Date of Award:

5-1-1983

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Department name when degree awarded

Life Sciences:Biology

Committee Chair(s)

Keith L. Dixon

Committee

Keith L. Dixon

Committee

Lloyd W. Bennett

Committee

Ivan G. Palmblad

Abstract

Behavioral interactions between Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) and Marsh Wrens (Cistothorus palustris) were studied in northern Utah. Marsh Wrens were important predators on Yellow-headed Blackbird eggs. On marshes where Marsh Wrens were present, they were implicated in the destruction of between 5.3 and 15.3% of all active Yellow-headed Blackbird nestings, but only nests within 25 m of Marsh Wren home ranges were depreciated by wrens. Films of Marsh Wrens destroying Yellow-headed Blackbird eggs suggest that wrens consume egg contents. Yellow-headed Blackbirds crushed only one of 19 Marsh Wren nests tied 2 m from prelaying Yellowhead nests. Wren nests provide convenient perches for blackbirds nesting in sparse vegetation, and nests probably are crushed inadvertently. Yellowheads responded significantly more strongly to broadcast Marsh Wren songs than they did to either Song Sparrow (Melospiza melodia) or Common Yellowthroat (Geothlypis trichas) playbacks. Male Yellowheads chased live Marsh Wrens at territorial - boundaries significantly more often than females did, but females responded aggressively more frequently than males did to Marsh Wren playbacks presented beside nests containing eggs. The proportion of aggressive approaches by male Yellowheads to Marsh Wren playbacks was low and did not change with the stage of the nest beside the loudspeaker, but females responded aggressively to 53% of the Marsh Wren playbacks beside nests containing eggs, indicating a significant increase in nest defense at that stage. Thus, female Yellow-headed Blackbirds defend their nests against a specific predator, whereas males may defend their territories with a more general response. The variability in male nest defense against Marsh Wrens was unrelated to harem size. Males with small harems were as responsive to broadcast Marsh Wren songs as males with larger harems, and the relationship between female nest defense and harem size was not significant. The responsiveness of female Yellowheads to Marsh Wren songs also was related to proximity to Marsh Wren activity. Broadcast Marsh Wren songs elicited a greater proportion of aggressive approaches from female Yellowheads when playbacks were farther than 25 m from Marsh Wren home ranges than when playbacks were closer. Coloniality in Yellowheads may be advantageous partly since it maximizes the effectiveness of an active, aggressive defense against wrens.

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