Scott F. Pearson

BREEDING BIRD RESPONSE TO RIPARIAN BUFFER WIDTH IN MANAGED PACIFIC NORTHWEST DOUGLAS‐FIR FORESTS

We examined the relative importance of riparian vs. upland habitats to breeding birds by comparing species abundance, richness, and similarity of bird communities in managed Douglas-fir forests in western Washington State, USA. We also examined whether forested buffer strips along second- and third-order streams effectively maintain the pre-logging riparian breeding bird community by comparing species abundance, richness, and turnover among three treatments: (1) unharvested controls; (2) sites that were clear-cut, leaving a narrow (∼14 m) forested buffer on both sides of the stream; and (3) sites that were clear-cut, leaving a wide (∼31 m) forested buffer along both sides of the stream. Deciduous trees, berry-producing shrubs, and other deciduous shrubs less common in adjacent upland forest characterized streamside zones. Despite different vegetation features, riparian and upland habitats did not differ in any measures of bird species richness and composition. No species or species group was more abundant...

Wintering Yellow-rumped Warblers ( Dendroica coronata ) track manipulated abundance of Myrica cerifera fruits

Abstract Food availability during winter may determine habitat use and limit populations of overwintering birds, yet its importance is difficult to judge because few studies have experimentally tested the response of nonbreeding birds to changes in resource abundance. We experimentally examined the link between fruit availability and habitat use by manipulating winter abundance of Myrica cerifera L. (Myricaceae) fruits in managed longleaf (Pinus palustris) and loblolly (P. taeda) pine stands in South Carolina. Myrica cerifera is a common understory shrub in the southeastern United States and provides lipid-rich fruit in late winter (February and March), when insects and other fruits are scarce. On treatment plots, we covered fruiting M. cerifera shrubs with netting in early winter to prevent birds from eating their fruits. In late February, when M. cerifera fruit crops were largely depleted elsewhere on our study site, we uncovered the shrubs and documented the response of the bird community to those patches of high fruit availability. Relative to control plots, total bird abundance (excluding the most common species, Yellow-rumped Warbler [Dendroica coronata]) and species richness did not change after net removal. Yellow-rumped Warblers, however, became significantly more abundant on treatment plots after net removal, which suggests that they track M. cerifera fruit abundance. We suggest that M. cerifera plays a role in determining the local distribution of wintering Yellow-rumped Warblers at our study site. To put these results into a management context, we also examined the effect of prescribed fire frequencies on M. cerifera fruit production. Across pine stands with different fire regimes, M. cerifera fruit abundance increased with the number of years since burning. It takes 4–6 years for individuals to recover sufficiently from a burn to produce large quantities of fruit. Thus, shorter intervals between burns will reduce winter fruit availability. Taken together, these results suggest that within those pine plantations, the local winter distribution of at least one common migratory bird is closely tied to fruit abundance, which in turn is tied to the frequency of prescribed fires.

FRUIT ABUNDANCE AND LOCAL DISTRIBUTION OF WINTERING HERMIT THRUSHES (CATHARUS GUTTATUS) AND YELLOW-RUMPED WARBLERS (DENDROICA CORONATA) IN SOUTH CAROLINA

Abstract We conducted winter censuses of two short-distance migrants, Hermit Thrushes (Catharus guttatus) and Yellow-rumped Warblers (Dendroica coronata), over seven years in five different habitats to determine whether their local abundances could be predicted by fruit pulp biomass. Sampled habitats were stands of upland and bottomland hardwood, loblolly pine (Pinus taeda), longleaf pine (P. palustris), and young (<10 years) longleaf pine. Hermit Thrush abundance, which was highest in bottomland hardwood habitats, was positively related to total dry mass of fruit pulp. Those results are consistent with the hypothesis that resource availability affects the local distribution of migrant passerines on their wintering grounds. Our results also indicate that bottomland hardwood habitats in the southeastern United States may be especially important to wintering Hermit Thrushes. Yellow-rumped Warbler abundance was correlated with ripe-fruit pulp dry mass of Myrica cerifera, a major source of winter food for that species. However, because M. cerifera pulp dry mass was confounded with habitat type, we could not distinguish the relative importance of fruit resources and habitat for Yellowrumped Warblers. Our results underscore the importance of fruit to wintering birds. However, the overall percentage of variation in winter bird abundance explained by differences in ripefruit biomass was modest, indicating that other factors are also important.

Cold temperature increases winter fruit removal rate of a bird-dispersed shrub.

We tested the hypothesis that winter removal rates of fruits of wax myrtle, Myrica cerifera, are higher in colder winters. Over a 9-year period, we monitored M. cerifera fruit crops in 13 0.1-ha study plots in South Carolina, U.S.A. Peak ripeness occurred in November, whereas peak removal occurred in the coldest months, December and January. Mean time to fruit removal within study plots was positively correlated with mean winter temperatures, thereby supporting our hypothesis. This result, combined with the generally low availability of winter arthropods, suggests that fruit abundance may play a role in determining winter survivorship and distribution of permanent resident and short-distance migrant birds. From the plant’s perspective, it demonstrates inter-annual variation in the temporal component of seed dispersal, with possible consequences for post-dispersal seed and seedling ecology.

