Variation in prevalence of avian heiatozoais related to taxonomic affDiation at the level of the family or subfamily but not of the genus within families. Prevalence is comparatively insensitive to the influences of habitat and season; however, temperate species have higher incidences of infection than tropical species belonging to the same himilies. Among taxa of nonraptorial altrical landblrds, hematozoan prevalence is inversely related to the length of the incubation period but shows little relationship to body size and rate of postnatal development. This finding suggests a possible link between the duration ofembryonic development and the ability to resist or control infection, possibly due to maturational processes in the avian immune system.The proportion of individuals with microscopically detectable infections of blood parasites (hematozoan prevalence) varies widely among species of birds (1)(2)(3). This variation suggests differences in exposure to parasite vectors, vulnerability to parasites, or ability to resist or control infection. To the extent that resistance falls under genetic control, parasites have been implicated in the evolution of sexual reproduction (4), genetic polymorphism (5), and sexually selected characters (6-9); certainly, they may influence survival and reproductive success (10), even causing extinction of susceptible populations (11). Some ofthe variation in prevalence of hematozoa has been linked to locality (1, 12), habitat, diet (13), season (14, 15), and, in tropical latitudes, to plumage brightness (13,16). Other factors have been shown to be unimportant. For example, although hematozoa appear to infect a larger proportion of colonially than solitarily nesting birds in one African sample (17), prevalence in north temperate species appears generally unrelated to nest location, mating system, habitat type, and elaboration of secondary sexual characteristics (18)(19)(20). Prevalence of avian hematozoa does, however, bear a strong association with taxonomic group at the family level (1,12,13). At present, the causes of this variation are not understood.In this report, I relate hematozoan prevalence to embryonic and postnatal development rate and demonstrate that prevalence is inversely related to the length of the incubation period. I speculate that this correlation arises from a direct relationship between immunocompetence and period of embryonic growth. Long incubation periods might allow increased proliferation of B stem cells and greater diversification of the variable region of immunoglobulin light chain genes before they are expressed as antibody.
MATERIALSParasite data were obtained from literature surveys of field studies in the nearctic (1) and the neotropical regions (2, 3).To restrict the scope of the survey with respect to habitat, diet, and development type, I consider only nonraptorial (i.e., excluding birds of prey) altricial (i.e., those with highly dependent neonates, thereby excluding gallinaceous birds and ducks and geese) terrestrial birds. Neotropical...