Effects of Time of Sampling on Oocyst Detection and Effects of Age and Experimentally Elevated Testosterone on Prevalence of Coccidia in Male Dark-eyed Juncos

The hormone testosterone (T) has been linked to enhanced expression of secondary sexual traits and acquisition of mates (Ligon et al. 1990, Alatalo et al. 1996, Raouf et al. 1997). However, high plasma levels of testosterone also have been suggested to suppress immune function (Folstad and Karter 1992, Zuk et al. 1995, Hillgarth and Wingfield 1997; but see Ros et al. 1997, Hasselquist et al. 1999). Thus, an obvious question is whether testosterone enhances reproductive success while simultaneously affecting susceptibility to disease. In the Dark-eyed Junco (Junco hyemalis), experimental maintenance of plasma testosterone levels at their natural early season peak for the entire breeding season causes males to sing more frequently, and testosterone-treated males (T-males) in captivity are more attractive to females than are control males (Cmales; Enstrom et al. 1997, Hill et al. 1999). However, T-males also have elevated plasma levels of the steroid hormone corticosterone (Ketterson et al. 1991, Klukowski et al. 1997) and show a greater response to handling stress than do C-males (Schoech et al. 1999), which can translate into suppressed immune function (Sapolsky 1992, Besedovsky and del Rey 1996, J. Casto pers. comm.). To investigate possible effects of elevated levels of plasma testosterone on the junco’s susceptibility to disease, we asked whether free-living males treated with testosterone would be more likely than controls to be infected by coccidia (Isospora spp.), a protozoan gut parasite belonging to the Eimeria complex. We also asked whether yearling juncos were more likely than older adults to be infected and whether our ability to detect infection was affected by the time of day when fecal samples were collected. Methods.—This research was conducted at the University of Virginia’s Mountain Lake Biological Station (378229N, 808329W; see Chandler et al. [1994] for description of study area). We captured male juncos in mist nets and Potter traps during the early spring of 1995 and 1996 and classified them as second year (younger) and after second year (older) based on