Monitoring of Wintering Bald Eagles

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					                       Monitoring of Wintering Bald Eagles
                        -Westchester and Rockland Counties 2009-

                        A Project of the Bedford Audubon Society




Prepared by Tait Johansson
Bedford Audubon Society
October 20, 2009




           Bylane farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org
Bedfford Audubon Sociietty
Bed ord Audubon Soc e y



Introduction
         The Bedford Audubon Society conducted its third Winter Eagle Monitoring Project from January 5,
2009 to March 23, 2009. This year, the project surveyed roosting Bald Eagles (Haliaeetus leucocephalus) at
three sites, two in northwestern Westchester County, New York, and one just across the Hudson River in
Rockland County, New York. The purpose of this ongoing project is to help monitor the significant
concentration of wintering Bald Eagles in the Lower Hudson River Important Bird Area. As part of this
project, three Bald Eagle nests located at two of the roost sites were also monitored.

         The Bald Eagle, our national symbol, fell victim to a serious decline in numbers in the middle part of
the last century, primarily due to the widespread use of chlorinated hydrocarbons such as DDT (Dichloro-
diphenyl-trichloroethane) as pesticides. These chemicals were taken up into the eagles’ bodies through the
prey they ate and interfered with their breeding physiology, resulting in eggs laid with too-thin shells (or
sometimes no shells) that broke during incubation. Since the ban on DDT for most uses took effect in 1972,
this eagle’s population has slowly but steadily climbed. In New York State, the number of breeding pairs
had increased from one in 1965 to 145 in 2008 (Nye 2008). Although recently taken off the Endangered
Species List, the Bald Eagle is still a species of conservation concern that warrants careful monitoring of its
population.

         The lower Hudson Valley is one of the most significant Bald Eagle wintering areas in the state
(Burger and Liner, 2005). Although roosts in the area are surveyed twice per winter by the New York
Department of Environmental Conservation, Bedford Audubon’s more frequent monitoring in 2007 revealed
large, sudden, frequent fluctuations in numbers at these sites (Bielfelt 2007), underlining the importance of
more regular monitoring to truly reveal the extent of eagle usage of these sites.



Methods

         Observations were taken on a standardized data sheet at all three main sites, weather permitting,
(Verplanck, George’s Island, and New Croton Reservoir) every Monday and Thursday during the
monitoring period (January 5- March 23), with a few weather- related exceptions to this schedule.
Monitoring, conducted by volunteers, occurred from about 1½ hours before sunset to dusk. Observers
recorded the date, time spent monitoring, percentage of cloud cover, percentage of visible water covered by
ice, temperature, total numbers of adult eagles roosting onsite, total number of immature eagles roosting
onsite, and general observations on the birds’ movements. Efforts were also made to age the eagles to year if
possible (often it was not possible), and to record any other noteworthy birds or observations occurring
during the monitoring period.

The data sheet is reproduced below.




            Bylane Farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org
Bedfford Audubon Sociietty
Bed ord Audubon Soc e y
___________________________________________________________________________________________________
Eagle Project                                Observer______________________
Date:___/___/2009                            Location: Cr.Pt / Cr. Dam/ Gr.Is/ Iona Is.

Start Time__________________                 End Time_______________

% Cloud Cover_________                       Temperature_____________

% Ice Cover on the Water (that you can see)_____________________

How many were in the trees upon arrival? ______________________

Approximately how many flew in from the following directions?

East____       North____      West____       South____      _____ ______
                                                            (other)

Could you clearly see all the eagles? Y/ N
If not, what obstructed your view?______________________________

Total Roosting Adult Eagles: _____           Total Roosting Immature Eagles: ____

Other interesting birds or observations:
__________________________________________________________________________________
Eagle Project                                Observer___________________
Date:___/___/2009                            Location: Cr.Pt / Cr. Dam/ Gr.Is/ Iona Is.

Start Time__________________                 End Time_______________

% Cloud Cover_________                       Temperature_____________

% Ice Cover on the Water (that you can see)_____________________

How many were in the trees upon arrival?

