From Egg to Breeding Adult
Shasta Eagles
Steve Zack,
PhD
Steve Zack is a PRBO research associate who
is also affiliated with the Wildlife Conservation Society.
Just after dawn
on a June day last year, on Shasta Lake in northern California, I spotted
an Osprey with a fish flying particularly fast overhead. Its haste was
made clear a few seconds later. An immature Bald Eagle was bearing down
on the Osprey: the eagle wanted the fish. The Osprey quickly surrendered,
jettisoning its catch laterally to the demanding eagle with an aerial pass
of sorts.
But wait... another bird, this one an adult Bald Eagle, was coming on fast
to the scene. The immature eagle had no chance; it passed the still wiggling
fish to the adult. End of drama. The result: one unlucky fish, passed through
six talons, to another happy adult Bald Eagle on Shasta Lake. I was happy,
too. It's a great thing to watch our national symbol in action.
It hasn't been this good for Bald Eagles for very long. The world of Bald
Eagles has changed radically in the last century everywhere, including
Northern California. The widespread use of DDT and similar chemicals devastated
Bald Eagle populations throughout North America in the 1960s and 1970s.
The Bald Eagle was declared endangered in 1966 over much of the United
States, and in 1978, all were protected in the lower 48 states.
Bald Eagle numbers have rebounded dramatically in Northern California and
the rest of the Pacific Northwest since their nadir in the mid-70s. At
Shasta Lake, the number of breeding pairs has doubled since the first reliable
survey in 1980; there are 18 pairs today - the largest aggregation of breeding
pairs anywhere in the state. Yet the nest productivity of Bald Eagles at
Shasta Lake still warrants concern, because of its clear relationship with
the lake level there: drought years and corresponding low lake levels are
associated with more nest failure than in high lake level years.
Because of the need to closely evaluate this relationship, I have been
able to collaborate with the U.S. Forest Service and the Bureau of Reclamation
since 1996. For two years now I have directed this research and have had
the opportunity to study the lives of these striking birds. I have done
so with Nancy Hutchins and Jeff Wood of the Forest Service, and PRBO interns
Kerry Mehl, Naomi Nichol, Hilary Cooke, and David Juliano. Together, we
are monitoring the eagles' nesting and foraging activities, assessing the
numbers and kinds of fish taken, and mapping each bird's territory and
perch sites. We are collecting these and other data that will, in the long
term, allow us to understand which feature of the eagle's ecology is most
affected during low lake level years.
|
 Most
exciting to me, however, is the chance to watch eagles, particularly immature
birds, play out their life history. Northern California is one of the few
regions of the country where Bald Eagles are year-round residents (most
populations are migratory). At Shasta Lake, there are immature eagles present
throughout the year. Bald Eagles require four or more years to attain the
adult plumage of white head and tail we are so familiar with. Immature
Bald Eagles can be identified through four different age categories by
their plumage and beak characteristics. We use this information to interpret
the behavioral patterns of the immature eagles within or between adult
territories.
I hope, through time, to come to understand how immature Bald Eagles become
breeding adults - how they find mates and establish or take over territories,
and if their behaviors when they are younger influence where and how they
find a mate and a territory. Intriguingly, there is virtually no information
available on this critical feature of Bald Eagle life history. At Shasta
Lake, I think we can begin to piece together this puzzle for the first
time.
This year we have recruited the assistance of Ron Jackman, an experienced
eagle researcher, who has provided us with his insights, color bands, four
radio collars, and one satellite collar. With his help, we are color-banding
this year's Bald Eagle nestlings and will be able to follow at least some
of them with radios and one with satellite telemetry for their first three
to four years of life. There is some evidence that first-year eagles migrate
in the fall to British Columbia and then return to or near their natal
region. I suspect that some Lake Shasta young will return; if they do,
we may be able to monitor their travels and will certainly be on the scene
at Shasta to study their interactions. |