aswan at rockisland.com
Sat May 1 12:08:36 PDT 2004
I've been enjoying reading your birding observations. Wayne's question about Osprey nesting success related to closeness of BAEA neighbors and whether it had been studied, reminded me of something in Gerrard and Bortolotti's book, The Bald Eagle, an fine 1988 Smithsonian Nature Books summarization of their BAEA research in northern Saskatchewan. Looking at osprey and bald eagle nest distribution, they basically found that the more eagles nesting around a lake the fewer ospreys. "Osprey nests are relatively rare in Saskatchewan. They can generally be found in the northern part of the province where roads have been built, but are rare in wilderness areas suitable for eagles. In contrast, nesting eagles are absent from the human-developed areas but are abundant in remote locales. It is possible that Ospreys have a tough time living on a wilderness lake because they are no match for the eagles there; instead, they establish themselves where eagles have been driven out by people....After watching the two species interact, I can't help but favor the idea that eagles actively exclude Ospreys. Consistent with this concept, the few remote lakes where Ospreys do nest are marginal habitat for eagles because of their late thaw (Ospreys nest later than eagles and so can breed successfully)." Palmer's section on Ospreys in Vol. 4 of the Handbook of North American Birds describes the same competition: "...when the reproductive timing of the two species coincides, (piracy) may be regular....In Florida, the establishment of a breeding pair of Bald Eagles led to the relocation of nest sites and to reduced breeding success by Ospreys." And, "Perhaps it is worth noting that Osprey numbers are few in Alaska and nw. Washinton, where Bald Eagle numbers are high."
Gerrard and Bortolotti describe an "encounter':
"As we boated across the lake, an Osprey flew low directly over our heads carrying a fish ten to twelve inches long. This was an uncommmon sighting; no "fish hawks" breed on the lake. As we started to record the time and location of the observation in our notebook, a shadow passed suddenly over us. It was an adult Bald Eagle flapping so hard that we could hear its wingbeats. In a matter of seconds, the pursuer was upon his target. The size difference between the two birds was astonishing. The eagle attacked from a few feet above and to one side of the Osprey--once, twice, and then a third time. On each pass the Osprey visible flinched, for the eagle's talons barely cleared its back. We anxiously awaited the outcome. Many observers had described how typically the Osprey drops its fish, and with surprising agility the Bald Eagle snatches the fallen prey in midair. We were not prepared for what happened. After three unsuccessful attacks, the eagle turned to brute force. This time coming up fast from behind and below, the eagle flipped onto its back, thrust its talons upward, and ripped the fish right out of the Osprey's grasp. What a sight! After quickly righting itself, the eagle turned and flapped leisurely to deposit the booty on its nest."
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