Rouge River Bird Observatory
Wood Thrush Nesting Success in an Urban Forest Fragment
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Annual summaries
Introduction Methods Results
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Annual summaries
Introduction Methods Results
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| The Wood Thrush (Hylocichla mustelina) is a Neotropical migrant
whose populations have been declining over the last two decades.
A major reason for this decline is forest fragmentation on the eastern
North American breeding grounds. This makes the Wood Thrush vulnerable
to nest parasitism by the Brown-headed Cowbird, as well as to many predators
-- such as cats, raccoons, and Blue Jays -- that are more common in fragmented
forests.
From 1994-2000, the Rouge River Bird Observatory at the University of Michigan-Dearborn looked at the nesting success of Wood Thrush in this 118-hectare (292-acre) forest surrounded by urbanization. |
For more information on Wood Thrush, you can read Cornell's "All About Birds" Wood Thrush account . There is information on Wood Thrush population trends, life history data, and related topics from the National Biological Survey's Breeding Bird Survey database. Listen to a clip of the song of the Wood Thrush (504 kb, .au format). |
Methods
Each year, efforts are made to locate all Wood Thrush nests in the
Natural Area. This is accomplished by locating the territories of
all singing males using tape-recorded playback. Birds are observed
and followed to locate the nests. Nests are monitored through fledging
of young or nest 
failure.
If possible, all adult birds and nestlings of accessible nests are banded
with a uniquely-numbered, standard U.S. Fish and Wildlife aluminum band,
and one or two color bands, which enable individual identification in the
field (see picture at right). Some adults are color banded as part of regular
banding operations if the birds are captured in May and appear to be in
breeding condition; however, some of these birds may be migrants.
After a nest has been vacated, data on height, tree species, distance from edge and water, and other microhabitat characteristics are recorded.
Results
While our data set is very small, some of our results were surprising.
Our data on site fidelity, preferred nest tree species, and reasons for
nest failure have all been unexpected.
Nesting success -- Thirty-one nests were located of
which 9 were successful in fledging a total of 27 young. This gives
an apparent nesting success rate of 32.3%. However, it is likely
that more nests are found further along in the nesting cycle (and thus
more likely to survive) and that successful nests are more likely to be
found than unsuccessful nests. Therefore, scientists use a standardized
procedure, the Mayfield method, of calculating nesting success to
compensate for this. The nesting success using the Mayfield method
for 22 nests that were followed closely is 26.6% (the probability of a
nest surviving the entire period from egg laying to fledging is only 26.6%).
Nesting success of Wood Thrush (using the Mayfield method) has been
measured at less than 30% small (generally less than 80 hectare) plots.
Several studies calculate a rate of about 26 to 30%. While their
nesting success is lower in fragmented areas, plots of this
size are frequently occupied. In fact, a Cornell Lab of Ornithology study
revealed that Wood Thrush are actually more likely to nest in fragmented
forests. Thus, fragmented forests are "ecological traps" for this
species.
While our area totals 118 ha, two factors act to "reduce" its size from the Wood Thrush's perspective. First, the physical characteristics of the area (a linear configuration following the Rouge River, openings created by meadows and a lake on the Henry Ford Estate property, many foot trails) greatly reduce the core area, or the amount of area that is at least 100 meters (328 ft) from an edge. Second, with only two exceptions, the Wood Thrush in the study concentrate their territories in a very small area (<6 ha, or 15 acres).
The mean distance of a nest in our study site from an edge (an opening of greater than 0.2 ha, or about half an acre) for 23 nests is 30.3 m (99.4 ft). Mean distance a trail for 20 nests is 12 m (39.1 ft). Nesting success increases as distance from edge increases, yet the Wood Thrush in our area consistently nest quite close to edges.
Nearly all nests were located in a < 6 ha area, rather than being more evenly distributed throughout the available habitat. The males occupying this area were often not males that have nested here before -- new males as well as returning males are attracted to this limited area, which is located between the edges of the UM-D and Henry Ford Community College campuses and the lake. Part of the attraction may be that microhabitat characteristics are more important than distance to edge in this population of Wood Thrush. Another fascinating aspect may be the concept of "the hidden lek." Females of many monogamous songbird species seek out copulations with males other than their mates, especially if a neighboring male is of higher social rank or somehow more fit than her own mate. In some species, it has been determined that females choose nest sites close to the territories of other males to facilitate these extra-pair copulations.
Predation and cowbird parasitism -- While cowbird parasitism
in Michigan has been measured at over 32%, only 4 nests have had cowbird
eggs in our study (rate of 12.9%), despite a healthy cowbird population
in the area. We believe -- due to the fact all of our failed
nests remained intact and were located on branches 
unable
to support most mammilian predators -- that our nests were predated on
by other birds, almost certainly Blue Jays. Blue Jays and American
Crows are the predators that typically remove the eggs from the nest before
consuming them.
Researchers have found that predation in general accounts for a very high percentage of Wood Thrush nest failures. One study indicated that 56% of the nests in forest fragments <80 ha (198 acres) failed due to predation. Smaller forest fragments have a greater number of avian predators because there are fewer large predators to keep their numbers in check.
Preferred tree species for nest -- Despite being maligned as an invasive species with no wildlife value, the European or Common Buckthorn (Rhamnus carpathica) has been the nest substrate for all but 7 of the 31 nests; only 2 of the successful nests was not in buckthorn. Buckthorn provides many of the nest site characteristics preferred by Wood Thrushes such as dense shrub layer with a sparse herbaceous layer (shaded out by buckthorns). Buckthorns also favor damp areas that the thrushes like, and are a dominant shrub in our area. Therefore, while buckthorn is a favored nest substrate, other native species share these characteristics and would, if present more abundantly, substitute for this exotic species.
Site fidelity -- Wood Thrushes we suspect may be staying in the area to nest are color banded. 63 Wood Thrush (35 adults and 28 young) have been color banded for this study through 1999. We are not able to catch all breeding adults, and not all young are accessible enough to band. Eight birds banded prior to 1999 have returned in subsequent years:
This data is in agreement with other studies that have shown that males are more apt than females to return to breeding areas, and that young birds less likely to return to their natal areas the next year than adults.
Discussion
Our data indicate that our area, like many small, isolated woodlots,
acts as a population "sink" for the Wood Thrush. In other words,
Wood Thrush in our location are not producing enough young to replace the
adults and maintain the population. Scientists use a calculation that takes
into consideration the number of broods and nesting attempts a species
will make in a breeding season, the adult mortality based on previous studies
for that species, and the number of female young produced per successful
nest based on the Mayfield method for the study population in question.
Using this calculation, the Wood Thrush in our area do not produce enough
young to compensate for adult mortality. In a population "source" area,
the reproduction would produce a surplus of young, which would then in
turn be able to colonize other areas. This situation, repeated over
and over again as forests continue to become fragmented, threatens the
status of this wonderful species in our eastern forests.
Annual summaries in an eggshell
1994: 3 nests, all in buckthorn, 1 successful fledging 3 young
1995: 1 successful nest fledging 3 young in buckthorn
1996: 2 nests, 1 successful fledging 3 young in buckthorn (other nest
in box elder)
1997: 3 nests, two in buckthorn, one in dead tree, none successful
1998: 4 nests, 1 successful fledging 3 young in buckthorn, 2 other
nests in buckthorn, one in viburnum
1999: 9 nests, 2 successful fledging 6 young. Successful nests
in buckthorn and hawthorne; failed nests in buckthorn (6) and a deadfall
(1).
2000: 9 nests, 4 successful fledging 9 young.
Please note: All data is provisional and not to be cited without permission.
