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

Summary

We used sequential removal experiments to test whether the resource-holding potential (RHP) of territory “owner” red-winged blackbirds, Agelaius phoeniceus, was superior to that of their first replacements (“shallow floaters”) and subsequent replacements (“deep floaters”). Among the removals were secondyear males, which were morphologically inferior to adults and which also tended to be competitively inferior in aviary contests. The highest proportion of secondyear males occurred in the deep floater class. Thus, the RHP of some deep floaters was inferior to that of owners and shallow floaters. However, among adults, owners, shallow floaters, and deep floaters had equivalent morphological and competitive RHP. Furthermore, replacement males that had defended territories for many days were neither morphologically nor competitively superior to males that had defended territories for only a few days. Our results suggest that RHP distinguishes adults from second-year males, but does not separate owners from floaters. The only hypothesis that is potentially supported by our observations is that owners have a greater expected payoff from their territory than intruders, and in this way owners are able to maintain site dominance. The nature of that payoff remains to be determined.

Methodology

We studied a migratory population of red-winged blackbirds near the Queen's University Biological Station, located 50 km north of Kingston in southeastern Ontario, Canada. The males in the population defended territories that were either isolated from one another or were contiguous with other territories on small marshes. The isolated territories generally consisted of small (50-200 m 2) patches of cattail (Typha sp.). Some isolated territories were within sight of one another, but no suitable breeding habitat existed between these sites. The marsh territories also had cattails and shared boundaries with up to three neighbors. Territories were adjacent to, and were irregularly spaced along, 30 km of highways.

Location