Finch, Small Tree-Finch) in two very different vegetation zones. In the wet Scalesia zone, the species overlap in their feeding techniques, but the proportions of observed feeding techniques differ significantly. In the arid zone this difference becomes even more pronounced: The Large Tree-Finches specialise on only two feeding techniques, whereas the Small Tree-Finches enlarge the proportion of plant food in their diet. For the Woodpecker Finch tool-use becomes the most important foraging technique, which makes food resources available that are out of reach for all other species. We conclude that the ecology of those species causes specialisation both in the choice of habitats and in the feeding behaviour.

Key words: tool-use, Darwin finches, feeding ecology, specialisation

Hashmi, D. 1998. Global distribution of species richness of seabirds as a quasi-experimental test of the energy hypothesis. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 224.

The causes of species richness variation are often elusive. Many exploratory data analyses show that large-scale species richness is highly correlated with primary productivity in a variety of taxa. Quasi-experimental conditions to test the energy hypothesis, however, are rarely if at all met, because large-scale primary productivity often co-varies with other possibly important factors like habitat structure, Pleistocene glaciations or area. Defining seabirds as species regularly foraging in marine pelagic waters during the breeding season, geographically global primary productivity patterns contrast with other independent variables possibly limiting species richness. Crucially, pelagic primary productivity is highest in upwelling zones and subarctic/-antarctic latitudes where seabirds might have been able to respond to climate changes (ice ages) by latitudinal shifts of breeding ranges. Seabird species richness has been measured for different categories of coastlines of varying length and is compared with a number of factors including primary productivity, latitude, wind conditions and human population density. The results strongly suggest an important effect of primary productivity on species richness. This may reflect an ideal free distribution of populations or a linkage between primary productivity and speciation rates. The latter might be traceable by molecular clocks.

Key words: species richness, energy hypothesis, seabirds, productivity, biodiversity

Van Zyl, A.J. & Amutete, G. 1998. Time-energy budgets of Common Kestrels over latitudes: Energy minimisation or maximisation of intake rates? In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 224.

In the Netherlands, Common Kestrels Falco tinnunculus minimise energy expenditure during periods of low food availability (winter) and maximise intake rates during periods of food abundance (summer). Prey populations of Common Kestrels in the tropics and subtropics do not have the large population booms during spring and summer months as their temperate counterparts. Small mammal prey availability is not as high at lower latitudes as in the temperate regions. It can thus be hypothesised that Common Kestrels at lower latitudes would have a time budget that minimises energy expenditure rather than maximises intake rates. We applied this model to studies of South African (32 °S) and Kenyan (0°) Kestrel time budgets to assess whether they follow patterns of energy minimisation or intake rate maximisation during the early breeding season, a time when food is important in building up reserves for egg-laying. Kenyan and South African Kestrels spent more time perch hunting than Dutch Kestrels. Kenyan Kestrels spent a very low proportion of their time hover hunting while South African Kestrels spent similar times to Dutch Kestrels hover hunting. Kenyan and South African Kestrels caught more small prey items than Dutch Kestrels. The results suggest that Kenyan and South African Kestrels avoid using energy expensive methods to catch small prey. The high energy expenditure of South African Kestrels can possibly be explained by the low rainfall during the study season, forcing them to use an energetically more expensive hunting method but with higher yields.

Key words: life history, latitude, Falco tinnunculus, time-energy budget, raptor

Roos, S., Söderström, B. & Pärt, T. 1998. Avian nest predators affect the spatial distribution of nesting Red-backed Shrikes Lanius collurio. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 225.

Accipiter-hawks and falcons Falconidae are known to affect the spatial distribution of their prey. Avian nest predators, however, have, with few exceptions, not been reported to have any influence on prey distribution. In a farmland area in Central Sweden, the open-nesting Red-backed Shrike Lanius collurio suffers from high nest predation rates (30% of all nesting attempts were predated during 1994 to 1997), caused by avian nest predators (i.e. Corvidae). Could these high rates of predation influence Red-backed Shrikes to nest in Corvidae-free sites? Our results show that Red-backed Shrikes avoided to nest in pastures where Black-billed Magpies Pica pica were recorded. Furthermore, an experiment with artificial Red-backed Shrike nests with quail and clay eggs suggests that Red-backed Shrikes adjusted their habitat choice in order to avoid the most effective nest predator, the Magpie. This study is one of the first to report that the actual presence of nest predators, and not habitat characteristics associated with high nest predation risk (which is the case with most other studies), affect the spatial distribution of prey species.

Key words: Lanius collurio, Corvidae, nest predation, spatial distribution, predator-free sites

Thibault, J.C. & Martin, J.-L. 1998. Introduced species and biogeography: Impact of rats on bird distribution in the western Mediterranean. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 69: 225.

Islands were seminal in developing theories in ecology and biogeography. Species distribution on islands is usually analysed as the result of natural processes. Recent findings on individual species ecology on islands and the discovery of numerous subfossil birds that vanished after alien species were introduced on islands by humans shed new light on bird distribution on islands. We analyse how the introduction of the Black Rat Rattus rattus in the Western Mediterranean two thousand years ago has modified the distribution of colonial birds at a regional and local scale. Impact varies in relation to species size, biology and island area. Smaller species are restricted today to small rat free islands. Larger species are able to maintain populations on rat infested islands. On small rat infested islands where rat densities are highest and interannual rat density fluctuations lowest, reproductive success in these species is negligible. On larger islands where rat densities are lower and interannual fluctuations high these long-lived species are able to reproduce successfully at regular intervals. This is enough to maintain their populations both on larger and smaller islands. These differences in response to the presence of rats are key components of their current distribution. They also improve the understanding of their past distribution.

Key words: biogeography, introduced species, species distribution, colonial birds, Mediterranean islands