Site tenacity, mate retention and sexual dimorphism in black terns

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Between 1982-1984 I studied site tenacity and mate retention in Black Terns at Sycan Marsh, Lake County, Oregon. I banded 778 adult Black Terns and used this marked population to test the hypotheses that site tenacity and mate retention would be less for Black Terns which nest in variable marsh habitats than for other larids that nest in stable habitats. Based on 76 recaptures of previously banded terns, I estimated that only 40% of the banded Black Terns survived and returned to Sycan Marsh. Seventy-one percent of the recaptured individuals nested in the same Primary Nesting Area, but only 38% of the terns nested in the same colony site in consecutive years. Overall, I estimated that approximately one half of the surviving Black Terns returned to Sycan Marsh, indicating low site tenacity. Of 89 pairs of banded terns, only 5 pairs maintained their pairbonds during consecutive seasons. In 21 instances, terns were nesting with a new mate. There were no instances of terns nesting with a new mate when the old mate was also known to be present. These results indicate that if both members of a pair returned to the same breeding area, pairbonds were likely to be maintained. However, site tenacity in Black Terns was low, and consequently relatively few pairbonds were maintained for consecutive breeding seasons. In 1984, I examined sexual dimorphic size traits and trap bias in Black Terns. Based on two traits, culmen length and total head length, males were significantly larger (p<0.05) than females, though not all males were larger than all females. Within pairs, however, the male was always larger than its mate. I also found that males were more likely than females to be the first individual of a pair captured in a nest trap. I used three variables (culmen length, total head length and trap order) in a discriminant function to correctly predict the sex of individuals approximately 80% of the time.

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Reference Code: U87STE02IDUS

Full Citation: Stern, M. A. 1987. Site tenacity, mate retention and sexual dimorphism in black terns. Unpublished thesis, Oregon State University, Corvallis. 45 pp.

Location: ANIMAL EF: CHLIDONIAS NIGER