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McKearnan, J.E. and S.J. Maxson. 1994. Reproductive success and nest attentiveness of common terns at Pine and Curry Island, Minnesota in 1993. Final report submitted to the Nongame Wildife Program, Minnesota Department of Natural Resources. 23 pp.


The breeding status of common terns on Pine and Curry Island at Lake of the Woods, Minnesota in 1993 was determined by conducting a census, monitoring reproductive success, and observing nest attentiveness and disturbances. The census was conducted by counting all nests when it was believed that the greatest number of adults would be incubating. A sample of 23 nests inside two wire enclosures were monitored for reproductive success. Nest attentiveness was measured by observing 1-4 nests from blinds during 36 4-hr diurnal observation periods between 26 May-28 July. The number of subcolony or whole colony disturbances was also recorded during these 4-hr observation periods. Eight nocturnal observations were conducted to ascertain if disturbances occur at night.

We counted 153 active nests on 21 June. Forty-one percent of the eggs laid in the enclosures survived to fledging and 1.04 chicks fledged per nest. These estimates indicate that reproductive success was higher than the observed success for the last few years and were only slightly below the success needed to maintain the colony. The predominant causes of fledging success were dying or missing chicks. The entire colony or parts of the colony were disturbed an average 2.1 times per hour and terns were off their nests 1.7 minutes per hour on average. The source of disturbances was unidentified 93.8% of the time; identified sources of disturbance were predominantly corvids, Canada geese, or gulls. A great horned owl caused desertion of the nests for most of the colony on at least four occasions before the owl was removed 28 June. After the owl was removed, some terns still deserted their nests at night, but another owl was not believed to be the cause. Terns with full clutches incubated eggs 97.1% of the time on average and the non-incubating mate was present 9.9% of the time.

Management recommendations include continued discouragement of gulls, removal of mammalian and owl predators, and prevention of human disturbance. Monitoring reproductive success and censuses should be used to determine success of management recommendations.

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