Annual spring relapse of blood parasite infections in birds is believed to be the result of hormonal changes associated with breeding. As part of a larger study on the epizootiology of Haemoproteus danilewskyi in blue jays in south-central Florida, we studied the relationship between H. danilewskyi infections and levels of luteinizing hormone, prolactin, progesterone, testosterone, estradiol, and corticosterone. We found a positive association between intensity of H. danilewskyi infection and corticosterone levels in females but not in males. We also found no association between infection and levels of prolactin, luteinizing hormone, progesterone, testosterone, or estradiol in males or females. In addition, we found a positive relationship between levels of corticosterone and handling time and between corticosterone and testosterone levels. We suggest a possible influence of corticosterone on spring relapse of Haemoproteus spp. infections in birds but provide no support for the influence of breeding hormones on relapse of these parasites.

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