Stopover Ecology of Ruby-throated Hummingbirds (\u3ci\u3eArchilochus colubris\u3c/i\u3e) During Autumn Migration

The Ruby-throated Hummingbird (Archilochus colubris) is one of the smallest Nearctic-Neotropical migrants and the only species of hummingbird that breeds in Eastern North America, yet few studies have investigated the biology of Ruby-throated Hummingbirds when they stopover during migration. Given their small size, high metabolism, aggression, and specialized diet, hummingbirds may represent a migrant that operates on the physiological edge. Therefore it is important to understand the factors that influence their stopover as well as the decision to resume migration. Towards this end, Ruby-throated Hummingbirds were captured during autumn migration along the northern coast of the Gulf of Mexico to investigate (1) differences between age and sex classes during stopover, (2) resource use and defense during stopover, (3) the feasibility of using radio telemetry, and (4) the conditions influencing both an individual’s departure time and direction upon resuming migration. Data provided strong evidence of age-dependent migration and only weak evidence of sex-dependent migration. Older birds arrived earlier, had larger fuel loads, had shorter stopover durations, and longer estimated flight ranges than younger birds. In younger birds there was no effect of sex, but in older birds males arrived with larger fuel loads than females. When ages were combined, males were estimated to have longer flight ranges compared to females. During stopover, males tended to use aggressive behaviors in order to obtain priority access to resources. Resource use was influenced by defense, fuel load, conspecific density, and sex. Resource defense was influenced by conspecific density and fuel load. An aviary study to determine the influence of radio-tags on hummingbirds suggested that individuals are capable of carrying the additional weight; however effects were less when tags were smaller. When birds departed coastal Alabama, the majority left between sunrise and noon and timing was dependent on both wind direction and wind speed. The distribution of departure bearings were uniformly distributed, although most individuals departed parallel with the coast line. Overall, these results suggest that the stopover and migration of Ruby-throated hummingbirds are influenced by the interaction of both intrinsic and extrinsic variables as they circumnavigate the Gulf of Mexico to tropical wintering grounds