Wood Thrush nesting success

From 1994-2000, the Rouge River Bird Observatory examined nesting success of Wood Thrush (Hylocichla mustelina) in the 118-ha natural area on the campus of the University of Michigan-Dearborn.

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.

Methods

Each year, efforts were 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 were observed in order to locate the nests.  Nests were monitored through fledging of young or nest failure.  When possible, all adult birds and nestlings were banded with a uniquely-numbered, standard U.S. Fish and Wildlife aluminum band, and one or two color bands, which enabled individual identification in the field. Some adults were color banded as part of regular banding operations if the birds are captured in May and appeared to be in breeding condition; however, some of these birds may have been migrants.

After a nest had been vacated, data on height, tree species, distance from edge and water, and other microhabitat characteristics are recorded.

Results and Discussion

While the 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 on the north end of campus. 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
— 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.

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.

Please note:  All data is provisional and not to be cited without permission.

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