Seasonal decline in nestling cellular immunocompetence results from environmental factors — an experimental study.

In a seasonal environment, immune function in bird nestlings has been reported to decline with hatching date. Two groups of factors are expected to contribute to this decline: (1) seasonal deterioration of environmental conditions, e.g., food availability, and (2) differences in individual quality between parents breeding early and late in the season. To distinguish between these effects, an experimental manipulation of hatching date in great tits (Parus major L., 1758) was conducted. Whole clutches were swapped between pairs of nests with a 6-day difference in expected hatching date, while some nests remained nonmanipulated, constituting a control group. Nestling T-cell-mediated immune response to phytohaemagglutinin was negatively related to hatching date both within nonmanipulated control broods and all broods pulled together. Experimental change in hatching date produced changes in nestling immune response, as predicted from the seasonal trend observed in the control nests. Male and female nestlings did not differ in the level of immune response and the seasonal decline in immune response did not differ between sexes. Our results indicate that the seasonal decline in nestling immune function may be driven by date-dependent environmental conditions rather than differences in parental quality. [ABSTRACT FROM AUTHOR]