Determinants of partial bird migration in the Amazon Basin

Area of Science:

• Ornithology
• Animal Behavior
• Ecology

Background:

• Mechanisms driving bird migration at tropical latitudes remain poorly understood.
• Partial migration, where only some individuals migrate, is common in South American bird populations and offers insights into migration drivers.

Purpose of the Study:

• To test the Dominance and Body Size hypotheses for intraspecific variation in migratory behavior.
• To investigate the proximate mechanisms of partial migration in a Tropical Kingbird (Tyrannus melancholicus) population in the southern Amazon Basin.

Main Methods:

• Longitudinal study over 4 years using individually-marked Tropical Kingbirds.
• Analysis of factors influencing migratory decisions, including size, age, and sex.

Main Results:

• Confirmed the occurrence of partial migration within the studied Tropical Kingbird population.
• Found that the largest males and younger females exhibited the highest probabilities of migrating.
• Migration probability was linked to a complex interplay of size, age, and sex, exceeding predictions of existing hypotheses.

Conclusions:

• Migratory behavior determinants in tropical latitudes differ from those in temperate zones.
• Seasonal food limitation, particularly for insectivorous species, may drive partial migration in tropical seasonal habitats.
• Proposes a new hypothesis focusing on seasonal food limitation for insectivorous tropical migrants.