Conventional wisdom states that Ovenbirds (Sieurus aurocapillus), a small ground-dwelling songbird, reside deep in the interior of deciduous forests in eastern North America during their breeding season. However, I was surprised to find them exclusively in scrubby forest edge habitats at my study site in the Hemlock Hill Biological Research Area of northwestern Pennsylvania.
A glance at the map below shows where Ovenbird territories (red ovals) are located, along roads and field edges. They were NOT in the forest interior and had not been for the 28 years I’ve studied birds in these woods. Why were Ovenbirds acting so peculiar here?
Map Showing Ovenbird Territories (red ovals) at fringes of forest (darkly-colored areas) and playback experiment area (dashed oval)
I decided to find out why Ovenbirds choose such atypical habitat using a two-pronged approach. The first approach was to try to induce them to nest in mature interior forest habitat by using playbacks of their song to simulate occupied territories. The second was to determine what was so good about the roadside habitat here.
As with many migratory songbirds, Ovenbird territories are clumped, probably to facilitate an extra-pair mating system (see Sex, Song, and the Promiscuous Warbler). I hoped that song playback might simulate occupied territories and attract prospecting birds to settle. The broadcast, emanating from three speakers situated about 100 meters apart, covered 18 hectares (100 meters by 100 meters), the white dot oval in the picture (see above graphic), in mature interior forest.
The playbacks were first done during the summer of 2001. Would this simulation attract prospecting birds to settle in the following spring? Spring of 2002 came and went and no Ovenbirds appeared to settle. Next I tried the playbacks during the spring of 2003. Many Ovenbirds were sighted near the playbacks but, again, no settlement occurred. Since the playbacks were unsuccessful, I concentrated on trying to find out why scrubby forest edges were so appealing to Ovenbirds.
Assessment entailed sitting quietly on a folding chair for two minutes, to allow nature to absorb my presence, and then recording the time it took to hear or see a chipmunk during the next ten minutes. If no chipmunk was detected, a time of 600s was recorded. Eureka! It took much longer to detect chipmunks on Ovenbird territories.
In edge habitats not occupied by Ovenbirds, chipmunk detection time averaged 153 seconds compared with the virtual absence of chipmunks from occupied edge habitats. The forest interior seemed to be crawling with chipmunks, a decided disadvantage for a bird that nests on the ground.
So far we know that Ovenbirds in northwestern Pennsylvania prefer to nest in edge habitats. An attempt to attract them to settle in mature forest interior habitat through song playback failed. The playbacks performed in summer did not attract birds to settle the next spring nor did spring playbacks attract birds to settle the current spring.
Chipmunks, a known nest predator, were virtually absent from edge habitat occupied by Ovenbirds at Hemlock Hill Biological Research Area, but were common in forest interior and in unoccupied edge habitat.
I suggest the two results are related. Prospecting Ovenbirds base their settlement decision on factors that include the presence of conspecifics (other Ovenbirds) AND chipmunks. Chipmunks could be easily detected by Ovenbirds because both forage on the ground. Other ground nesting species, such as the Black-and-White Warbler and the Canada Warbler are also completely absent in the forest interior.
Chipmunks are very common at Hemlock Hill because the hoards of food they need to store away for the winter are easy to obtain. They eat a wide variety of seeds but two of the most important kinds are from Red and Sugar Maple trees. Red Maples produce seed in the spring, when chipmunk winter food caches are depleted whereas Sugar Maples produce seed in the fall, just when chipmunks need to store up. So, one can imagine, why, if they could, Ovenbirds would hate maple syrup!
I thank Friends of the National Zoo, SI Scholarly Studies program and the Christensen Fund for support.
This article summarizes the information in this publication:
Morton, E.S. 2005. Variation in breeding habitat use in the Ovenbird with special reference to breeding habitat in selection in northwestern Pennsylvania. Wilson Bulletin, 117: 327-344.
...Here, I report a study of habitat use by Ovenbirds in northwestern Pennsylvania, where they use only edge habitat contiguous to mature, deciduous forest-interior habitat. I show that this aberrant selection of breeding habitat appears to be influenced by predators, notably the eastern chipmunk (Tamias striatus). I also describe a playback experiment designed to attract Ovenbirds to settle in forest-interior habitat. My results, and those of others, show that habitat usage may vary across diverse geographic areas.
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