INDIVIDUALISTIC RESPONSES OF BIRD SPECIES TO ENVIRONMENTAL-CHANGE

We investigated how the population dynamics of the same bird species v aried in different environments, and how the population dynamics of di fferent species varied in the same environment, by calculating long-te rm trends for 59 insectivorous songbird species in 22 regions or strat a of eastern and central North America using data from the North Ameri can Breeding Bird Survey. Of the 47 species that occurred in more than one region 77% increased in some regions and declined in others. Of t he 22 regions 91% had some species that increased and others that decr eased. There were only slightly more significant correlations between strata in species trends and between species for stratum trends than w ould be expected by chance. Because of nonlinearities in the data, the actual patterns of population fluctuations of the same species in dif ferent regions and of different species in the same region were even m ore heterogeneous than suggested by our analyses of linear trends. We conclude that these bird species respond to spatial and temporal varia tion in their environment in a very individualistic fashion. These ind ividualistic responses show that the extrapolation of population trend s gained from a few local studies to a larger spatial scale, and the u se of a few indicator species to monitor the status of a broader commu nity, are suspect.