Our Living Riwoiincs — BinJs 



21 



Many studies have foLiiid significant 

 changes, pfiniarily declines, in popula- 

 tions of bleeding birds throughout the United 

 States. Most studies have focused on birds that 

 migrate to the Neotropics for winter. 

 Speculations about causes of observed declines 

 have primarily implicated habitat fragmentation 

 and loss (e.g.. deforestation) in Central and 

 South America. The National Audubon 

 Society's Christmas Bird Counts (CBC). begun 

 in the winter of 1900-01. provide the data need- 

 ed to discern consistent population trends in 

 birds wintering throughout the United States. 



For this study we used the CBC data to 

 examine population trends of songbirds with 

 ranges that apparently are limited by lower tem- 

 peratures in the North. We chose these species 

 to track populations of birds that could be in 

 peril in the future. These birds potentially will 

 be more quickly affected by changing climate 

 than other birds, and we need baseline informa- 

 tion on them to document possible conse- 

 quences of global climatic change. The species 

 that are indeed declining need to be monitored 

 because the possible synergistic effects of 

 declining populations and changing climate 

 could result in local and even regional extinc- 

 tions. 



Methods 



We examined 30 years of CBC data (winters 

 of 1959-60 to 1988-89) for 50 songbirds whose 

 northern range edges are associated with 



January minimum temperatures (Root 1988b). 

 For each songbird species or subspecies at each 

 count site, we calculated the number of individ- 

 uals seen per counting effort (e.g.. hours of 

 observation). Yearly averages for each of the 

 conterminous states were determined from 

 these values for each species. Data were used 

 from all count sites that were censused at least 

 25 of the 30 years. For details on the method we 

 used to calculate population trends, see Geissler 

 and Noon ( 1 98 1 ) and contact us. All of our con- 

 clusions rest on very conservative analyses. 



Trends 



Of the 50 songbirds examined. 27 (54%) 

 exhibited a statistically and biologically signifi- 

 cant trend in at least one state (Fig. I). Of these 

 27 species. 16 (59%) had populations declining 

 in more states than states in which they were 

 increasing; 12 exhibited only declines and 4 had 

 a population increase in at least one state. Ten 

 (37%) of the 27 species had populations 

 increasing in more states than states exhibiting 

 declines, with 7 exhibiting only population 

 increases. One (4%) species had populations 

 increasing and decreasing in the same number 

 of states. 



In general, the populations of birds that eat 

 seeds from grasses and forbs (e.g.. spanows and 

 meadowlarks) seem to be declining more fre- 

 quently than those of birds that eat seeds from 

 shrubs and trees, or berries (e.g.. tufted titmouse 

 [Parus bicolor] and cedar waxwing [Bombycilla 



Winter 

 Population 

 Trends of 

 Selected 

 Songbirds 



by 

 Terry L. Root 



University of Michigan 



Larry McDauiel 



National Center for 



Atmospheric Research 



