We documented nest habitat selection, nests success, and factors affecting nest success of Black Terns (Chlidonias niger) at Agassiz National Wildlife Refuge in northwestern Minnesota. During 1992-1994, 289 Black Tern nest sites and 400 random sites were sampled on five search areas totaling 1,325 ha. Four habitat characteristics were measured at each nest and random site: (1) mean water depth, (2) distance to open water, (3) dominant vegetation within a 2-m radius, and (4) amount of open water within a 2-m radius. Habitat variables were highly correlated with each other, making it difficult to estimate independent effects of each habitat variable on nest-site selection. However, conditional logistic regression models indicated that locations closer to open water and in deeper water were more likely to be associated with nest sites. Locations in bulrush (Scirpus acutus) and sedge/grass were preferred, although 68% of nests were in cattail (Typha spp.) reflecting the greater availability of that habitat in the study area. Nest success ranged from 48-69% (Apparent) and 33-62% (Mayfield) among years. Except for five nests that were abandoned or had infertile eggs, nests that failed to hatch apparently were depredated. Nest success was higher for nests with larger clutch sizes and for nests located farther away from other nests. Nests with 3-egg clutches were 2.8 times as likely to hatch as 2-egg nests. The odds of a nest being successful increased by 25% for each 5 m increase in distance to the nearest nest. Earlier nests were also more successful (the odds of a nest being successful was estimated to decrease 7% for each additional day that passed before the nest was initiated). Nest success was not related to nest cluster size and was negatively related to habitat factors associated with nest site-selection.
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Vol. 30 • No. 1