Nest Concealment and Predation in Hooded Warblers: Experimental Removal of Nest Cover

Abstract

-It is commonly assumed that passerine birds with open nests reduce the risk of predation by concealing their nests. At a forest study site in northwestern Pennsylvania, about 50% of the Hooded Warbler (Wilsonia citrina) nests were depredated-the primary cause of reproductive failure. To test the hypothesis that concealed nests are less likely to be depredated than conspicuous nests, we examined seven characteristics of the warbler's nest and nest microhabitat in relation to nest predation: nest visibility, vegetation density surrounding the nest, height of nest, height of nest substrate, nest-substrate species, height of cryptic dead-leaf base of nest, and proximity of nest to a microedge. Successful and depredated nests (n = 97) did not differ in any of these variables. In addition, we performed a vegetationremoval experiment in which manipulated nests (n = 15) with surrounding vegetation removed were on average 86% more visible overall than control nests (n = 15). The highly visible manipulated nests did not suffer higher predation than control nests. Contrary to our hypothesis, our results show that nest concealment is not an important factor in predator avoidance for Hooded Warblers. This outcome would be expected if nest predation is the result of nonspecialist predation that occurs by chance and is based on the location of available food in general. Received 19 September 1994, accepted 25 April 1995. THE MAJOR FACTOR influencing reproductive success of open-nesting birds is nest predation (Skutch 1949, Nice 1957, Ricklefs 1969, Gates and Gysel 1978, Martin 1992). Consequently, there should be selective pressure for birds to place their nests in sites that deter predation. It is clear that birds select nest sites nonrandomly (e.g. Peterson and Best 1985, Bekoff et al. 1987), and there is evidence that they can identify nest sites with characteristics that reduce the risk of predation (Stauffer and Best 1986, Martin and Roper 1988, Marzluff 1988, M0ller 1988). Nest concealment and characteristics of nest microhabitat, such as vegetation density and height I Present address: 3662 Kinter Hill Road, Edinboro, Pennsylvania 16412, USA. of nest from the ground, have been related to predation risk (Caccamise 1977, Murphy 1983, Martin and Roper 1988, Yahner and Scott 1988, Kelly 1993, Martin 1993) and, thus, could serve as criteria in nest-site selection. Although concealment of a nest by selecting an inconspicuous site is commonly assumed to reduce the risk of predation, there have been conflicting results in testing this hypothesis. Martin (1992) concluded in a review of 56 studies that dense foliage reduces the probability of predation by concealing a nest. However, of the 11 studies on passerines, 7 reported a negative correlation between predation and nest concealment, and 4 reported no correlation. Pertinent research, including those studies reviewed by Martin (1992), was conducted by Nice (1937), Nolan (1978), Martin and Roper