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Anatomy of the Dawn Chorus in a Neotropical Forest Bird CommunityEPA Grant Number: U916082
Title: Anatomy of the Dawn Chorus in a Neotropical Forest Bird Community
Investigators: Berg, Karl S.
Institution: Florida International University
EPA Project Officer: Just, Theodore J.
Project Period: January 1, 2002 through January 1, 2004
Project Amount: $53,686
RFA: Minority Academic Institutions (MAI) Fellowships for Graduate Environmental Study (2002) RFA Text | Recipients Lists
Research Category: Biology/Life Sciences , Academic Fellowships , Fellowship - Natural and Life Sciences
The objective of this research project is to investigate a neotropical dawn chorus to provide evidence for the importance of forest light in scheduling social communication in birds. The concentration of avian song at first light (i.e., the dawn chorus) is widely appreciated, but has an enigmatic functional significance. The most widely accepted explanation is that birds are active, but light levels are not yet adequate for profitable foraging so that social communication is selectively advantageous during this time period. As a consequence, the time of first song should be predictable from the light level, foraging behavior, and visual capacity for individuals participating in the dawn chorus.
To test these hypothesized relationships, I collected data on an avian community in a humid tropical forest of lowland Ecuador. Using an omnidirectional microphone, I recorded the dawn chorus at different sites on different mornings, involving more than 130 species across 12 avian orders. Light intensity was measured simultaneously. Height was estimated during peak foraging hours. Eye diameter was recorded from museum specimens and the literature. Species joined the dawn chorus at a rate of about two per minute and this was tightly correlated with the rate of light increase (R2 = .99). The time of first song (relative to local twilight) was a highly repeatable species-specific trait (r = .56). At higher taxonomic levels, phylogeny had a strong effect. Most data exist for the Suboscine branch of the passerines. Using phylogenetic independent contrasts, foraging strata was the most significant predictor of time of first song, with canopy-foraging species singing earlier than those specialized closer to the forest floor (R2 = .36). Contrary to studies of temperate bird communities, eye size (relative to body size) was not a reliable predictor of time of first song. One tenet of biodiversity theory is that the number of species increases with ecological diversity. The mechanisms explaining how increasingly complex ecosystem structure leads to reproductive isolation between sympatric populations are less obvious. If dawn song indicates a measure of reproductive fitness on which female selection is based, vertical habitat specialization may have given way to temporal divergence in fitness-related characters such as an earlier dawn song, opening the door to reproductive isolation. Although light level predicts the onset of singing in both tropical and temperate bird communities, the additional structural complexity and trophic specializations in tropical forests may mitigate against a linear relationship between eye size and time of first song.