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Figures show the numbers (%) of species showing different trends in three different regions of North America. From Peterjohn et al. (1995).

Figures show the numbers (%) of species showing different trends in three different regions of North America. From Peterjohn et al. (1995).

some partly different patterns, again reflecting regional variations in population trends (Hagan et al. 1992).

By use of radar, Gauthreaux (1992) measured the volume of spring migration across northern coastal areas of the Gulf of Mexico, as birds migrated northward past Lake Charles in Louisiana. Although the individual species could not be distinguished by this method, the migration observed consisted almost entirely of Neotropical migrants. The proportion of suitable days (because of favourable weather) on which trans-Gulf flights were observed during 8 April-15 May each year declined from 90%, 95% and 100% in 1965-1967 to 36%, 43% and 53% in 1987-1989. These radar counts thus provided an independent indication of a substantial net decline in the numbers of Neotropical migrants over this 22-year period.

Taken as a whole, then, the trend data from breeding bird counts and surveys, together with migration counts, reveal a recent marked decline in the numbers of some Neotropical migrant species that breed in the forests of eastern North America, but not in others which have shown stable or increasing trends. However, substantial regional variations were apparent. Declines occurred at a time when the total forest area in the eastern USA was on the increase, owing partly to regeneration of abandoned farmland.2 Other local factors, which seemed to have influenced the trends, included habitat change through forest maturation, and long-term fluctuations in the numbers of Spruce Budworm Choristoneura fumiferana and other caterpillars on which many species feed while in their breeding areas (see later).

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