SCALE OF INVESTIGATION AND THE DETECTION OF COMPETITION - AN EXAMPLE FROM THE HOUSE SPARROW AND HOUSE FINCH INTRODUCTIONS IN NORTH-AMERICA

Interference competition between the introduced Passer domesticus and native Carpodacus mexicanus observed at backyard nest boxes for 4 yr in the W USA may have resulted in habitat segregation on a larger scale along a rural-urban habitat gradient. Competition may be occurring in the E states since the introduction of C. mexicanus in 1940. Christmas Bird Count data were examined for patterns in species abundance predicted by competition theory, using 3 scales of investigation: local (area 250 km in diameter), and continental (between areas of sympatry and allopatry). Findings suggest that competition has not influenced patterns of changes in the abundances of the competitors on a local scale in urban and rural habitats in the E USA or on a regional scale in the E and W USA. On a continental scale, the presence of C. mexicanus may have accelerated widespread declines in the density of P. domesticus in areas of sympatry, possibly because of the energetic demands of interference competition. The dissimilar behavior and foraging strategies of the competitors may permit coexistence in all habitats, such that the niche of P. domesticus is included in the niche of C. mexicanus. Differences in the perception of habitat by the competitors may preclude the development of habitat segregation along a rural-urban habitat gradient. -from Author