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Many avian species are negatively impacted by urbanization, but other species survive and prosper in urbanized areas. One factor potentially contributing to the success of some species in urban areas is the reduced presence of predators or parasite vectors in urban compared to rural areas. In addition, urban areas may provide increased food and water resources, which can enhance immune capacity to resist infection and the ability to eliminate parasites. We determined patterns of blood parasitism, body condition, and immune cell profiles in urban and rural populations of five adult male songbird species that vary in their relative abundance within urban areas. Urban birds generally exhibited less blood parasitism than rural birds. This difference was particularly evident for the urban-adaptable Abert's towhee Pipilo aberti. In contrast, no difference in haemoparasitism was seen between urban and rural populations of the curve-billed thrasher Toxostoma curvirostre, a less-urban adaptable species. In two closely related species, the curve-billed thrasher and the northern mockingbird Mimus polyglottos, urban birds had a higher leukocyte count and a higher heterophil to lymphocyte ratio, which is often associated with chronic stress or current infection, than rural birds. Urban northern mockingbirds were in better condition than rural counterparts, but no habitat-related differences in condition were detected for other species. Parasitic infection was correlated with body condition in only one species, the canyon towhee Pipilo fuscus. Parasitic infection in most species was correlated with changes in leukocyte abundance and profile. The findings suggest that interspecific differences in parasitic infection cannot be attributed entirely to differences in vector abundance or body condition. Interactions between immune function, parasite infection risk, and resource availability may contribute to determining the relative ability of certain species to adapt to cities.

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Male and female redpolls (Acanthis flammea) showed marked increases in circulating corticosterone up to 1 hour after exposure to a common stress-apture, handling and restraint-indicating that their hypothalamo-pituitary-adrenal axis responded to acute stress in a manner similar to that of other vertebrates. We used this protocol as a measure of responsiveness of the adrenocortical cells to acute stress in general and for comparison with gender and across seasons. In both sexes the adrenocortical response to stress was reduced in January (at Fairbanks, 64"N) and maximal when birds were breeding in June at Tbolik Lake (69"N). The elevation of circulating corticosterone following capture and handling in breeding males at Barrow (71'") was significantly less than in breeding males at Toolik Lake. There were also considerable variations among individuals in the intensity of the adrenocortical responses, particularly in the maximum levels of corticosterone attained. This individual variation correlated significantly with fat score andor body mass in both sexes only in breeding birds at Barrow. This difference may be explained by generally lower, and thus reduced variability in body fat and mass in birds sampled in the warmer climate of Toolik Lake. A similar trend was seen in non-breeding birds, but this was not significant. Additionally, in January, baseline cortisterone levels in males were correlated with body mass, although this relationship did not hold when both sexes were considered. Body mass and fat score in winter were similar to those of redpolls sampled at Barrow in June. These data suggest that redpolls may be able to adjust their responsiveness to acute stresses in relation t o fat stores. Those with greater fat depots had reduced responsiveness to stress.