Avian frugivory on a gap-specialist, the red elderberry (Sambucus racemosa).

Publication: Wilson Bulletin

Publication Date: 01-DEC-05

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In the temperate zone, few plants produce fruit during the avian breeding season (Thompson and Willson 1979, Wheelwright 1988), and the typical pattern is for temperate zone plants to fruit in late summer and early fall (Morton 1973). Only a small fraction of the breeding bird community is even partly frugivorous, largely because of the high abundance and protein content of arthropods (although we note that many insectivorous migrants do eat fruit in the nonbreeding season). Little is known about the importance of fruit to temperate breeding birds (McCarty et al. 2002), or about the movements of territorial bird species in response to early-fruiting plants (Gorchov 1988).

Red elderberry (Sambucus racemosa pubens), typically found in forest gaps, is among the earliest woody plants to fruit in the northeastern region of the United States (Stiles 1980) and is available to forest birds while they are still nesting. In this study, we videotaped ripe elderberry shrubs to quantify which avian species fed on elderberry fruit and the rate at which plants were visited. We also analyzed radiotelemetry movements of one key species that eats elderberry, the Scarlet Tanager (Piranga olivacea)--to determine whether it shifts its territory use in response to fruiting elderberry or makes long distance movements off territory in search of fruit.

METHODS

Study area.--From 2000 to 2003, we studied avian frugivory on red elderberry at the Hemlock Hill Biological Research Area (41[degrees] 46' N, 79[degrees] 56' W), a 150-ha mixed forest in Crawford County, northwestern Pennsylvania. The fruiting period for elderberry was between mid-June and mid-July, although individual plants were sometimes depleted of fruit within 7-10 days of ripening in mid- or late June. We searched the study site for elderberry plants and found 54 different plants (0.36/ ha) at 19 different sites (defined as >50 m apart; 0.13 sites/ha). Fruiting plants typically had 20-50 clusters of fruit per plant (mean = 24, SD = 25.8, n = 49 plants), with about 200 individual fruits per cluster. Fruits are brilliant red, small (3-5 mm diameter; 0.05 g wet mass), and have a relatively high-energy content (68.8 kcal/100 g; Usui et al. 1994).

Bird visits and bird surveys.--We selected medium- or large-sized elderberry plants for videotaping; for a subset of these plants, the mean number of fruit clusters was 46 (SD = 33, range = 8-105 clusters per plant, n = 11). Some sites contained several adjacent elderberry plants that were videotaped separately, but were considered the same site because presumably the same individual birds fed on adjacent elderberry plants. We videotaped 14 different sites, 3 of which were taped in 2 different years, yielding a sample size of 17 sites. Video cameras (Sony Hi-8) were positioned so that most or all of one fruiting plant could be observed, and we collected 1-4...

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