




Each year that we spend time with Bald Eagles we learn new things about them.
From 1979 t0 1982, Gary Bortolotti (in the centre - bottom photo) did the field work for his Ph.D. thesis on Bald Eagles at Besnard Lake. One of the predictions from his thesis work (and subsequent studies by Elston Dzus) was that there might come a year when we would see a disproportionate number of nests with one young, and that a high proportion of such young would be male.
Although the full story is quite complicated, Gary's work predicted that when food resources were more limited, Bald Eagles (through normal biological processes) would have more male young (in these one young nests). He suggested that this adaptation would occur because males are smaller and take less food to raise. In the year's since Gary's work, a variety of other bird species have been found to adjust the sex ratio and the birth order of young.
This year, when we visited Gary, his wife Heather (on Gary's right hand side), his daughter Lawren (on Gary's left), and his graduate student at the summer home on Besnard Lake that they share with Elston and Connie Dzus, we had interesting news to report. This year, we found that 13 of 16 Bald Eagle nests with young had one young. The number of nests with two or more young was only 3 or 19% of successful nests. This is the lowest proportion of nests with two or more young we have ever seen. Normally about half (47% was the average for 1993-2005) of all successful nests have two or three young (in fact most of these nests have two young and very few nests have three young).
Furthermore, of the four nests with one young where we banded, three of the four were males.
This sample is too small to draw a firm conclusion, but the results are consistent with predictions from Gary Bortolotti's thesis work.
The photos above show (at the top) a Bald Eagle nest from above. The second photo shows the same nest closer up - so you can see one young in the nest. The third and fourth photos are close-ups of a young Bald Eagle in its nest.