Critical Variables in Successful
Nesting Habits of Sialia sialis: The Eastern Bluebird
A Study at Ward Pound Ridge Reservation, Westchester
County, N.Y.
By Devin Hefferon
Mentored by James Daly
Abstract
During this past century there has been a dramatic decline in bluebird populations, a decline, researchers argue, principally due to habitat destruction. Because Bluebirds need specialized nesting cavities found primarily in old trees, routine deforestation has adversely impacted the bluebird populations, causing their numbers to decline. However, due to the efforts of organizations such as the North American Bluebird Society (NABS), The Cornell Nest-Box Network (CNBN), and amateur birders and their nest-boxes, this past decade has shown a rising number of bluebirds in northeastern America. In order to reverse the declining populations of cavity-nesting birds, these agencies and their volunteers have expended much effort to ascertain the ideal conditions under which these birds prefer to nest. While previous analysis of collected data has suggested some important variables in cavity-nesting preference and success, the ideal nesting conditions for the eastern bluebird is still understudy. “Critical Variables in Successful Nesting Habits of Sialia sialis” is the result of a five-year study on the nesting habits of Eastern Bluebirds in Ward Pound Ridge Reservation in Westchester County, New York. An analysis of the data suggests some interesting information that impacts nest preference and success, as well as proposing a new and significant variable for determining the ideal nesting conditions for Eastern Bluebirds: geographic positioning. The data from Ward Pound Ridge Reservation suggests that there are variables yet to be studied that indicates significant nesting preferences for Sialia sialis. Studying these variables may allow ornithologists to create ideal conditions for the Eastern Bluebird to nest in, therefore achieving a greater nesting success for a breeding pair of Eastern Bluebirds.
Introduction
Previous studies of the nesting habits of cavity-nesting birds have suggested that there are significant factors that affect nesting and reproductive success. Variation in clutch size (i.e. reproductive success) has been attributed to latitude (Young 1994, Hendricks 1997), date of laying (Young 1994, Winkler and Allen 1996), and altitude (Jørvinen 1989). However, the data set from which these geographic variables were measured was small, and its reliability not assured. The Cornell Lab of Ornithology’s Nest-box Network (CNBN) was designed to gather data nationally from volunteers monitoring nest-boxes inhabited by cavity nesting birds. The Cornell protocol for gathering data includes measuring of all the above-mentioned variables as well as gathering other information that potentially impacts clutch size and successful nesting such as predation, pairing of boxes, and pests. A preliminary analysis of the national 1997-1998 CNBN results (Kast et. al 2000) supports earlier claims that tie variation in clutch size to the effect of latitude and clutch-initiation date. In a follow up study based on the same data, Dhondt and Kast argued that for the first time there was strong evidence that suggested a correlation between geographical variation (i.e. latitude and longitude) and successful nesting as represented by clutch size.
Other
variables are documented by The North American Bluebird Society (NABS). NABS suggests that nest-box preference
leading to successful nesting is determined by other variables including box
materials, size of the box and entry hole, mounting, colors, drainage, weather
protection, parasite control, and predator deterrence. In addition, Somershoe and Zegers (2000)
found a strong correlation between bluebird nest-box preference and shape of
the entry hole.
Volunteers
at the Ward Pound Ridge Reservation in Westchester County, New York began
monitoring bluebird next boxes in 1996.
The Bluebird population had been steadily declining and an effort was
underway to attempt to revitalize the bluebird population locally by building
and mounting 50+ nest-boxes in Ward Pound Ridge Reservation. Between 1996 and 1998, Jim Daly and other
volunteers followed the NABS specs and the CNBN protocols to collect information
about the nesting habits of the Eastern Bluebird. In 1998, I began collecting data on the nesting habits of the
Eastern Bluebird in Ward Pound Ridge Reservation. In 1999, after reviewing data from previous years, I suspected
that there were other factors affecting Eastern Bluebird nest-box selection and
nest-box success. At that point, I
designed my own set of protocols, using a combination of the CNBN protocols and
variables I suspected may impact nest preference and success. Hoping to test another geographic variable
in addition to the longitude and latitude studies previously undertaken, I
focused my study on determining if the geographic direction (i.e. north, south,
east, west) in which the nest-box entry hole faced had an impact on Sialia sialis’ nest-box selection and
overall nest-box success at Ward Pound Ridge Reservation. This data is included
in both the Reservation’s records and in Cornell University’ s Lab of
Ornithology’s nest-box study.
