Reproduction and Moult
There are many constraints on reproduction for birds that are not resident or that migrate between fixed areas. Nomadic birds, often dependent on fairly short-lived resources, need to be adapted to being able to fairly rapidly find nest sites, build nests
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Reproduction and Moult
There are many constraints on reproduction for birds that are not resident or that migrate between fixed areas. Nomadic birds, often dependent on fairly short-lived resources, need to be adapted to being able to fairly rapidly find nest sites, build nests, lay eggs, raise young and move on together with the young once the food begins to run out. Elaborate, prolonged courtship displays can use up valuable time. Nevertheless, courtship displays in one family of birds in which there is a high proportion of nomads (the Alaudidae) vary among the nomadic species from intricate aerial manouvres with complicated songs to perch displays with monotonous songs, to quite simple ground displays with simple calls. Pre-breeding displays in Stark's Lark Eremalauda starki and Grey-backed Sparrowlark Eremopterix vertical is may be quite synchronized, with hundreds of males calling and performing simultaneous aerial displays (Maclean 1970a, b; Willoughby 1971). Time can also be taken up with the construction of large and complicated nests (but there are exceptions) using particular materials that may be rare in the environment. The birds need to spend the minimum of time building a nest, so a simple nest construction is optimal; in some cases, there is no nest at all (Fig. 6.1). The birds also need to be able to adjust clutch sizes to the amount of resources available. When resources are marginally sufficient for breeding, the birds need to be careful with their investment in the breeding effort in case the resources suddenly dry up and the birds are forced to move. Fledged young need to be able to accompany adults away from the breeding area, because fairly high -density patches of breeding birds often attract predators (Chap. 5, Table 5.5); fairly close nearest-neighbour distances between nests or colonial nesting are frequent in nomads, and particularly in aquatic nomads.
W. R. J. Dean, Nomadic Desert Birds © Springer-Verlag Berlin Heidelberg 2004
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Reproduction and Moult
Fig.6.1. Eggs ofludwig's Bustard Neotis ludwigii laid on bare ground without any nest material or any further preparation of the site other than moving some of the stones to the edge of the "nest". Tierberg, southern Karoo
6.1 Nest Sites in Nomadic Birds Theoretically, nomadic bird species that move in response to environmental changes need to be independent of specialized nest sites - for example, birds that use the old nests of other birds, particularly range-restricted species, must remain within the area where that species is common, and nomadism is not an option. However, the old nests of common widespread species, such as crows, may be available in many areas. Many species that use old nests of raptors or crows are relatively unselective about the size or the condition of the nest. Greater Kestrels Falco rupicoloides in arid South Africa, for example, will nest opportunistically in the large disused nests of Secretary Bird Sagittarius serpentarius and Lappet-faced Vulture Torgos tracheliotus and in much smaller nests of crows
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