Approximately how many flew in from the following directions?

East____       North____      West____       South____      _____ ______
                                                            (other)
Could you clearly see all the eagles? Y/ N
If not, what obstructed your view? ______________________________

Total Roosting Adult Eagles: _____           Total Roosting Immature Eagles: ____

Other interesting birds or observations:

__________________________________________________________________________________
           Bylane Farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org
Bedfford Audubon Sociietty
Bed ord Audubon Soc e y

Results
Sites Surveyed: Descriptions
        Observations were collected at three main locations: George’s Island Park, New Croton Reservoir,
and Verplanck. The Verplanck observation site afforded views of two roost sites, which were monitored
simultaneously. Several other locations were checked during the monitoring season for possible usage by
roosting eagles, with no success: Iona Island in Stony Point, and Titicus Reservoir in North Salem.

New Croton Reservoir

         One large eagle roost was monitored at New Croton Reservoir in the towns of Yorktown and
Cortlandt, Westchester County. The monitoring was conducted from two separate locations, one on the dam
itself, and one about a mile east of the dam, just off of Croton Dam Rd. on the southern shore of the
reservoir. These two sites afforded different views of the same large eagle roost in tall, mostly deciduous
trees in and around a small cove on the north shore of the reservoir. There was also an active Bald Eagle nest
visible from the dam (see Eagle Nests section of report below). This was usually the largest roost surveyed,
and had the consistently highest numbers.

         This roost averaged 28.48 eagles (8.43 immatures, 15.14 adults) in 21 monitoring visits from 1/5/09 to
3/23/09, with a high of 76 on 2/5/09.

Average numbers by month:
January average: 22.86 roosting eagles (8.43 immatures, 10.86 adults) in 7 monitoring visits February average:
47.88 eagles (19.75 imms., 28.0 ads.) in 8 visits
March average: 9.17 eagles (6.17 imms., 3.00 ads.) in 6 visits



George’s Island Park
        George’s Island Park is a small county-managed park on the Hudson River in the town of Cortlandt,
Westchester County, New York. Observations were made of an eagle roost on the forested southern shore of
Montrose Point (also in Cortlandt) looking north from the area around the northwest corner of the parking
lot. The birds here mostly roosted in tall deciduous trees, many of them Tuliptrees (Liriodendron
tulipferum). Only a small part of Montrose Point was visible from the vantage point, and it was clear that
some eagles were roosting out of view just to the north on parts of the point that were out of view (see
separate description of George’s Island- North below).

         This roost averaged 18.74 roosting eagles (12.21 immatures, 6.16 adults) in 19 monitoring visits from
1/5/09 to 3/19/09, with a high of 57 on 1/22/09.
Average numbers by month:
January average: 27.00 roosting eagles (16.71 immatures, 9.71 adults) in 7 monitoring visits
February average: 18.38 roosting eagles (12.13 imms., 5.88 ads.) in 8 visits
March average: 5.00 roosting eagles (4.50 imms., 0.50ads.) in 4 visits


George’s Island-North (North side of Montrose Point)

         Monitored from a small park by the Hudson River in the village of Verplanck, town of Cortlandt,
this roost was on the forested north side of Montrose Point, the other side of the peninsula that also holds
the eagle roost monitored from George’s Island Park. Most birds here roosted in several large, tall Tuliptrees
(Liriodendron tulipferum). Frustratingly, there is a large area in the middle of Montrose Point that is not
visible from either of these two monitoring locations. This area almost certainly held more roosting eagles.
Some eagles that appeared to be flying in to roost in this non-visible part of the point were observed at both
George’s Island-North and at George’s Island Park. These two roosts may only be the two visible parts of


            Bylane Farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org
Bedfford Audubon Sociietty
Bed ord Audubon Soc e y

one larger roost on Montrose Point, but logistically feasible visual access to the middle appears to be
impossible.

