2002-03-06 P & R Packet - Laserfiche WebLink

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possmLE EXPLANATION OF COLONY ABANDONMENT HYPOTHESIS #1 Disease ASSESSl\1ENT - To my knowledge there is no evidence of any 11UlSSive die oft However, that does not preclude the possilJility that the hirds are getting sick during the early part of the hreeding season and that this causes them to abandon. I judge this tl3 highly unlikely. ACTION PLAN - We sh.ouJd prohably he on th:e look ou.t around tJre metro area for heron and e!f1'et carcosses. They cou.ld be evaluOUd forpesticids . levels, West Nile virus, de... 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 ASSESSl\1ENT - To date there is no direct ~nce th.atth.is~'I1J1LOhlr:m eit7ieriii-t7t.is cOlony or in Minnesota generally. Investigation of the Eagle nest this post year revealed no evidence of eagle predotion on herons and egrets. However, in other regions of the country there are records of colony abandonment os 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 quo.nt4v/qlllllity ASSESSl\'.IENT - Foodfor herons and egrets is unlikely to he 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 afew 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 hardfor me to imagine how ,this could result in the abandonrnentof 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.