Summary
In resource-defence mating systems we intuitively expect that the most competitive males should win the best resources so that, in territorial species, male quality and territory quality will be highly correlated. In the "polygyny threshold model" (PTM) of mate choice this expectation has become an assumption. We performed a removal experiment using red-winged blackbirds to test the validity of this assumption. On the basis of two morphological and two behavioral indices of competitive ability we found only weak correlation between male competitive ability and territory quality. Factors potentially contributing to this result include habitat quality perception, site dominance, and site fidelity. However, both our experimental design and measurement of male competitive ability may have caused us to underestimate the real correlation between male quality and territory quality. Nevertheless, our results suggest that male quality is not perfectly correlated with territory quality as is assumed in the PTM. Therefore, male quality and territory quality should be treated independently when modelling mate choice in this and other territorial species.
Methodology
We conducted the study in eastern Ontario in April and early May of 1984 and 1985. In 1984 we used two groups (based on removal date) of territorial males (n = 10, removed on May 3 and 9, removed on May 13) and a single group in 1985 (n=22, removed on April 25). All males were trapped and individually color-banded as soon as they established territories in early April. Flattened wing chord was measured as an index of body size (James 1970) and epaulet length as an index of epaulet size (Searcy 1979b). Both body size (Searcy 1979b) and epaulet size (Eckert and Weatherhead 1987b) have been shown to be associated with success in competition in this species. We also used aggression toward conspecific intruders as a third measure of competitive ability. Rohwer (1982) provides data which indicates this measure is a good correlate of RHP. We simulated an intrusion by placing in a male's territory a stuffed conspecific adult male accompanied by a taped playback of advertisement song and observed the response of each resident male for a period of 15 min from a car parked on the roadside. Each male was presented with a model three times over a 10-day period, once between sunrise and 08:00 EST, once between 08:00 and 10:00 and once between 16:30 and sunset. Each male had been resident on its respective territory for at least 10 days before testing. Individuals were never tested more than once a day and tests were only conducted in fair weather. A time budget of a male's activity during a model presentation was estimated by instantaneously sampling the male's behavior at 5 s intervals. All discrete displays (songs, display flights, etc.) were tallied and classified according to Orians and Christman (1968). Song spreads were classified with respect to relative intensity on an integer scale from 1 (no exposure of epaulets or extension of wings) to 5 (epaulets fully exposed and wings fully extended). Data from the three model presentations were pooled for each male. See Table 1 for all the behaviors we recorded.