05/01/2006 Council Packet - Laserfiche WebLink

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• • Discussion Despite substantial chick loss in 2005, cameras failed to directly show raccoon predation. Evidence on the ground and indirect video evidence suggest that raccoons did manage to kill at least a few chicks; which in a colony of 30 nests is still a sizeable percentage. Chick remains found on the ground were clearly eaten by raccoons but may have been scavenged once a chick fell from the nest. Video evidence from the 2004 nesting season suggested that once raccoons identified a vulnerable tree, they would methodically return until all nests in the tree had been raided resulting in the loss of all chicks. Usually, these raids would occur in rapid succession. During the 2005 season, this pattern of loss was seen in some trees but not in others. Though the relative lack of raccoon activity shown on video suggested that the flashing may have been marginally effective, high rates of nest failure still occurred in trees flashed with sheet metal. Aside from the relatively poor quality of video shots available over the 2005 season, it is also plausible that other causes of chick mortality played a more important role in 2005. Avian predators may have had a proportionately greater impact during the 2005 season. Great Horned Owls, a known predator of Great Blue Herons, were present and active at the Peltier Island colony in 2005. American Crows (Corvus brachyrhynchos), also well documented predators, particularly of eggs, were common in the area. The relative importance of owls and crows upon heron mortality is not currently known. Also noteworthy is the presence of Bald Eagles (Haliaeetus leucocephalus) in the area. Though in Minnesota there are no documented cases of eagles preying upon herons, there is ample literature on this phenomenon occurring in the Pacific Northwest. Obviously, metal flashed tree trunks would not impact the activities of these predators. Other important factors include human disturbance and random chance. As the colony size shrinks, the relative importance of random chance (stochastic events) becomes greater. Thunderstorms, siblicidal aggression, and accidents are all examples. This is also true with human disturbance due to the proportionately larger "footprint" caused by their presence.