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A Great Blue Heron (Ardea herodias) colony located at Peltier Lake, in east central Minnesota, was <br />formerly one of the states largest. Beginning in 2000, the nesting birds abandoned this site for the <br />first of five consecutive seasons. Because of its regional ecological importance and high community <br />value, the conservation of the Peltier Lake colony was prioritized by the Minnesota Department of <br />Natural Resources. <br />Early management efforts focusing on anthropogenic disturbance mitigation were unsuccessful in <br />halting colony abandonment. At its lowest point, the nesting population at Peltier Lake dropped <br />>90% from an estimated maximum of >1,100 nesting pairs. To determine the cause(s) of colony <br />abandonment, I used intra -nest video surveillance to monitor nesting behavior and document <br />events occurring within the colony. Video data demonstrated that, while human disturbance rates <br />at Peltier Lake were similar to those at another regional colony, predation by raccoons (Procyon <br />lotor) was an important cause of chick mortality. Great Horned Owl (Bubo virginianus) depredation <br />was also confirmed using physical evidence, but the impact of owls on the colony was not <br />determined. Based on these results, a strategy of predator management was recommended for <br />Peltier Lake. Methods included installing predator guards and relaxing furbearer trapping <br />restrictions within the county park where this colony is located. Scratches on the predator guards <br />demonstrated that raccoons attempted to climb trees containing active nests significantly more <br />often than inactive trees. Following predator management, the productivity of the Peltier Lake <br />colony increased significantly. Continued lack of productivity in unprotected nest -trees <br />demonstrated the efficacy of the predator guards; and also suggested that, despite removal efforts, <br />enough predators remained present to opportunistically exploit unguarded active nest - trees. The <br />events at the Peltier Lake colony and model results demonstrating the disproportionately large <br />impact that a small population of arboreal mesopredators can have support the predation <br />hypothesis as a cause for colony abandonment. The implications of this study underscore the value <br />of strong empirical evidence as well as the importance of predation to the success of colonial <br />waterbird conservation efforts. <br />Based upon video evidence, the sole cause of chick mortality directly observed at Peltier Lake was <br />raccoon predation (n = 9), accounting for the confirmed loss of 10 out of the 15 chicks detected. <br />Video showing a raccoon feeding upon chicks in the nest (though no predation event was observed) <br />suggested predation of two other nestlings. Finally, footage showing a single chick that appeared to <br />be chased from its nest onto a limb from which it fell, also suggested raccoon activity. This <br />interpretation is supported further by the observation that two other chicks from the same nest had <br />been killed earlier by raccoons. Based on these observations, raccoon predation was responsible for a <br />minimum of 66.7% and possibly as much as 86.7% of heron chick mortality in the study trees at <br />Peltier Lake. <br />Physical evidence found during surveys suggested that avian predators were present and active on <br />Peltier Island. Great Horned Owl predation (n = 3) was confirmed when the remains of at least two <br />