Illinois Natural History Survey - University of Illinois

Kentucky Warbler Population Dynamics in a Forest Mosaic

During the past 15 years there has been an ever-increasing concern for the fate of many species of nongame forest birds that winter in the tropics (neotropical migrants). Several lines of evidence suggest that they are declining over much of their range. Forest fragmentation and the creation of forest edges on breeding sites in the temperate zone have been implicated as possible causes of this decline. Areas in Illinois where continuous forest once stood are now dominated by a mosaic of forested and nonforested habitats. The combination of elevated levels of both nest predation and brown-headed cowbird parasitism in such a mosaic significantly diminishes reproductive success, and thus recruitment, of many forest-dwelling migrants.

Indeed, while small forest "islands" harbor large numbers of edge-associated nest predators, such as raccoons, skunks, and blue jays, the agricultural openings surrounding them maintain unusually high populations of nest-parasitic cowbirds. Cowbirds lay their eggs in the nests of other species, and are entirely dependent on the hosts for raising the cowbird young. Large populations of cowbirds can have serious effects on host reproductive success primarily because female cowbirds remove host eggs prior to laying their own and they can parasitize a single nest multiple times. Parasitized nests frequently fledge few or no host young.

With a few exceptions, the dynamics of neotropical migrant populations are poorly understood, especially for forest-interior species. Most studies rely on crude measures of nesting success and return rates of adult birds. We lack detailed demographic and life-history data about the diverse factors influencing reproductive output and age- and sex-specific return rates of adults in relation to previous nesting success. Until these critical gaps are filled, we will not know the levels of nest predation and parasitism that populations can tolerate. Such detailed, species-specific data are crucial before effective conservation plans can be formulated. I am currently investigating the impact that a mosaic consisting of intact forest, disturbed forest (clearcuts and tree plantations), and agricultural fields has on the reproductive success and dynamics of a population of Kentucky warblers in the Shawnee National Forest in southern Illinois.

Male and female Kentucky warblers. (from original painting by INHS researcher Solon Morse)

At many sites in Illinois, Kentucky warblers experience moderate but variable levels of both nest predation and cowbird parasitism and seem to be somewhat tolerant to fragmentation. Populations may be quite high in selectively logged tracts of forest, tree plantations, mid-successional clearcuts, and other disturbed forest where understory cover is abundant. Kentucky warblers also occupy territories adjacent to forest edges and choose nest sites usually within a meter or two of an edge created by a trail or stream. However, my results indicate that the production of Kentucky warbler young is very low in disturbed habitats and along agricultural edges -- even though Kentucky warblers may be drawn to them -- due to both elevated levels of nest predation and parasitism.

I found that predation of nests is the primary cause of nest failure, and that predation increases with increasing levels of forest disturbance. In undisturbed forest, nearly 45% of all nests successfully fledge young. Half of the females successfully fledging first attempts build second nests following fledgling independence. Some double-brooded females fledge as many as nine young in a single season. By contrast, in clearcuts, along forest/field edges, and in the tree plantation (which superficially resembles undisturbed forest) only 16%, 25%, and 30% (respectively) of all nests successfully fledge young. Most females will attempt to renest two or three times (and very rarely four) following failure, but because clutch size decreases and nest predation increases throughout the summer, renesting attempts rarely produce many young.

Kentucky warbler nest parasitized with cowbird eggs. (photo by Solon Morse)

Unlike nest predation, the level of cowbird parasitism is not related to forest disturbance but is determined largely by proximity to agricultural fields bordering the forest tract. In the forest closest to these fields, 56% of the nests are parasitized. Parasitism drops to 20% at 1 km and only approaches 0% at 2 km away from the fields. Cowbird parasitism does not preclude the production of host young entirely. Parasitized nests usually fledge one or two Kentucky warblers as long as a few host eggs remain in the nest to hatch. However, unparasitized nests fledge 2.5 times more host young than do parasitized nests. The major costs of parasitism are in the initial 50% egg removal and in a 20% decrease in hatchability of the remaining host eggs.

The highest fledging success recorded for Kentucky warblers on my site is low compared to that measured elsewhere (70%), and productivity--a function of fledging success and cowbird parasitism--is only slightly more than half that measured for other neotropical migrants in unfrag-mented forest (2.4 versus 4.5 young produced per female per year). Coupled with high post-fledging and migration mortality, it is unlikely that these birds are producing enough young to replace themselves. Furthermore, the return of adults to breed the following year is much lower at this site than measured elsewhere (43% versus 65% returning) and may be related to low nesting success. Both males and females are half as likely to return following failure during their first breeding season compared to birds that successfully fledge young. Low adult return rates mean that this population is critically dependent on immigrants from other areas for persistence.

My data are consistent with those collected by Scott Robinson of the Survey at other sites in southern Illinois. Collectively, these results suggest that Kentucky warbler reproductive output would benefit from large continuous tracts of undisturbed forest and they imply that caution must be used in assuming the viability of populations based on the presence of adults alone. Although Kentucky warblers will use a variety of disturbed habitats -- and are frequently at high densities in them-- such habitats may act as population "sinks" in that adults are lured in from the surrounding landscape but subsequently fail to reproduce. For a forest tract to provide some measure of cowbird-free space, the nearest agricultural edge must be more than 2 km from the center of the tract. Continuous forests of this size are rare in southern Illinois, and all but absent to the north. In other parts of the Kentucky warbler's range, however, nest predation and cowbird parasitism are much lower. As long as these distant source populations continue to subsidize our local populations, the Kentucky warbler is likely to persist in Illinois.

Solon Morse, Center for Wildlife Ecology 

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Subject: INHSPUB-2163
Last Modified 3/19/96

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