Oilbirds performing non-seasonal intra-tropical migrations reduce flight effort compared to individuals that frequently return to the colony roost.

Individual behavioural variation is a driver of population dynamics and key for the adaptation and persistence of species. In colonial species, individuals share the same ecological niche and aggregate in space, but they might use different foraging strategies and incur differential costs. Using location and accelerometer data from 21 adult oilbirds (Steatornis caripensis), which are colonial frugivores, we characterized nocturnal movement patterns and measured total nightly flight time, as a proxy of movement effort, for different movement strategies during their non-breeding period. We collected location and accelerometer data using 22 g loggers (e-obs GmbH; Munich, Germany), between August 22nd, 2008 and February 14th, 2009. The tracked oilbirds displayed three distinct movement strategies for the entire sampling period: Intra-tropical migrants (ITM), which performed roost switches that lasted between 17 and 73 continuous days, consistent with partial nomadic migrations rather than seasonal movements; venturers, which performed long-distance excursions but returned to the main cave within 3 days, and roost-faithful individuals that never performed such excursions and never switched roosts. Among the three strategies, we estimated that ITM showed a lower movement effort, based on total nightly flight time, throughout the sampling period. Despite travelling long distances, ITM birds reduced overall flight effort by day-roosting near foraging sites, minimizing the need for long commuting flights. The oilbirds showed site fidelity to multiple foraging areas, without among-individual spatial overlap. These findings give insights into how behavioural flexibility could facilitate resource partitioning and coexistence in this colonial species, as well as the consequences of inter-individual variation for adapting to habitat change. Such information is relevant to evaluate the role of oilbirds as ecosystem service providers and habitat connectivity.