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Thread: Eagles Eating Rare Birds In Maine

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    William Malacarne
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    Default Eagles Eating Rare Birds In Maine


  2. #2
    Axel Hildebrandt
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    Thanks for sharing the article. That sounds like quite a dilemma.

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    Raptors are coming back from recent lows in populations sometime in the 1970s. We had better get used to them eating things we like and/or value! A friend of mine has been banding Semipalmated Sandpipers since the early 1980s but can't anymore due to the almost constant presence of raptors going after the sandpipers. On one day at the banding site a year or two ago we had Peregrines, Merlins, Marsh Harriers and Bald Eagles all "spooking" and hunting the birds. I rarely go to a tern colony these days without seeing a Peregrine sometime during the day. A possible 7th gannet colony in North America in the outer Bay of Fundy (one pair in 1999) was quashed by families of Bald Eagles hunting there in late-summer. Increases in White-tailed Sea Eagle populations on the coast of Norway have been linked to to long-term declines in kittiwakes there.

    As I said we are going to have to get used to this. One bright light is that top predators are generally rarer than the species they eat because they are exactly that- at the top of the food chain- and therefore do not generally have such an extensive food base.

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    Not nearly as dramatic as the Maine situation but eagle predation on a Sandhill Crane recently made the news at the oft discussed location of Homer.

    http://www.homertribune.com/article.php?aid=4205

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    I believe the increase in the number of raptors is a direct result of an increased prey base. Predator species off spring numbers generally increase in response to increased food supply following an increase numbers of prey species. As prey numbers decrease predator numbers will decrease and predator reproduction numbers will subsequently fall as well. Prime example being the population relationship between Lynx and Snowshoe Hares.

    Jim

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    You are right in bringing up the importance of food availability and how it controls populations of consumers Jim. Predator-prey cycles are relatively short-term and as the name implies are characterised by cyclical increases and declines in populations. Cycles of predator and prey tend to be out of phase, one being lagged behind the other. As prey increases, it takes a while for this to feedback into the predator population.

    The situation with raptors is a little different in that we have seen a dramatic and steady increase in populations over the past 30-40 years. It is generally thought that this increase has been caused by decreases in toxic chemicals such as DDT and PCBs in the environment. Raptors are at the top of their food chains and these chemicals concentrate at each level in the chain so by the time you get to raptors they end up receiving quite a large dose in their lifetimes. Raptors suffered a "double-whammy" effect because they also seem to be particularly susceptible to the effects of these chemicals compared to other species of birds. These chemicals appeared to have the effect of thinning eggshells and thus caused much lower hatching success and overall breeding success than was normal. Survival of adults and immature birds may also have been affected. As the concentration of these chemicals has declined in the environment so has raptor body burdens and in turn eggshells have thickened and their breeding success has come back to normal.

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