§
Environmental Concern: How to revitalize waning populations of Sialia sialis
in Ward Pound Ridge Reservation, Westchester County New York.
§
Key Question: Does nest box direction (i.e. north, south,
east, west) have a significant impact on Eastern Bluebird nest-box selection
and success as determined by clutch size and number of fledglings?
Methods
In order to better understand the nesting habits of Sialia sialis and in turn increase their
populations in the park, the nest-boxes were monitored throughout Ward Pound
Ridge Reservation in Westchester County, New York over a four-year period. During each breeding season, which lasted
roughly from April through July, boxes were monitored and data was collected
weekly. Collected data included the type of species inhabiting the box; numbers
of eggs, nestlings, and fledglings; physical characteristics of the boxes, such
as indications of parasites or predators; whether the box had an existing nest
or if it had been cleaned out before a new nest attempt was made; the elevation
of the box from the ground; the distance of the box from other trees and from
human activity; the structure upon which the nest-box was mounted; and the
direction in which the box faced (West, East, North or South). The information gathered in this study was
analyzed for patterns that might suggest significant variables not yet studied
or recommended, that impact successful Eastern Bluebird nesting.
Results and Discussion
Eastern
Bluebird 28%
Nest-box Habitation: The Cornell Nest Box study of 1997 that monitored hundreds
of boxes in North America and found that only 28% of the active nest-boxes were
inhabited by Eastern Bluebirds, a success rate that was much
lower than at Ward Pound Ridge Reservation.
Between 1996 and 2000, bluebirds occupied 61% of active
|
1996 |
1.57 |
|
1997 |
2.25 |
|
1998 |
2.7 |
|
1999 |
3.5 |
|
2000 |
3.9 |
boxes at Ward Pound Ridge Reservation (see Fig. 1). In the Cornell North American study, at least fifty percent of the nest attempts reported for five cavity nesting bird species reported at least one young fledgling from each active nest. In contrast, the rate of successful fledging at Pound Ridge Reservation was consistently higher with 2.7 bluebird fledglings for all active nests (see Table 1, right).
Overall Trends in Eastern Bluebird Nesting Activity: While the numbers
of nest-boxes in the reservation did not stay constant between the years 1996
and 2000, it is interesting to note that numbers of nest-boxes used by
bluebirds grew fairly consistently as did the number of nesting attempts (See
Figure 2, 3).
Fledgling Success:
The absolute number of bluebirds that fledged grew fairly consistently over the
five-year span (see Fig. 4). There are
always environmental and climatic variables that may have an effect on the
nesting success. (See Dunn 2000 and Black 2000 on climate changes affecting
bluebird populations.)
Sialia sialis sensitivity to nest-box disturbance: Data indicates that
predation or disturbance to nest-boxes occurs year after year. Sialia
sialis demonstrates a strong
preference for boxes that have not been vandalized or
disturbed. 70% of boxes vandalized with
bluebirds in one breeding season were not successful in the following
year. 50% of those boxes vandalized
were not successful for at least two years afterward. Data also indicates that Sialia
sialis will not return to a vandalized nest until it has been successfully
inhabited by another species, in this case Tree Swallows (Tachycineta
bicolor). Once success in nesting
has been demonstrated by a fellow cavity nesting species, bluebirds will return
to nest. It is interesting to note that
Tree Swallows do not discriminate in this way.
Data has shown that Tachycineta bicolor will use the same box
year after year regardless of whether the nest is vandalized repeatedly. Moving vandalized bluebird nest-boxes to a
new location after the first case of vandalizing may improve the chance for
successful bluebird nesting in the next breeding season.
Mounting Results:
No significant
results were found to indicate that bluebirds prefer that their nest-boxes have
particular mountings. However, data
shows that 88% of boxes mounted on trees produced no fledglings. Although a more expansive study needs to be
undertaken on this subject, we might suggest that the remarkably high failure
rate associated with boxes mounted on trees is most likely because trees offers
a predator easier access to the nest-box than does an artificial mounting, a
metal pole mounting, for example.
