Many birds in breeding seasons engage in vigorous dawn singing that often turns to a prominent chorus. We examined dawn chorus variation of avian assemblages in a tropical montane forest in Taiwan and tested the hypothesis that onset sequence is affected by eye sizes, foraging heights, and diet of birds. Chorus onset and duration varied between sampling months, but generally peaked in the prime breeding season. Overall dawn chorus length increased with, but mean duration per species was negatively correlated with, species richness. The inter-correlation among trait variables in phylogenetic independent contrasts was calculated and examined. Both foraging height and relative eye size were selected as positively explanatory factors, but no dietary effects were detected, for chorus onsets. Dawn singing onsets also tended to delay as decreasing log (eye sizes) that, however, was not selected with enough explanatory power. The positive relationship of chorus onsets versus relative eye sizes contradicts our prediction, yet the negative correlation of relative eye sizes with log (eye sizes) indicates an allometric constraint on eye sizes along with increasing body sizes. Lower-layer species initiated singing earlier than upper-layer species, which complies with the positive correlation of onsets with foraging heights and supports our prediction. This pattern may be condition-specific and more likely occurs in forests lacking a sufficient canopy height and layering and a distinct light difference among forest layers. Foraging heights were additionally found negatively correlated with canopy coverage and slightly positively correlated with ground coverage of perches. Our results concord that foraging height is a relevant factor as eye size in determining the onset and sequence of dawn chorus, yet, suggest that forest settings and vegetation structure may likely complicate the prediction.
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