Causes Of Population Declines

Over the past several decades, various species of intercontinental landbird migrants have declined in Europe and North America, but the main causal factors seem to have differed between the two regions (Chapters 24 and 25). The European species winter in the Afrotropics, and most of those that have declined occupy arid scrub habitats prone to drought. Their declines have been widely attributed to events on wintering areas, notably reductions in rainfall and associated food supplies. The North American species winter in the tropics of Central and South America, and most of those that have declined occupy forest, with smaller numbers in scrub and grassland habitats. Their declines have been widely attributed to events in breeding areas, notably forest fragmentation and the associated increases in densities of predators and parasitic Brown-headed Cowbirds Molothrus ater. The lines of evidence proposed in favour of these various hypotheses are summarised in Chapters 24 and 25, along with some alternative explanations applicable to at least a minority of species.

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