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Birds 



Overview 



Migratory bird popula- 

 tions are an international 

 resource tor which there is special federal 

 responsibility. Moreover, birds are valued and 

 highly visible components of natural ecosys- 

 tems that may be indicators of environmental 

 quality. Consequently, many efforts have been 

 directed toward measuring and monitoring the 

 condition of North America's migratory bird 

 fauna. The task is not an easy one because the 

 more than 700 U.S. species of migratory birds 

 are highly mobile and may occur in the United 

 States during only part of their annual cycle. 

 Some species annually make round-trip migra- 

 tions spanning thousands of kilometers or 

 miles, others engage in short or irregular migra- 

 tions of tens or hundreds of kilometers, and 

 even resident species are capable of moving 

 great distances over short intervals. One often 

 cannot tell whether a bird observed at a given 

 moment is a resident, a migrant, a visitor from 

 another locality, or the same individual seen 10 

 minutes earlier. 



Determining status and trends is further 

 complicated by the fact that each of these 

 species has its own patterns of distribution and 

 abundance, and each species has populations 

 that respond to different combinations of envi- 

 ronmental factors. Finally, the sheer abundance 

 of birds estimated at 20 billion individuals in 



North America at its annual late-summer peak 

 (Robbins et al. 1966) may make it difficult to 

 obtain accurate counts of common species, and 

 the absolute abundance of some may mask 

 important changes in their status. 



Biologists have developed many different 

 approaches to determining abundance and 

 trends in abundance, and nearly all of the recog- 

 nized census methods applicable to birds are 

 represented by the articles in this section. Not 

 suiprisingly, trends among the large number of 

 populations treated are mixed. 



Results from the nationwide Breeding Bird 

 Survey (Peterjohn et al., this section) and a por- 

 tion of the large-scale Christmas Bird Count 

 (Root and McDaniel, this section) show that 

 some populations are declining, others increas- 

 ing, and many show what appears to be normal 

 fluctuations around a more or less stable aver- 

 age. Overall, approximately equal numbers of 

 species appear to be increasing and decreasing 

 over the past two to three decades. Groups of 

 species with the most consistent declines are 

 those characteristic of grassland habitats, appar- 

 ently reflecting conversion of these habitats to 

 other types of vegetative cover. 



Waterfowl populations are monitored close- 

 ly as a basis for regulating annual harvests at 

 levels consistent with maintenance of popula- 

 tions. Goose populations (Rusch et al.. 



by 

 Science Editor 



RussellJ.Hall 



National Biological 



Service 



Division of Research 



Washington, DC 20240 



