Kentucky Ornithological Society
Kentucky Warbler 74:64-67, 1998
Nest Site Selection and Nesting Success in White-eyed Vireos
Danny R. Peake and Gary Ritchison
Department of Biological Sciences
Eastern Kentucky University
Richmond, KY 40475
Introduction
Among birds, predators are often the cause of nest failures. For example, Lack (1954) estimated that 75% of all eggs and nestlings lost from open cup nests are taken by predators. In response, many birds build their nests in sites inaccessible to or camouflaged from predators. However, several factors must be considered by birds selecting nest sites. For example, higher nests may be less accessible to snakes and large mammalian predators, while lower nests may be less accessible to avian predators such as Blue Jays (Cyanocitta cristata) and Common Crows (Corvus brachyrhynchos) (Best and Stauffer 1980).
The risk of brood parasitism is another factor that may influence nest site selection by birds. Brown-headed Cowbirds (Molothrus ater), well-known brood parasites, have become more abundant over the past 30 years (Brittingham and Temple 1983) and, as a result, many songbirds may be at greater risk of parasitism. For example, recent studies suggest that cowbirds may parasitize three-fourths of the nests of neotropical migrants in small forest fragments in Illinois (Robinson 1992).
To avoid predation and brood parasitism, birds may build nests in locations where vegetation conceals nests. Well-concealed nests may be more successful than less concealed nests. For example, recent investigations suggest that well-concealed Hermit Thrush (Catharus guttatus) nests are more successful than less concealed nests (Martin and Roper 1988). In contrast, however, a study of nest site selection by Northern Cardinals (Cardinalis cardinalis) revealed no correlation between the degree of nest concealment and nest success (Filliater et al. 1994). Additional work is needed to determine those characteristics of nest sites that might influence nest success. The objective of our study was to examine nest site selection by White-eyed Vireos (Vireo griseus), an open-cup, edge-nesting species. Nest site selection by vireos may be particularly important because predation rates may be higher in edge habitats and because cowbirds frequently parasitize their nests.
Methods
Our study was conducted from May - August 1996 at the Central Kentucky Wildlife Management Area in Madison County, Kentucky. During May and June, vireos were captured in mist nets and banded with unique combinations of colored, plastic bands to permit individual identification. Pairs were observed 2 -3 times per week to delineate territories and to determine nesting status. Nests were located by observing females during nest construction, searching sites where vireos were frequently seen, and following adults when they were feeding nestlings. Once located, nests were monitored to determine whether they were successful or not. Successful nests were those from which at least one young vireo fledged, while unsuccessful nests were those from which no young fledged due to either predation or cowbird parasitism.
For all nests, we determined the species of plant in which the nest was located and, after nests fledged young or were lost to predation, we also determined: the height of the nest above ground, distance to the nearest edge, and nest visibility.
Visibility was measured at 1, 2, and 3 meters from nests in the four cardinal directions, and at 1 and 2 meters above nests. At each point, we estimated how much of the nest was visible. A nest that was completely visible was 100% visible, while one that could not be seen was 0% visible. Other measurements taken for each nest included nest width and height, cup depth, distance from main trunk, and the diameter of supporting branches.
Analyses were conducted using the Statistical Analysis System (SAS Institute 1989). All values are presented as mean +/- standard error.
Results and Discussion
We located 31 vireo nests. Fourteen nests, or 45%, were successful. Three unsuccessful nests were lost due to cowbird parasitism, while 14 were lost to predation. Even this success rate may be higher than typical for White-eyed Vireos. For example, investigators in Virginia monitored 47 nests over a 7 year period and found that only 30% successfully fledged young (Hopp et al. 1995).
The 31 vireo nests in our study were located in 11 different species of trees and vines, with most in Rough-leaved Dogwoods (Cornus drummondii; N = 11) and Eastern Redcedars (Juniperus virginiana; N = 6). The mean height of nests above ground was 0.80 +/- 0.05 (SE) meters, while the mean distance of nests from the nearest edge was 3.4 +/- 0.6 meters. Vireo nests averaged 5.57 +/- 0.18 cm in diameter, 6.8 +/- 0.3 cm long, and 4.3 +/- 0.1 cm deep. Also, nests were an average of 63.5 +/- 7.7 cm from the main trunk and attached to branches with mean diameters of 2.5 +/- 0.2 mm.
