The T–cell–mediated immune response and return rate of fledgling American kestrels are positively correlated with parental clutch size

Abstract

Life-history theory predicts that parents face a trade-o¡ between the number and viability of the progeny they produce. We found evidence for an apparent trade-o¡ in a free-living population of American kestrels (Falco sparverius), as larger clutches produced more but lighter £edglings. However, while the body mass of £edglings has traditionally been used as a measure of survival prospect, o¡spring immunocompetence should also play an important role. We thus measured the T-cell-mediated immune response of £edgling kestrels in relation to brood traits and nest-rearing conditions through a cross-fostering experiment. The immune response was positively correlated with the body condition of £edglings, but was also higher in those hatched from ¢ve-egg than four-egg clutches. These results were not in£uenced by other brood traits, nor by current exposure to stressors and infectious agents, as measured by serological variables. Such ability to resist pathogens may account for why the probability of o¡spring returning to the study area in subsequent years, when controlling for brood size, was higher for ¢ve-egg than four-egg clutches. These results suggest an optimal clutch size through maternal e¡ects on o¡spring immunocompetence rather than a trade-o¡ between the number and quality of the o¡spring.

Thyroid Hormone Suppression and Cell-Mediated Immunomodulation in American Kestrels ( Falco sparverius ) Exposed to PCBs

“…Total WBC count (WBC ϫ 10 9 /L) was estimated based on the average number of leucocytes in five fields. The WBC count was standardized to the number of red blood cells (RBCs); to control for variability in smear thickness, only the fields with an evenly distributed monolayer of RBCs were counted (described in Tella et al 2000). The proportions of heterophils, lymphocytes, monocytes, eosinophils, and basophils were determined from a sample of 100 leukocytes counted at the feathered edge of the smear.…”

“…Counts were log-transformed for statistical analyses. To provide a frame of reference for the blood cell counts, we present data on wild American kestrels during the breeding season from north central Saskatchewan, Canada, in 1997 (see Tella et al 2000).…”

Genetics, local environment and health as factors influencing plasma carotenoids in wild American kestrels (Falco sparverius)

“…We then performed a crossfostering study in 1997 to partition the variance in carotenoid concentrations of nestlings into genetic and environmental components. We used four-and ¢ve-egg clutches (by far the most common clutches, Tella et al 2000a) and manipulated them when the oldest chick was ¢ve days old, which assured us all chicks had hatched and avoided disturbance during the hatching span which could provoke stress (Wiebe & Bortolotti 1994; see also Tella et al 2000b). We individually marked and weighed each nestling and ranked them by size.…”

“…Nesting chronology was determined accurately each year by repeated visits to all nest-boxes starting before laying and continuing until the eggs hatched (Tella et al 2000a).…”

“…the concentration of carotenoids. Each explanatory variable was tested for signi¢cance in turn following the stepwise branching modelling procedure (Tella et al 2000a). The result was the most adequate model for explaining variability in carotenoid concentration, where only signi¢cant explanatory variables were retained.…”