[va-richmond-general] Rare Bald Eagle 4-chick brood on James River
- From: "Wilson, Michael D" <mdwils@xxxxxx>
- To: "Va-bird@xxxxxxxxxxxxx" <va-bird@xxxxxxxxxxxxx>, "va-richmond-general@xxxxxxxxxxxxx" <va-richmond-general@xxxxxxxxxxxxx>
- Date: Fri, 22 Apr 2011 13:56:26 +0000
Extremely Rare Bald Eagle 4-chick Brood Documented on James River
On 18 April, 2011 while flying eagle productivity surveys along the James
River, Bryan Watts and Mitchell Byrd from the Center for Conservation Biology
and pilot Fuzzzo Shermer documented a 4-chick brood in a nest on Bailey's Creek
in Hopewell, Virginia. The breeding territory had been occupied for more than
10 years and the current nest is in the top of a large scarlet oak. The four
chicks appeared in good condition and ranged in ages from approximately 32 to
42 days.
More than 8,000 breeding attempts have been monitored in Virginia as part of
the annual survey and this is the first 4-chick brood ever documented in the
state. As far as we are aware, this represents only the third 4-chick brood
ever documented for this species. The first was in 1986 on the Eastern Shore
of Maryland and the second was in 2010 in the water gap portion of the Delaware
River.
Brood size is a life history trait that reflects a broad range of evolutionary
tradeoffs. For many large raptors, sibling competition and brood reduction are
behavioral mechanisms that maximize chick production under poor resource
conditions. Under such conditions, sibling competition imposes downward
pressure on brood size to match available resources. When resources are
abundant and provisioning exceeds demand, sibling dominance hierarchies are
relaxed, the last chick disadvantage is reduced, and brood reduction is less
likely. Under such conditions, large clutches provide an advantage by allowing
pairs to raise above-average broods
Bald Eagle broods hatch asynchronously, often form dominance hierarchies, and
may exhibit brood reduction depending on prey availability. Under normal
circumstances, the degree of hatching asynchrony for unusually large clutches
likely creates such a disadvantage for the last chick that it sets an upper
bound on brood size.
A perfect storm of conditions must come together in order for a Bald Eagle pair
to raise four chicks to independence. The female must produce a four-egg
clutch, the equivalent of 10% of her body mass, in a narrow time window. If
the time span of laying is too long, the level of asynchrony will be too great
for the forth chick to overcome. The female must brood and feed the older
siblings for several days while maintaining the viability of the forth egg
until hatching. Shortly after the last egg hatches, ambient conditions must
allow for the female to leave the brood frequently enough to assist the male
with foraging and provisioning. Lastly, foraging conditions must allow for
high per capita provisioning rates to reduce the strength of a dominance
hierarchy and the likelihood of brood reduction.
posted for Bryan by:
Michael Wilson
Center for Conservation Biology
College of William and Mary & Virginia Commonwealth University
P.O. Box 8795
Williamsburg, VA 23187-8795
phone: 757-221-1649
fax: 757-221-1650
email: mdwils@xxxxxx
web: www.ccb-wm.org
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