02/11/2002 Council Packet - Laserfiche WebLink

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• • • POSSIBLE EXPLANATION OF COLONY ABANDONMENT HYPOTHESIS #1 Disease ASSESSMENT — To my knowledge there is no evidence of any massive die off However, that does not preclude the possibility that the birds are getting sick during the early part of the breeding season and that this causes them to abandon. I judge this as highly unlikely. ACTION PLAN — We should probably be on the look out around the metro area for heron and egret carcasses. They could be evaluated for pesticide levels, West Nile virus, etc... We could attempt to capture adults (away from the colony would be best) and take blood samples to investigate health. If this is the cause of colony abandonment then examination of only a few individuals should be sufficient. HYPOTHESIS #2 Eagle Presence ASSESSMENT — To date there is no direct evidence that this is a problem either at this colony or in Minnesota generally. Investigation of the Eagle nest this past year revealed no evidence of eagle predation on herons and egrets. However, in other regions of the country there are records of colony abandonment as a result of Eagle activity. ACTION PLAN —Make observations of Eagles around the island to determine whether there are any negative interactions between the species. Examination of nest contents for evidence of eagle predation on herons and egrets would be good to do again this year. HYPOTHESIS #3 Food quantity /quality ASSESSMENT — Food for herons and egrets is unlikely to be a limiting resource in the Chain of Lakes Park region. However, it is possible that the quality of that food has changed without our knowledge. ACTION PLAN — We could attempt to capture adults (away from the colony would be best) and examine body weight and blood chemistry to document individual health. If this is the cause of colony abandonment then examination of only a few individuals should be sufficient. HYPOTHESIS #4 Health of nest trees ASSESSMENT — We didn't discuss this at all and I've never been on the island However, it is hard for me to imagine how this could result in the abandonment of the entire colony. ACTION PLAN — We could monitor tree health (could be correlated with the risk of nest destruction due to branch breakage) and density of canopy cover (could be correlated with risk of predation). HYPOTHESIS #5 Human Disturbance (other than noise) ASSESSMENT —It is possible that noise is not much of an issue but that there is a sufficiently high level of human disturbance (e.g., speed boats, canoes, people walking on the island, people fishing) that the herons and egrets have abandoned the island I don't think we have enough information on level of human disturbance to evaluate how likely this explanation might be. ACTION PLAN — Monitor human use of the island and adjacent waters. Limit use until more evidence is in hand