Our Lrvini^ Rfsoinvcs — Hinls 



17 



section), and the Puerto Rican parrot {AnuKxnui 

 vittaur, Meyers, this section), are reasonably 

 well known. Through censusing these species, 

 biologists have tracked declines, often to a few 

 indi\ iduals. and slow recoveries resulting from 

 intensi\e management activities. Other rare 

 species have populations that are depleted or 

 vulnerable because of recent trends, but which 

 can be censused with far less certainty. For 

 example, willow tlycatchers (Fjupidomix tniil- 

 lii: Sogge. this section) breed sparsely m parts 

 of the Grand Canyon where exotic species have 

 displaced natural riparian vegetation; likewise, 

 the status of the red-cockaded woodpecker 

 {Picoidcs hoivalis) appears closely tied to the 

 decline of the longleaf pine {Pinus paliistrls) 

 ecosystem (Costa and Walker, this section). 



Broad-scale programs such as the Breeding 

 Bird Survey, annual waterfinvl surveys, and 

 wintering surveys such as the Christmas Bird 

 Count may provide information on status and 

 trends for as many as 75% of U.S. bird species, 

 at least to the extent that they would provide 

 evidence of catastrophic declines. Remaining 



species may be censused only with difficulty 

 and often with imprecision because they are 

 secretive, rare, highly mobile, or occupy poorly 

 accessible areas. Specialized surveys provide 

 information on some of these groups but. as 

 indicated by the articles in this section, they do 

 so with varying degrees of success. Much work 

 remains to be done on obtaining better informa- 

 tion and developing better ways of inteipreting 

 a\ailable information on difficult-to-census 

 species. 



If any overall conclusion is possible on the 

 wide array of information now available on sta- 

 tus and trends of bird populations it is this: 

 apparent stability for many species; increases in 

 some species, many of which are generalists 

 adaptable to altered habitats; and decreases in 

 other species, many of which are specialists 

 most vulnerable to habitat loss and degradation. 



Reference 



Robbins. C.S . B. Bruun. and HS. Zim. 1966. Birds of 

 North .'\nierica. Golden Press. .New \'ork. .^40 pp. 



The North American Breeding Bird Survey 

 (BBS) was begun in 1966 to collect stan- 

 dardized data on bird populations along more 

 than 3.400 survey routes across the continental 

 United States and southern Canada. The BBS 

 has been used to document distributions and 

 establish continental, regional, and local popu- 

 lation trends for more than 250 species. 



We summarize here survey-wide patterns in 

 the 1966-92 population trend estimates for 245 

 species of birds observed on a minimum of 40 

 routes with a mean relative abundance of 1 .0 

 bird per route. Survey-wide trend estimates are 

 also summarized for six groupings of birds, pro- 

 viding insight into broad geographical patterns 

 of population trends of North American birds. 



Methods 



The BBS routes are located along secondary 

 roads and surveyed each year during the peak of 

 the breeding season by observers competent in 

 bird identification. Each route is 39.4 km (24.5 

 mi) long, with 50 stops placed at 0.8-km (0.5- 

 mi) intervals (Robbins et al. 1986). To estimate 

 population change, we used a procedure called 

 route regression, described in greater detail by 

 Geissler\indSauer(1990). 



We examined population change in several 

 ways. First, we estimated overall population 

 change for individual species over the entire 

 survey area. Second, we looked for temporal 

 and geographic patterns in individual bird 

 species (e.g.. Sauer and Droege 1990). 



Additionally, we analyzed overall patterns of 



population change for several species of partic- 

 ular management interest. Groups of birds were 

 defined by migration status (nonmigratory. 

 short-distance, and Neotropical migrants) or by 

 breeding habitat (grassland, shrubland. or 

 woodland: see also Peterjohn and Sauer 1993). 

 For each group, we determined the percentage 

 of species with positive (> 0) trends. If popula- 

 tion change is not consistent within the group, 

 about half (50%) of the species should show 

 positive trends. Clearly, some species will show 

 very significant declines (or increases) over the 

 interval, and these species can be identified in 

 the Appendix. However, the percentage of 

 species with positive population trends is a con- 

 venient summary of information from all 

 species within the group to demonstrate overall 

 trend patterns. 



Finally, to display regional patterns of popu- 

 lation change, we calculated the mean trend for 

 the species in each group for each survey route. 

 We used an Arc/Info geographic infomiation 

 system to summarize and display geographic 

 patterns of population change (Isaaks and 

 Srivastava 1989: ESRI 1992). 



Trends 



Of the 245 species considered. 130 have 

 negative trend estimates, 57 of which exhibit 

 significant declines. Species with negative trend 

 estimates are found in all families, but they are 

 especially prevalent among the mimids (mock- 

 ingbirds and thrashers) and sparrows. A total of 

 115 species exhibits positive trends, 44 of 



Breeding Bird 



Survey: 



Population 



Trends 



1966-92 



by 



Bruce J. Peterjohn 



John R. Sauer 



Sandra Orsillo 



National Biological Service 