Influence of Breeding Phenology and Clutch Size on Hybridization between Hermit and Townsend's Warblers

-In avian hybrid zones, differences in timing of breeding or clutch size may confer a selective advantage for one species over the other. We compared clutch sizes, arrival dates, pairing dates, and clutch initiation dates among Hermit Warblers (Dendroica occidentalis), Townsends Warblers (D. townsendi), and their hybrids from areas within and adjacent to their hybrid zones in Oregon and Washington. Adult males of all phenotypes arrived first, with yearling males and females following by 10 and 18 days, respectively. Arrival was significantly later at higher elevations for adult males. The phenotypes did not differ in arrival dates when the effect of elevation was removed. Pairing dates were similar for the three male phenotypes, and clutch initiation dates were similar for the three female phenotypes. Thus, breeding phenology seems not to influence competition between these warblers and their hybrids. However, the clutches of Townsends Warblers were 0.6 eggs larger than those of Hermit Warblers and 1.15 eggs larger than those of hybrids, suggesting a selective advantage for Townsends Warblers. Hybrid females produced the smallest clutches we recorded (though not significantly smaller than Hermit Warbler clutches), suggesting that hybrids are inferior to both parental species. Received 16 July 1997, accepted 5 February 1998. HERMIT (DENDROICA OCCIDENTALIS) and Townsends (D. TOWNSENDI) warblers are sister species that apparently diverged in Rocky Mountain (Townsends) and coastal (Hermit) refugia during the middle Pleistocene (Bermingham et al. 1992). Three geographically isolated hybrid zones connect the ranges of these species, one in the Olympic Mountains of Washington, another in the southern Cascade Mountains of Washington, and a third in the Cascade Mountains of Oregon south of Mt. Hood (Rohwer and Wood 1998). The two Washington hybrid zones have been described in detail. For both zones, transitions from one parental phenotype to the other occur over 100 to 125 km, a distance only three to four times greater than estimates of root mean square dispersal (Rohwer and Wood 1998). These rapid character transitions imply that strong selective forces prevent these zones from becoming wider (Barton and Hewitt 1985, 1989). Rohwer and Wood (1998) evaluated a variety of models that might explain these narrow zones, and they concluded that both hybrid in3Present address: Department of Zoology, P.O. Box 118525, University of Florida, Gainesville, Florida, 32611, USA. E-mail: spearson@zoo.ufl.edu feriority and parental fitness asymmetries are keeping these zones narrow and, further, that parental fitness asymmetries are causing them to move. Hybrid zones will remain narrow when selection against hybrids is balanced by immigration toward the center of the zone by parentals (Barton and Hewitt 1985, 1989, Barton and Gale 1993). Asymmetric charactertransition curves also suggest that the Washington zones are moving southward, owing to the competitive superiority of Townsends Warblers over both hybrids and Hermit Warblers (Rohwer and Wood 1998). Because this inference was based only on the shape of the character-transition curves, the competitive ranking of hybrids and Hermit Warblers could not be inferred. Several alternative models that can explain abrupt character transitions are not applicable to these hybrid zones (Rohwer and Wood 1998, Pearson 1997a). These include recent contact (Endler 1977, Barrowclough 1980), hybrid superiority within an ecotone (Moore 1977, Moore and Price 1993) and habitat modification (Gill 1980, Wake et al. 1980, Hewitt 1989: table 4). These warbler hybrid zones are interesting because the inferred fitness asymmetry implies that Townsends Warblers are replacing Hermit

INFLUENCE OF NICHE OVERLAP AND TERRITORIALITY ON HYBRIDIZATION BETWEEN HERMIT WARBLERS AND TOWNSEND'S WARBLERS

Abstract Hybrid zones between Townsends Warblers (Dendroica townsendi) and Hermit Warblers (D. occidentalis) in the Pacific Northwest are narrow relative to estimated dispersal distances and appear to be moving, with Townsends replacing Hermits. We examined whether the habitat-transition and parental-fitness asymmetry models can explain why these zones are narrow and moving by comparing habitat variables associated with warbler territories in the Washington Cascades hybrid zone. Habitat variables did not differ among phenotypes, suggesting that the habitat-transition model cannot explain the narrow and dynamic nature of this hybrid zone. Habitat characteristics of Hermit Warbler territories did not differ inside versus outside the hybrid zone, also suggesting that this zone is not associated with a region of habitat transition. The lack of difference in habitat use could be the result of comparing variables that are not important to pairing success. However, warblers tended to select territories on west-southwest aspects. South aspects in the southern Washington Cascades are dominated by Douglas fir (Pseudotsuga menziesii) and true fir, which is a habitat selected by female warblers when choosing among territories and males. The parental-fitness asymmetry model does not necessarily make predictions about habitat use within the hybrid zone but predicts the superiority of one parental species over the other. However, if significant overlap occurs in habitat use or niche (as in these warblers), then competition between parental species is likely to occur. To determine whether these species compete, we mapped 12 warbler territories and monitored an additional 94 territories throughout the breeding season and found that all males with neighbors compete for and hold exclusive territories. Thus, the pattern of habitat use and territoriality is consistent with the parental-fitness asymmetry model.