Bird diversity was assessed by point-transect-count sampling during the dry and the wet season in riverine gallery woodland and in Colophospermum mopane woodland with different levels of elephant impact. Dramatic woodland degradation did not result in a dramatic overall reduction in bird diversity, but resulted in substantial changes in bird species composition and capacity for migrant birds. Less affected, more dense woodlands functioned as dry season refuges for 'resident' Afrotropical species, but this function was lost in degraded woodlands. In contrast, secondarized habitat had a much higher capacity for long distance migrants in the wet season. Species endemic to the sub-region and subspecies with restricted ranges centred in the area were not negatively affected by woodland degradation. The cumulative interference of past and current elephant numbers with the conservation of bird diversity appears to be insignificant in northern Botswana.

Effects of woodland modification by African elephant Loxodonta africana on bird diversity in northern Botswana

Previous ecological studies of Palearctic passerine migrants in Africa have claimed to reveal some general features with respect to habitat use, foraging ecology and interspecific relationships with Afrotropical residents. In this review we discuss apparent contradictions between earlier generalisations and more recent results from more detailed field studies and explore in which areas our ecological knowledge and theoretical understanding remain poor and have given rise to misconceptions. For example, it has been claimed that migrants use structurally more diverse and open habitats and that they forage higher and in more peripheral parts of the vegetation than their ecologically similar Afrotropical counterparts, yet in the past these characteristics were often not clearly defined and not always correlated in practice. It has also been stated that migrants are more flexible in habitat use, occupying a wider range of habitat types and employing a higher diversity of foraging techniques, both of which were assumed to be adaptations to permit coexistence with Afrotropical residents by using untapped resources that are only seasonally available. Yet results from studies of the role of competition in shaping migrant-resident communities remain largely unconvincing. While flexibility may facilitate migrant-resident coexistence, it may also favour the evolution of migration because specialists are less able to use their advantages in different environments. We note that definitions of flexibility and specialisation may themselves depend on the ecological or evolutionary approach adopted by researchers. We conclude that few generalisations can safely be made about the ecology of Palearctic migrants in Africa and that adaptive explanations for the behaviours observed are largely lacking, as are studies of the fitness consequences of different migrant strategies such as have been conducted in the Nearctic-Neotropical migration system.

Palearctic passerines in Afrotropical environments: a review

In bird migration systems, the question of coexistence and competition of migrants with residents in the nonbreeding season and their role in shaping the evolution of present avian communities is much debated However, conclusions are often drawn in a speculative way In the Palearctic-African bird migration system, many studies have addressed the question of coexistence of Palearctic breeding birds with Afrotropical species in the former's wintering grounds These studies have led to some generalizations concerning habitat selection and foraging ecology about the traits that might enable migrants to coexist with residents Migrants were therefore assumed to forage in more open habitats, in more peripheral parts of the vegetation and with a higher foraging speed than residents Furthermore, they were also assumed to be more flexible in foraging behavior by using a wider range of foraging tactics, but some studies revealed contradicting results We studied the ecology of Palearctic migrants in Comoe´ National Park, Ivory Coast, West Africa, during three successive winters to explore the factors of niche partitioning between migrants and residents and to test the hypothesis of whether there are common behavioral traits in migrants Therefore, we compared the ecology of two Palearctic breeding species: pied flycatcher, Ficedula hypoleuca, and willow warbler, Phylloscopus trochilus, with resident members of the respective guilds With respect to use of microhabitat, foraging speed, and intake rates, we could not confirm the above-mentioned generalizations, showing that care has to be taken into account when drawing conclusions from few studies for a whole migration system on a huge continent However, both migrating species were more flexible in foraging behavior than were their Afrotropical counterparts As there are hints that this is also the case when migrants are compared with residents on their breeding grounds, we suggest that this flexibility enables migrants to partition resources with residents and, therefore, coexist with Afrotropical species We discuss, however, whether this flexibility is an adaptation to migratory behavior or a prerequisite for the evolution of migration The role that competition plays in present communities cannot be solved with a few observational studies because of the following problems First, it is difficult to detect competition in the field Second, there are constraints of performing field experiments that have not previously been performed in Africa, and third, there are several possible hypothetical scenarios about the role of competition in shaping present communities, including factors that might have been important in the past and are therefore impossible to detect at present Copyright 2003

Niche partitioning of two Palearctic passerine migrants with Afrotropical residents in their West African winter quarters

In this paper I discuss factors that influence the habitat choice of small migrant birds on their Afrotropical wintering grounds and the ecological isolation of migrants and residents. The main characteristic of migrants is the use of resources which are sporadic in space and time. The majority of migrants occur in seasonal savannas and open woodland, mostly using temporarily and locally abundant food sources generally unused by residents. Migrants are more eurytopic and exploit more open parts of the habitats than ecologically similar tropical species. In some guilds, the foraging speed and rate of migrants is higher and they use their wings more often in comparison to residents. There is weak evidence that niche shifts of residents are induced by the arrival of migrants. Overt interspecific interactions seem to be infrequent, except in a guild of ground-feeding birds which rely on a rather predictable but poor food supply. In this guild, residents, but not intra-African migrants, dominated Palaearctic migrants in aggressive encounters irrespective of body-size. In contrast to this, dominance was size-dependent among wintering Palaearctic migrants.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1474-919X.1992.tb04736.x

Habitat selection and coexistence of migrants and afrotropical residents

The objectives of this chapter are to discuss which habitats migrants choose for the winter quarters, and how they use these habitats. This discussion will focus on small migrant land birds that move from high to tropical latitudes. Therefore, the term “migrants” refers to north temperate species that winter at tropical and subtropical latitudes, and the term “residents” will refer to local, tropical, resident species.

Selection and Use of Habitat of Wintering Migrants

The ecology of the land-birds of Tsavo East National Park, Kenya

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The ecology of the land-birds of Tsavo East National Park, Kenya