         This roost averaged 6.74 roosting eagles (3.84 immatures, 1.74 adults) in 19 monitoring visits from
1/5/09 to 3/19/09, with a high of 32 on 1/26/09.

Average numbers by month:
January average: 12.29 roosting eagles (6.57 immatures, 3.71 adults) in 7 monitoring visits
February average: 4.43 roosting eagles (3.00 immatures, 0.43 adults) in 7 visits
March average: 2.20 roosting eagles (1.20 immatures, 0.80 adults) in 5 visits


Verplanck (Stony Point)
        This roost, monitored from the same small riverside park in Verplanck that George’s Island- North
was, was a small, intermittently used roost mostly in a group of dead Eastern Hemlocks (Tsuga canadensis)
situated on mostly forested Stony Point in Stony Point State Park, in the town of Stony Point, Rockland
County. There was also an easily seen, active eagle nest located on this peninsula next to the roosting area
(see Eagle Nests section of report below).

         This location averaged 2.70 roosting eagles (1.75 immatures, 0.55 adults) in 20 monitoring visits from
1/5/09 to 3/19/09, with a high of 13 on 2/5/09.

Average numbers by month:
January average: 3.57 roosting eagles (2.14 immatures, 1.29 adults) in 7 monitoring visits
February average: 3.63 roosting eagles (2.50 immatures, 0.25 adults) in 8 visits
March average: 0 roosting eagles in 5 visits




            Bylane Farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org
Bedfford Audubon Sociietty
Bed ord Audubon Soc e y

Graphic Representations of Data
Fig. 1: Total eagles at Croton Dam (CD) compared to total eagles at George’s Island (GI)




         This graph compares total roosting eagle numbers at Croton Dam, George’s Island, Verplanck
(George’s Island North), and Verplanck (Stony Point). The Croton Dam roost’s general status as the largest
roost is apparent, especially later in the season.




            Bylane Farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org
Bedfford Audubon Sociietty
Bed ord Audubon Soc e y

Fig. 2: Immatures vs. adults at Croton Dam




         This graph compares numbers of immature eagles roosting at Croton Dam with numbers of adults
roosting there. As was the case last year, immature numbers here fluctuated more widely than those of
adults, perhaps indicating greater adult roost site fidelity (see Bielfelt’s suggestion that this is the case in
Bielfelt and Hannan 2007). Still, numbers of immatures and adults seemed to track each other fairly closely
at this site, in contrast to George’s Island (see Fig. 4 below).




            Bylane Farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org
Bedfford Audubon Sociietty
Bed ord Audubon Soc e y

Fig. 3: Immatures vs. adults at George’s Island




        As was the case with last year’s data, this year’s showed fairly consistently higher numbers of
immatures compared to adults, unlike the numbers at Croton Dam. As at Croton Dam, there were wider
fluctuations in the number of immatures at George’s Island than there were of adults. This was also true last
year (Johansson 2008).

       No similar relationships were visible in the data collected at any of the other sites, perhaps because
monitoring started later at these sites, and as a result, not as many data were collected.




            Bylane Farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org
Bedfford Audubon Sociietty
Bed ord Audubon Soc e y


Eagle Nests
        There were three active eagle nests visible from roost monitoring sites during the monitoring period.
Observations were taken during roost monitoring of any eagle activity and behavior seemingly associated
with either nest.



New Croton Dam Nest

Monitored from New Croton Dam, 1/5/09-3/23/09
Located on northwest shore of Croton Reservoir

         This nest, occupied for the last two years (Bielfelt 2007, Johansson 2008) appeared to be occupied by
two adult-plumaged eagles, likely the same pair as nested at this location last year. This nest was partially
concealed behind tree branches, and it was often difficult for observers to see what was happening around it
clearly. Surprisingly, there was only one observation at this location of what was clearly nesting activity
during the monitoring period, when on 3/16 2 adult eagles were seen on the nest, one of which appeared to
be incubating.


