Roosting by Pelagic Seabirds: Energetic, Populational, and Social Considerations

Great Frigatebirds (Fregata minor) and Red-footed Boobies (Sula sula) roost in large numbers on the guy wires of the LORAN-C tower on Sand Island, Johnston Atoll, in the central Pacific Ocean. We quantified the diurnal pattern of movement to and from the atoll by the roosting, but nonnesting birds. The total number of birds using the atoll is difficult to determine but must be considered when making population estimates. By counting the roosting birds at 10 min after sunset a reliable estimate can be made of the maximum number of birds which will roost that evening. The number of roosting birds increases significantly when the trade winds decrease in velocity. Utilizing energy from winds and thermals is critical to these species for efficient flight, and energetic considerations may determine roosting patterns. Social interactions probably are secondary and result from the scarcity of suitable roost sites in the pelagic zone.