Foraging Habits of Ruby-Crowned and Golden-Crowned Kinglets in an Arizona Montane Forest

Abstract

--Foraging behaviors of breeding Ruby-crowned (Regulus calendula) and Golden-crowned kinglets (R. satrapa) in an Arizona mixed-coniferous forest were compared to assess foraging similarity and the use of available habitat components. The species were not significantly different in five of the eight foraging variables examined. Foraging behavior was significantly different (Chi-square test, P < 0.02), for three variables: method of prey attack (Ruby-crowned Kinglet hovered more, Golden-crowned gleaned more), tree species selection (Goldencrowned was more restricted to certain tree species), and relative foraging location (Ruby-crowned used the upper thirds of the trees more frequently). Mean foraging height was similar. Both kinglets differed from a random distribution of use when compared to resource availability for all foraging variables tested (distance from branch tip, selection of tree species, tree height use, foraging height, and relative location). Foraging repertoires of these similar congeners were strikingly alike. Niche theory predicts that two such similar species will modify their foraging behavior to partition the habitat and minimize potential competition. In the case of the kinglets, limited environmental subdivision was demonstrated. Under less favorable habitat conditions, competitive influences may be more important in modifying the foraging behavior of these species. Ecological theory predicts that similar sympatric species will act in such a way as to reduce the overlap in niche foraging characteristics. Studies suggest that habitats can be partitioned either through differential use of vertical or horizontal portions of the vegetation (e.g., Hartley 1953, Balda 1969, Morse 1980), or through differences in foraging substrate preferences such as perch types or sizes (e.g., Hartley 1953, Morse 1967a, b), or plant species (Balda 1969, Jackson 1970, Willson 1970, Franzreb 1978, Holmes and Robinson 1981, and others). Ruby-crowned Kinglets (Regulus calendula) and Golden-crowned Kinglets (R. satrapa) may coexist during the breeding season. They are similar morphologically and in body size, although the Golden-crowned Kinglet is slightly smaller (see the Appendix for morphometric and weight data). Their diets also are similar, consisting primarily of animal prey (insects, spiders), and to a much lesser extent, plant matter (fruits, seeds; Beal 1907, Laurenzi et al. 1982). The primary objective of my study was to determine if breeding Ruby-crowned and Golden-crowned kinglets foraged randomly in a mixed-coniferous forest, and if not, to determine their preferences. Secondly, I wished to assess the similarity of foraging behaviors between the two kinglets in order to ascertain the extent of habitat partitioning. I anticipated finding clear differences in food habits that minimized potential competition.