Flock formation of two parids in relation to cyclical seed production in a pinyon-juniper woodland
1990; Oxford University Press; Volume: 107; Issue: 3 Linguagem: Inglês
ISSN
1938-4254
Autores Tópico(s)Wildlife Ecology and Conservation
Resumo-Cyclical production of seeds in a pinyon-juniper (Pinus edulis-Juniperus monosperma) woodland enabled us to assess the effects of resource abundance on the winter foraging ecology and flock formation of two avian seed predators, the Mountain Chickadee (Parus gambeli) and the Plain Titmouse (P. inornatus). During the winter of low seed abundance (1986/1987), chickadees and titmice converged in microhabitat use. Chickadees exhibited a coarse-grained response by selectively foraging in areas with greater ponderosa pine (Pinus ponderosa) density during the winter of high seed production, but shifted to a fine-grained use of microhabitat during the following year by randomly foraging in different tree species. Titmice displayed a fine-grained response in microhabitat use irrespective of seed abundance, but foraged significantly more in ponderosa pine during the winter of low seed production (becoming more like chickadees in use of tree species). Conversely, foraging was consistent between years, and the two species were separable based upon the use of juniper substrates (e.g. chickadees foraged more on juniper needles than titmice, titmice foraged more on the ground beneath juniper). The convergence in microhabitat use by chickadees and titmice during the winter of low seed abundance may be attributed to the prevalence of mixed-species flocks. During the year of a mast seed crop, chickadees and titmice foraged singly or in pairs 80% of the time and were never observed together. Half of all individuals were observed in flocks the following year, and two thirds of flocking chickadees and nearly all (88%) gregarious titmice participated in mixed flocks. Monospecific flocks of chickadees used less juniper and foraged distinctly from titmice (e.g. probed more, pecked less, gleaned from an inverted position), but converged in these characteristics in the presence of titmice. Only two titmice ever occurred in mixedspecies flocks, yet up to six chickadees formed these mixed-species flocks. Chickadees apparently join titmice, as substantiated by the observed shifts in foraging ecology by chickadees in the presence of titmice, and perhaps gain knowledge of resource locations from resident titmice (chickadees are potentially altitudinal migrants within our study area). We observed an increase in sociality during periods of low seed abundance, which supports the proximate role of resource levels in promoting flock formation, but does not preclude the possibility that other factors that are a consequence of low resource abundance (e.g. decreased time available for vigilance) provide the primary impetus for flocking behavior. Received 18 July 1989, accepted 18 January 1990. PINYON-JUNIPER woodlands exhibit marked cycles in the production of seeds and berries. Mass production of seeds occurs every 5-6 years in pinyon pine (Pinus edulis) and every 2-3 years in juniper (Juniperus monosperma), such that in some years no seeds or berries are produced (Balda and Masters 1980). This local synchrony in seed production may be related to escaping 3Present address: Department of Biology, Colorado State University, Fort Collins, Colorado 80523 USA. depletion by overwhelming birds and mammals that feed upon seeds (flooding the system [e.g. Balda 1987]). Two avian seed predators that winter in the pinyon-juniper woodlands of northern Arizona are the Mountain Chickadee (Parus gambeli) and the Plain Titmouse (P. inornatus) (Shrout 1977, Balda and Masters 1980). Given the extreme annual variation in resource production, we were interested in the responses of these two seed predators in terms of flock formation and foraging ecology (microhabitat use, foraging be522 The Auk 107: 522-532. July 1990 This content downloaded from 207.46.13.122 on Thu, 19 May 2016 05:16:44 UTC All use subject to http://about.jstor.org/terms July 1990] Flocking Behavior of Two Parids 523 havior) during a winter (1985/1986) with a mast crop of both pinyon seeds and juniper berries, and during a winter (1986/1987) with almost no seed and berry production. Specifically, we examined the flocking of chickadees and titmice in relation to seed production during each winter, to determine whether differences in foraging ecology existed between these congeners, and to ascertain whether foraging ecology changed between winters in response to differences in food abundance. If one subscribes to the foraging efficiency hypothesis as the primary consequence of flock formation (Krebs et al. 1972; Caraco 1979a, b), then it is expected that Mountain Chickadees and Plain Titmice will form flocks during the winter of low seed abundance to increase the likelihood of locating scarce and patchily distributed resources. Formation of flocks will bring individuals and species into closer contact, and competitive interactions may counter potential benefits due to increased foraging efficiency. Species that flock during times of low resource levels (ecological crunches, sensu Wiens 1977) therefore should forage more distinctly in mixed-species flocks than when flocking with conspecifics. Species within flocks of oak woodland birds in Arizona were spatially segregated in microhabitat use. For example, Bridled Titmice (P. wollweberi) altered their foraging ecology (foraging stance, substrate use, and position in canopy) when in the presence of Bushtits (Psaltriparus minimus) (Austin and Smith 1972). Willow Tits (Parus montanus) avoided trees in which Crested Tits (P. cristatus) foraged, and shifted their position within the canopy of the tree in the presence of either Crested or Great (P. major) tits (Alatalo 1981). Conversely, or behavior predicts increased similarities between species when foraging in mixed flocks. Morse (1978) observed several unambiguous cases of copying in Blue Tits (P. caeruleus) attracted to sites previously or concurrently occupied by other foraging individuals (conspecifics as well as other species). Krebs (1973) experimented with mixed-species flocks of Black-capped (P. atricapillus) and Chestnutbacked (P. rufescens) chickadees in an aviary to demonstrate that flock members (of both species) converged in foraging in response to successful individuals (of either species). This led Krebs to propose that learning played an important role in flock and contributed to the increased advantage of heterospecific over monospecific flocks because total scanning range of the flock could be increased because of the presence of different species that foraged in different locations. Thus, we made several predictions in regard to flock formation and foraging ecology of Mountain Chickadees and Plain Titmice in response to changes in annual productivity of seeds: (1) chickadees and titmice will alter their foraging ecology in response to differences in food abundance between winters; (2) both species will exhibit increased sociality during the winter of low seed abundance; and (3) consequently, each species would be expected to diverge in foraging characteristics, particularly when participating in mixed-species flocks as compared with when foraging in monospecific flocks or alone. Alternatively, if sociality facilitates resource acquisition, then convergence in foraging ecology would be expected of species in mixed-species flocks. It should be possible, therefore, to distinguish between the contrasting predictions of the competition hypothesis and social facilitation hypothesis regarding mixed-species flock formation.
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