Directional Results:
An analysis of five
years of data strongly suggests that Sialia
sialis shows a significant preference for nest-boxes whose entry holes face
in an easterly direction or 45 degrees northeast to 180 degrees south (see Fig
5, 6). After noting a trend in the low
success rate of boxes facing west (i.e. bluebirds not preferring to nest in
boxes facing in a Westerly direction), I turned five of the westerly-facing
boxes so their entry holes faced directions I hypothesized to be more
preferable for bluebird nesting. Two of
the five boxes I moved had nesting attempts that same season, especially
remarkable because those nest-boxes had had no nesting attempts in previous
years. This bluebird nesting preference at Ward Pound Ridge Reservation
complements other data I collected
suggesting that Tree Swallows have a preference for nest-boxes facing in a
northwesterly direction.
Conclusions
In the years between 1996 and 2000, the Eastern Bluebird
population in Ward Pound Ridge Reservation has risen, therefore achieving the
goal of the volunteers at the park.
Whereas the national CNBN data found 21% of active nest-boxes to be
inhabited by Bluebirds, this study found that Bluebirds inhabited 61% Ward
Pound Ridge Reservation’s active boxes.
While the number of nest-boxes used by Bluebirds has increased, so, too,
has the average number of Bluebirds that have fledged from those nests. The CNBN data reports 50% of its nests with
at least one fledge; Ward Pound Ridge Reservation reports 68% of its active
bluebird nests with an average 4 fledges, 2.7 average for all active nests.
Also demonstrated in this study was Sialia
sialis’ sensitivity to disturbances in nest-boxes. Data indicates that one
should change a box’s location or allow for other species to use the nest-box,
if its Bluebird nest is vandalized or disturbed. This increases the chance for a successful Sialia sialis nesting in the following year. After finding that 88% of boxes mounted on
trees were unsuccessful, or unused, research also indicates that Sialia sialis strongly favor boxes
mounted on un-natural bases.
Most interesting may be the results that demonstrate
significant directional nest-box preferences by Eastern Bluebirds. Sialia sialis seems to demonstrate a
clear preference for nest-boxes with easterly-facing entry holes, a preference
that may correlate to successful nesting.
This study indicates that in order to have a successful Eastern Bluebird
nesting, the nest-box entry hole should be facing northeast, east, southeast,
or south. However, follow-up studies on
a scale larger than was available at Ward Pound Ridge are indicated to support
the results of this conclusion.
Literature Cited
Black, Conley C. 2000. 1999 Nesting
Box Report. Volume 22, Number 3:
19-20.
Dhondt, A.A. Kast, T. L., and Allen, P.
E. 2000. Clutch-size Variation in
Eastern Bluebirds. Birdscope,
Volume 14, Number 2: 3-5.
Dunn, Peter 2000. Climate change will affect bluebirds and
swallows. Bluebird. Volume 22, Number 3: 7-8.
Hendricks,
P. 1997. Geographical trends in clutch size: a range-wide relationship with laying date. American
Pipits. 114: 773-778.
Jørvinen, A. 1989. Clutch-size
variation in the Pied Flycatcher Ficedula hypoleuca. Ibis 131: 572-577.
Kast, T. L., P. E. Allen, P. Senesac,
A. A. Dhondt, and D. W. Winkler. 1998. Geographic Variation in the breeding biology of North American
cavity-nesting birds: Preliminary Results from the Cornell Nest
Box Network. BirdScope.
Kast, T.L. and Allen, P.E.
1998. Nest Box Network
Efforts Pay Off. Birdscope,
Volume 12, Number 1: 4-5.
North
American Bluebird Society, 1999. Fact Sheet: Recommended Bluebird Nest- Box
Specifications. Online:
http://www.nabluebirdsociety.org/boxspecs.htm
Somershoe, Scott G., and David A. Zegers, 2000. Nest-box preference and productivity by Eastern Bluebirds, Tree Swallows studied. Blubird. Volume 22, Number 2:14-17.
Winkler,
D. W., and P. E. Allen. 1996. The seasonal decline in Tree Swallow clutch
size: physiological
constraint or strategic adjustment? Ecology 77:922-932.
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B. E. 1994. Geographic and seasonal patterns of clutch-size variation in
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Acknowledgements:
I'd
like to thank my mentor Mr. James Daly for all of his guidance and for helping
start my life in field biology. I would
also like to thank and commend my Science Research
teacher
Mr. Bartel for his direction, guidance, and understanding over the past three
years. Finally, I want to thank my
parents. Without their encouragement to
persevere,
follow
my heart, and work for all my goals in life I would be lost. A special thanks to my Mom for all of her
help on this mammoth project.