Abstract
The annual flooding in the Amazon basin transforms the understory of floodplain forests into an aquatic environment. However, a great number of non-aquatic bird species occupy the understory and midstory of these forests. In general, these birds are thought to be sedentary and territorial, but the way they adapt to this dramatic seasonal transformation has never been described in detail. In this study, we describe avifaunal strategies to cope with seasonal flooding in the lower Purus region, central Amazonia, Brazil. We conducted focal observations of five insectivorous species occupying the lowest forest strata in two types of floodplain forest (black- and whitewater) during the low- and high-water seasons. For each observation, the height of the bird above the substrate (ground or water), its vertical position in the forest, and vegetation density around the bird were noted. All species remained present in the floodplain forests during the two seasons and were not recorded in adjacent unflooded (terra firme) forest. In general, birds migrated vertically to higher forest strata and most species (three of the five) occupied similar vegetation densities independent of water level. Despite the tendency of all species to rise in relative vertical position at high water, there was a reduction in height above substrate for four of the five species, suggesting that their position relative to water was not an important microhabitat element for them. Responses were similar in the two floodplain forest types. It is likely that the decrease in available space during the flood, combined with similar vertical displacement in arthropods, leads to increased prey density for understory insectivorous birds and permits year-round territoriality without major habitat shifts.
Highlights
We asked: (1) Do the birds remain in floodplain forests at high water, move to upland forest, or leave the area altogether? (2) If they remain in the floodplains, do they move vertically? (3) Do they use specific microhabitats associated with height above ground or water, forest stratification, or vegetation density? (4) Do bird responses differ between black- and whitewater floodplain forests? We discuss our findings in the context of known natural history of these species and the generalizability of these results to other Amazonian floodplain forest birds
The region presents an ideal opportunity to investigate the responses of understory floodplain birds to seasonal flooding in a closed-canopy forest matrix, because it contains an extensive mosaic of juxtaposed terra firme and floodplain forests inundated by white- and blackwater
It was motivated by the concept of vegetative strata being a structural element of the forest thought to be perceived by birds as distinct microhabitats
Summary
Amazonian floodplains are important for the diversity of birds in the Amazon, with approximately 15% of the non-aquatic Amazonian avifauna considered floodplain specialists [11] Most of these species are typical passerine insectivores and are not known to be migratory [12]. We attempt to redress this shortfall by focusing on the behavioral responses of floodplain understory birds to annual flooding. To describe these responses, we performed bird censuses in three major forest types (blackwater and whitewater floodplains and terra firme) and measured specific habitat parameters during the low- and high-water seasons in a representative set of five floodplain understory bird species. We asked: (1) Do the birds remain in floodplain forests at high water, move to upland forest, or leave the area altogether? (2) If they remain in the floodplains, do they move vertically? (3) Do they use specific microhabitats associated with height above ground or water, forest stratification, or vegetation density? (4) Do bird responses differ between black- and whitewater floodplain forests? We discuss our findings in the context of known natural history of these species and the generalizability of these results to other Amazonian floodplain forest birds
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