Katsufumi Sato, Yoshihisa Mori and Ken Yoda

Abstract

Bout analysis searches for a bout-ending criterion (BEC) to determine whether successive events are part of the same bout. Methods widely used for finding the BEC are logsurvivorship, log-frequency, and log-normal analyses. These analyses are based on the assumption that frequency distributions of event intervals can be described by two or three random distributions, and that the mean interval within a bout is common to all other bouts. Diving typically occurs in bouts. Since a dive bout is a sequence of complex behaviours in which the duration, depth, and interval between dives may all be adapted for optimal foraging, it is unreasonable to assume that the mean dive interval within a bout is common to all other dive bouts. Furthermore, one should not assume that dives might be split into bouts based only on dive interval without considering other characteristics, such as dive depth. Here we propose a new method, the sequential differences analysis, to find the BEC for dive bout analysis. This method has two features: (1) the frequency of differences in dive characteristics between two successive dives is used instead of the frequency of dive intervals, and (2) along with the dive interval, other characteristics are used to determine the BEC. Compared with the log-frequency analysis using dive intervals, the sequential differences analysis results in bouts with less variation (i.e. a smaller coefficient of variation) in dive characteristics, and a smaller number of dives within a bout. This suggests that our method splits dive sequences into bouts at a finer scale than the existing method. The sequential differences analysis is useful for dividing a sequence of complex behaviours with several characteristics into more meaningful bouts.

Yoshihisa Mori, Yoko Mitani, Yuuki Watanabe and Katsufumi Sato

We developed a simple behavioral indicator of prey patch conditions in diving animals. This indicator is a proportion of observed patch residence time to the "standard" residence time (POS). The standard residence time was defined as an optimal patch residence time maximizing the proportion of bottom time to the duration of dive cycle. We quantified the relationship between the POS and the prey richness index calculated from the number and size of prey-like objects on recorded still images from an animal-borne digital camera logger, and found significantly positive correlation between them. We found a similar positive correlation between the POS and a behavioral index (index of patch quality) developed previously in diving animals. These suggest that the POS is indeed a behavioral index of prey patch condition. Although this behavioral indicator was developed and tested in the context of foraging ecology of diving animals, this indicator can be applied to other central place foraging animals.

Takashi Yamamoto, Akinori Takahashi, Katsufumi Sato, Nariko Oka, Maki Yamamoto and Philip N. Trathan

Many animals migrate between breeding and wintering areas; however, whether each animal behaves consistently in space and time between consecutive years is less well understood. Furthermore, previous breeding state (successful or failed) is often not considered when attempting to understand consistent individual differences in behaviour that are likely to impact upon the subsequent behaviour. Between 2006 and 2010, we used geolocators to track the migratory movements of a pelagic seabird, the streaked shearwater Calonectris leucomelas, with individuals (N=46) being followed for two years or more, including 23 birds that had chicks in two seasons and 23 birds in just one season. All individuals, except for one bird, migrated to the same broad wintering areas, and their migratory route as well as the centre of wintering distribution did not change in relation to the previous breeding outcomes. Migration schedules (dates of departure from the breeding colony, southward and northward migrations, and first return to the colony) did not differ significantly between years for individuals that had chicks during both years, while failed individuals left the breeding colony and appeared to start the southward migration at an earlier date than the previous successful year. Nonetheless, the timing of the southward migration was consistent within individuals, including both males and females, over successive years regardless of the previous breeding outcome, and also the timing of first return back to the colony for females that had chicks in the both previous years and eggs in the both following season. This may imply the existence of individual-specific broad time schedules, possibly a circannual rhythm, though ecological conditions might affect the exact timing of the actual departure event. Our results present evidence for high levels of individually consistent behaviour for this pelagic seabird outside the breeding season.

Kozue Shiomi, Katsufumi Sato, Nobuhiro Katsumata and Ken Yoda

Abstract

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.