Authors
  • Shutler, Dave
  • Weatherhead, Patrick J.
Universities

Summary

Male red-winged blackbirds (Agelaius phoeniceus) vary substantially in how much they sing. We tested whether song rate reflected the male’s quality, the quality of his territory, or his frequency of interaction with potential competitors. Song rate declined with the seasonal decline in nest initiations, suggesting that song is used to attract females. However, males with higher song rates did not obtain larger harems. This suggests that females do not use male song rate to assess either male quality or territory quality. If song rate signals male quality to male rivals, and if territory owners are of higher quality than floaters, then owners' song rates should be higher than those of floaters. However, when we removed territory owners temporarily, the song rate of their floater replacements was similar to that of the original owners. Finally, a male’s song rate was not affected by how frequently he chased intruders, or by the proximity or number of neighbors he had. In short, the only information that basal song rate of redwinged blackbirds appears to provide to conspecifics is that the territory is occupied. In simulated territory takeover attempts, however, song rate was significantly higher than the basal rate. Thus, more may be revealed by the singer in these situations. 

Methodology

Our study area was in southeastern Ontario near the Queen's University Biological Station (45°37' N, 76°13' W). The territories in our population of red-winged blackbirds were irregularly spaced along the sides of approximately 30 km of highway. We used most of these territories in both years of the study. All analyses were performed on a microcomputer version of the Statistical Package for the Social Sciences (Nie, 1988). All means reported are ± 1 SD. 

We observed territory occupants from inside a vehicle parked 20 to 30 m from the territory. We determined song rates (number of songs per minute) while the male occupant was visible and within hearing range. Ninety-four percent of our observation periods (N=l 850) were 5 min. We only included observation periods for which the male was visible and within hearing range a minimum of 10% of the time (N = 1011). We also did not include assessments where males gave alarm calls in response to our presence; this generally did not occur until females were feeding nestlings. We visited each territory repeatedly (mean = 6.5 ±3.9 visits per year) to determine the occupant's song rate. In most cases, territories were visited in systematic order (i.e., starting one territory beyond the one we had last observed in the previous visit to the field), but we made more visits to territories where we had performed removals. Observations were made between 0500 and 1000 h and between 1800 and 2030 h from early April to mid-July in both years.

We tested whether males with higher song rates attracted larger harems. Harem size was defined as the maximum number of simultaneously "active" nests (having one or more eggs or nestlings) on a territory. Red-winged blackbirds have conspicuous nests, and we are confident that we found all nests on the territories that we used in this analysis.

Location