July 21, 2006: More questions

There have been several questions about sibling mating and what might prevent it; tough question! In the late 1960's and 70's, when eagle numbers were greatly depressed, there may well have been some interbreeding due to the lack of alternative partners. But that has all changed with literally thousands of immature eagles currently occupying the centeral and northern Atlantic coastlines. Most of our immatures leave Maine their first fall and wander for the next four years, often returning to Maine in the summer months. Others overwinter here when there is an abundant and predicable source of food throughout the winter. For example, here in Orono, at Pushaw Lake, there is a small group of mixed aged eagles that attend the ice fishermen on a daily basis. Anglers regularlly place pickeral and perch on the ice for the eagles. Young eagles are mixing continually during these years and it is unlikely that siblings would encounter each other much during this period except, perhaps, occasionally if they visit their natal area. It is also possible that siblings recognize each other after several years and there is some innate process that discourages interbreeding.

As with most bird species, female eagles select their male partners. With many species, males have established territories and advertise their wares with colorful plumage, displays and song; females then make their choice. Not so with eagles, pairing appears to occur between their 4th and 5th years during times when immature eagles are continously interacting with each other. Recent information, based a great deal on osprey research, suggests that the male then escorts his mate to his natal geographic area. She, in turn, does the final nest tree selection. Theory says that since the male is the prime food provider during the nesting period, he makes the territory choice based on food availablity. Here in Maine we have a great deal of data on former eagle territories and actual nest trees. In the 80's and 90's, as our population grew and new territories were established, we were amazed at the number of old territories that were reoccupied, even 20-40 years after they had been abandoned! Old nest trees, decades after nests had disappeard, were used again. Obviously, there were aspects of these territories that the new male recognized, probably food availability, such as runs of anadromous fish (e.g. alwives). The female, in turn, selected many of the old nest trees that still afforded strong limbs for nest support, easy access and good visibility.

We have limited data on dispersal and nesting locations of our own immatures. One bird Charlie and I banded as a nestling in Ellsworth (Hancock County) ended up breeding along the north shore of New Brunswick. It was 27 years old at its death, the 2nd oldest banded eagle ever recovered! Another breeding eagle we radio-tagged in the Bar Harbor region 15 years ago was banded as a nestling in Michigan and this year, we read the band number of a breeding adult in northen Maine that was banded as a nestling in New York. These are just few examples of the geographic mixing that occurs in our northeastern population. I suspect that it is typical, promotes genetic exchange, and sibling pairing under normal conditions is rare, at best. One last observation highlighting the extent of continental eagle movement was a recording by Mark during his PhD work. He was able to read the band number of an eagle feeding at one of his winter feeding stations in coastal Maine; it was from Saskatchewan!!

Finally, who was that bird seen at the nest on July 15th? I suspect it could have been a former nestling from that nest just checking things out once it was safe to come near. The nest has been productive for the last 14 years so there are a number of immatures out there somewhere. If you check our first eagle image from the 5th of February you will see another visitor. Charlie related to me that he has found several fledglings dead under eagle nests (not their own). There is probably some attraction to all eagle nests for youngsters and if they blunder into some resident adults it may be tough going. Our eagles mate for life and are at or near their territories most of the year. The chances of another individual or pair taking over that site would be very rare. -- Bucky Owen, Unniversity of Maine emeritus