The Abundance and Seasonality of Forest Canopy Birds on Barro Colorado Island, Panama

Abstract

The outer canopy of a lowland tropical forest has a less buffered microclimate and more seasonal leaf phenology than the understory. Censuses were conducted from a canopy tower on Barro Colorado Island, Panama, to assess the composition and degree of seasonality of the canopy avifauna. The canopy avifauna shares many species with scrubby second growth; many common canopy species were also found frequently in more open areas. The rarest visitors to the canopy were primarily those from lower strata. Most common species were omnivorous, and restricted insectivores were poorly represented when compared to lower strata. This distribution is correlated with a taxonomic shift from the antbirdwoodcreeper-dominated understory to a tanager-dominated canopy avifauna. A majority of common canopy species was significantly seasonal in abundance; the most seasonal resident species tending to be the omnivores. Two sources of overall fluctuation in birds using the outer canopy were an influx of small omnivorous tanagers in the early dry to early wet season, and the presence of temperate-zone migrants, mainly Dendroica castanea, from late wet through the dry season. THE OUTER CANOPY OF A TROPICAL FOREST is a world distinct from the somber understory it shades. The protective shell formed by the foliage of massive trees receives the full brunt of the weather: wind speeds are higher, solar radiation and rainfall are most intense (Allee 1923), and temperatures average 2-5? C warmer than in the understory only 5 m below (Smithsonian Environmental Sciences Program, unpubl.). The biotic environment also differs markedly; large trees cover areas that might have supported many small plants on the forest floor, creating patches of young leaves, flowers, and fruits on a larger scale in the canopy than in the understory below. Furthermore, canopy trees display more pronounced seasonality in leaf loss than understory shrubs in semideciduous tropical forests (Croat 1978, Leigh and Smyth 1979). As Karr (1976b) argued, birds using different levels of a tropical woodland face radically different seasonal regimes, and the diversity and seasonality of the avifauna of each stratum should differ accordingly. Karr (1976b) showed that in a late scrub community in Panama, the upper strata had a less diverse and more fluctuating bird population than lower strata. Various authors (Orians 1969, Pearson 1971) have noted the taxonomic affinity of canopy and clearing avifaunas and the dissimilarity between canopy and understory birds. These observations suggest that the influence of differences in phenology, microclimate, or foliage structure of different forest strata may be profound. Few data are available on the abundance and seasonality of species in the outer canopy. The major problem in analyzing bird use of a tropical forest canopy is the difficulty of observing birds 25-40 m above ground. This difficulty is exacerbated by the similarity of the high sibilant calls of many small tanagers. One solution to this problem is to census birds from a canopy tower or walkway. Lovejoy (1975) censused from such a tower in the Brazilian Amazon but did not separate canopy census results in his published analysis. In this paper I present an analysis of census data taken from a 40 m canopy tower in secondary forest of Barro Colorado Island, Panama (BCI). Censuses were conducted to quantify the seasonal use of a small piece of outer canopy.