Stony Point Nest

Monitored from 1/5/09- 3/23/09 from a small park in Verplanck
Located near lighthouse on Stony Point, Stony Point State Park, town of Stony Point, Rockland County

         This nest, built last year, appeared to be occupied by a pair of adult-plumaged eagles. The first sign
of activity was noted on 1/12, when 2 adults were seen “performing maintenance on nest” (this same activity
also noted on 1/26). By 1/29, 1 adult appeared to be incubating. The last activity at the nest noted was on 2/9,
when 1 adult was reported “at nest”. On 2/19, no activity was noted at the nest at all, and it appeared empty,
its fate unknown.


Montrose Point Nest
         This nest was newly built this year on the north side of Montrose Point, and was easily visible from
the small park the roost was monitored from. Nest activity was first noted on 1/5, when 1 adult was seen
sitting close to the nest continuously for about an hour and a half. 1 immature bird was in the nest on 2/12,
“poking its head down [into the nest] and then up again”, possibly rearranging or adding nest material. The
last observation for this nest was on 3/18, when 1 bird was observed flying “into” the nest.




            Bylane Farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org
Bedfford Audubon Sociietty
Bed ord Audubon Soc e y


Conclusion
        As in the two previous years of the project, data collected in 2009 found no relationship between ice
cover and numbers of eagles. Nor did our data show an inverse relationship between temperature and
number of eagles, as was found in 2007. Bielfelt and Hannan (2007) speculate that the relationship found in
2007 may have been due to eagles using secondary roost sites on colder days, or perhaps eagles feeding later
on cold days, and returning to roost in the dark, when they cannot be detected. With many more data having
been collected over the past two years, the lack of such a relationship over that time indicates that 2007’s
result may just have been due to the fact that all data that year were collected from late February to mid-
March, a time of year when rising temperatures coincide with increased eagle migration out of the area.

         As with last year’s data, numbers of immatures at both Croton Dam and at George’s Island
fluctuated more than numbers of adults, in line with the possibility mentioned by Bielfelt and Hannan
(2007) that adults may be more roost-specific than immatures. And, at both George’s Island and Croton
Dam, there was a rough inverse relationship between the total number of eagles at each site on a given day,
suggesting that there is some interchange between these two sites.

         The second season of Bedford Audubon’s Winter Eagle Monitoring Project saw a significant increase
over last year in the amount and scope of data collected on the area’s roosting eagles. This project should
continue in the future: the past three years’ close monitoring of these roosts has revealed large numbers of
eagles using them, but in numbers that can fluctuate significantly even over the course of several days.
These sudden fluctuations may well have had the effect of masking the true extent of the area’s importance
for wintering eagles, if past, less frequent surveys happened to occur on “off days”. Frequent monitoring of
these sites is therefore necessary. This charismatic bird merits continuing careful attention to its numbers
and habitats in this region.


Acknowledgements
        First of all, I’d like to thank our dedicated monitoring volunteers, without whom this project would
truly have been impossible: Peter Becker, Michelle Cranwell, Arthur Green, Mike Hannah, Tony Ianniello,
Laura Jaffe, Matt Jaros, Jack Kozuchowski, Allen Kurtz, Paul Lewis, Jean Luca, Ken Mirman, Jim Nordgren,
Cathy O’Neil, Nannette Orr (for both monitoring and data entry help), Helle Raheem, Ted Raymond,
Christine Smith, and Emily Wong. Your efforts are greatly appreciated. Special additional thanks to Arthur
Green for constructing the graphs.


       Thanks also to the DEP-NYC police for their willing and helpful cooperation.


Sources
       Bielfelt, Brian, and Hannan, John. Monitoring of Wintering Eagles in Westchester County 2007.
       Burger, Michael F., and Liner, Jillian M. Important Bird Areas of New York. 2005 Audubon
       New York.
       Johansson, Tait. Monitoring of Wintering Eagles in Westchester and Rockland Counties 2008.
       Nye, Peter. New York State Bald Eagle Report 2008.




end



            Bylane Farm • 35 Todd Road • Katonah, NY 10536 • 914-232-1999 • bedfordaudubon.org

				
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