Temporal and spatial determinants of route selection in homing seabirds

Time of day is recognized as an important behaviour modulator of wild animals mainly via physical environmental changes such as temperature and light intensity. These temporal factors can also affect animal movements (i.e., changes of locations) and consequent distribution. However, while it is commonly observed in diverse taxa that an animal's attendance at a specific site concentrates within a narrow time window, how time influences the route selections of traveling animals in the course of their movements through to the end point is still not well understood. In this study, we quantified temporal and spatial patterns in the homing paths of streaked shearwaters Calonectris leucomelas, which are present at the breeding colony exclusively after sunset, to investigate how time constrains their movement strategy for homing from at-sea foraging areas. We tracked the foraging trips using GPS loggers in chick-rearing seasons for five years. In addition, in one year we conducted displacement experiments, releasing birds at sea at three different times of the day (midday, sunset and night-time) to impose time constraints on homeward movements. The movement paths revealed that the time of sunset was key to their decision of timing and in route selections during homing. Most birds returning from foraging trips reached the coastal area around sunset by adjusting travel timing and directions, despite variation in foraging areas, and flew along the coastline to the breeding island. Meanwhile, most birds released offshore stopped flights around sunset and waited on the water surface for sunrise before restarting their homeward movements. Birds therefore avoid offshore traveling at night, appearing to preferentially use diurnal cues for homing at sea. This study demonstrates the importance of timing as well as geographic features for homing decisions of streaked shearwaters and confirms that both spatial and temporal cognitive abilities are well developed in seabirds.