Another factor of concern, which has been mooted as possibly contributing to population declines, is the mortality that occurs on migration (described in detail in Chapter 28). One type of mortality occurs at communication masts, which in certain weather conditions can kill large numbers of nocturnal migrants. Individual towers have been estimated to kill thousands of birds in a season, and collectively such towers have been estimated to kill 4-5 million birds per year in the USA alone (Chapter 28). Most of these casualties involve Nearctic-Neotropical migrants. The second type of mortality occurs during storms at sea, which can kill many thousands of birds at a time, as they migrate between the breeding and wintering areas. These storms affect mainly birds that breed in the eastern half of North America and winter on Caribbean Islands or in South America. They have increased in numbers and severity in recent decades, in association with global climate change (Chapter 28). There is no firm evidence that either of these mortality factors has contributed importantly to population declines of any species, but the possibility cannot be excluded (see Butler 2000).
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