Posted by: rlukei | 2010/04/13

Gender of Bald Eagle Broods at NBG

NBG Eagles Adults Jan 24-1

Dr. Bryan Watts addresses the question about the gender of the eaglets that this pair of bald eagles has produced at Norfolk Botanical Garden: 

A question was recently asked by Nancy Dooley of Iowa about patterns in the gender of broods from the Norfolk Botanical Gardens bald eagle pair.  We have not analyzed such patterns in this pair or actually even within the many broods that we have banded in the Chesapeake Bay.  I suspect that the data does exist to address this question across all the banders working throughout the species range but this data has not been compiled and examined.  She was very insightful to ask the question however, and this is one of those emerging areas of research where the closer you look the more interesting it becomes.

Many diurnal raptors are sexually dimorphic where males and females differ in body size and development rates.  In the Chesapeake Bay, for example, female eagles are 20-30% larger than males.  One obvious result of these differences is that males are the ‘cheaper’ sex to produce.  For this reason, if females could manipulate gender then you would expect them to do so when such actions would result in producing more young.  Such adaptive “decisions” could take on many forms and lead to a long list of research questions.  How should females manipulate or adjust sex ratio under different prey conditions?  How should females manipulate sexes within the laying order?  Should females manipulate sex ratio as a function of when a clutch was laid?  The adaptive answer to these questions ultimately depends on which strategy would result in the largest number of chicks that survive to reproduce themselves.

It turns out that several bird species have been shown to be able to manipulate gender during meiosis.  This gives them the capability to actually adjust sex ratio on several scales.  A classic example of this is the European Kestrel.  It has been shown that sons hatched early in the season have a greater chance of breeding as yearlings but that daughters do not.  It has also been shown that females that breed early produce more sons compared to females that breed later in the season to take advantage of these differences.  Here in Virginia, we have seen that sex ratio varies widely between years in Peregrine Falcons.  During some years sex ratio is highly skewed to males while in other years it is not. 

Much research is left to be done in this area and many of the stories that will result are likely to be unexpected and interesting.


Bryan D. Watts, Ph.D.
Mitchell A. Byrd Professor of Conservation Biology
Director, Center for Conservation Biology
College of William and Mary
Virginia Commonwealth University
P.O. Box 8795
Williamsburg, VA  23187

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