- Carleton University
- Memorial University
Summary
We examined associations among parasite infections, secondary sexual traits and testosterone in male red-winged blackbirds sampled at the start of the breeding season. Parasites quantified included ectoparasitic lice and mites and endoparasitic blood protozoans, nematodes, trematodes and cestodes. Secondary sexual traits that we quantified included body size, epaulet size and color, song repertoire size and song switching rate, and behavioral responses to male and female models. Overall we found few significant associations between parasites and secondary sexual traits, between secondary sexual traits and testosterone, or between parasites and testosterone. In addition, most parasite taxa appeared to infect birds independently, although the low prevalence (<50%) of many of the parasites meant that our sample sizes were too small to detect weak associations. Our most promising results were obtained for ectoparasitic mites, which tended to occur on birds uninfected with other parasites, on birds with longer epaulets, and on birds with higher levels of testosterone. Epaulet length and testosterone are both probable correlates of dominance in this species. Further research will be required to determine whether there is a causal link between the immunosuppressive effects of testosterone and the mite infections, and between testosterone, epaulet length and male mating success.
Methodology
We banded a large sample of males when they first began defending territories in the spring. We then collected behavioral information from those males over a period of several weeks prior to and into the start of the breeding season. We than attempted to recapture as many of those males as we could within a 10-day period to assay other secondary sexual traits, parasites and testosterone. The recapture period was limited to 10 days to avoid potential temporal biases in parasite prevalence or intensity (e.g. Weatherhead and Bennett 1991, 1992). During the recapture period we also captured unbanded territorial males to supplement our data (with the exception of behavioral variables). Unbanded males are much easier to catch than males with recent experience of our traps, mist nets and playbacks. All males were territory owners and were at least 2 years old based on their plumage. Most males acquire territories in their 3rd year (D. Shutler and P.J. Weatherhead ms.) and approximately 70% of males acquiring territories retain them for only 1 or 2 years (Orians and Beletsky 1989; Weatherhead, unpubl.). Therefore, although the males we studied here were of unknown age, most would have been either 2 or 3 years old.
Data were collected in April and May, 1991, at field sites near the Queen's University Biological Station in eastern Ontario. All the males included in the study held territories either in one large marsh 25 km from the station or in small marshes along roadsides within 15 km of the large marsh. We began capturing and banding the males as soon as they began defending territories in early April. At the time of this initial capture we collected a blood smear (Bennett 1970) for assessment of haematozoa. We also captured and collected blood smears from some males in the study area that had been banded as part of studies done in previous 15 years (Shutler and Weatherhead 1991a; Metz and Weatherhead 1991). We released all males back on their territories immediately after banding them and then quantified their behavior over the next several weeks.