Mean estimated visibility of vireo nests in the four cardinal directions was 31.8 +/- 4.2%. Above nests, mean visibility was 45.6 +/- 7.4% at 1 meter and 39.2 + /- 7.4% at 2 meters. A comparison of successful and unsuccessful vireo nests revealed no significant differences in visibility, height, or any other measured characteristic (Mann-Whitney U tests, P > 0.1). Similarly, a recent study in Virginia found no apparent patterns among vireo nest sites in the density of vegetation above, below, and adjacent to nests (Hopp et al. 1995).
Although we found no differences between the characteristics of successful and unsucessful nests, vireos do appear to follow certain behavioral rules when selecting nest sites. These include: (1) build the nest within a meter or so of the ground, (2) build the nest at the end of a relatively long Y-shaped branch, and (3) provide some concealment for the nest. Other birds also appear to follow certain rules when choosing nest sites. For example, simple rules for selecting nest sites appear to be followed by Northern Cardinals (Filliater et al. 1994), Field Sparrows (Spizella pusilla; Best 1978), and White-crowned Sparrows (Zonotrichia leucophrys; Morton et al. 1993). In none of these species, however, are specific behavioral rules associated with an increased probability of nest success.
Many songbirds, particularly those like White-eyed Vireos that nest in edge habitats, suffer relatively high rates of nest predation and parasitism. This is not surprising because such habitats support a diversity of potential predators with a variety of searching strategies and, often, large populations of Brown-headed Cowbirds (Gates and Gysel 1978, Brittingham and Temple 1983). Some authors have suggested that this diversity of predators and parasites may essentially eliminate predictably safe nest sites. However, the apparent absence of such sites does not mean that the behavioral rules just described do not represent "an evolutionary response to selection for safe sites" (Filliater et al. 1994:766). Rather, these rules will probably be of limited benefit in predator-rich communities because different predators search in different ways and a site safe from one predator may be more vulnerable to a different predator.
Although not quantified, our study site does contain a diversity of potential nest predators, including but not limited to black rat snakes (Elaphe obsoleta), eastern chipmunks (Tamias striatus), long-tailed weasels (Mustela frenata), Blue Jays, and Common Crows. Given this diversity of predators and the apparent absence of safe nest sites, the appropriate strategy for species that suffer high rates of nest predation may be to limit defense of nests with eggs or young because defense carries the risk of injury or even death (Filliater et al. 1994). Thus, once a nest is lost to predation, vireos should attempt to re-nest as quickly as possible and, if needed, attempt to re-nest as many times as possible during the breeding season.
White-eyed Vireos in our study population do appear to follow this strategy. Adults are weak defenders of nests, typically uttering a few vocalizations but not closely approaching potential nest predators (Ritchison, pers. observ.). Once a nest is lost, female vireos can construct a new nest in 3 - 5 days and begin a new clutch shortly thereafter (Ritchison, pers. observ.). With a nesting season that extends from mid-April through late July, vireos can make at least three or four nesting attempts per breeding season.
In summary, only 14 of 31 vireo nests in our study successfully fledged young and we found no differences between the characteristics of successful and unsuccessful nests. Our results, and those from studies of other edge-nesting species, suggest that many nesting birds, in an attempt to improve their nesting success, follow a few simple behavioral rules for placement of nests, exhibit weak defense of eggs and nestlings, and re-nest quickly after losing a nest to predation. Such a strategy may be best in areas with a wide array of predators and no predictably safe nest sites.
Acknowledgments
We thank Jeff Hawkins, Pari Beigi, and Eric Williams for assistance in locating nests. Our work was supported by a Kentucky NSF EPSCoR grant (to Gary Ritchison).
Literature Cited
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