Activity time budget during foraging trips of emperor penguins

Abstract

We developed an automated method using depth and one axis of body acceleration data recorded by animal-borne data loggers to identify activities of penguins over long-term deployments. Using this technique, we evaluated the activity time budget of emperor penguins (n = 10) both in water and on sea ice during foraging trips in chick-rearing season. During the foraging trips, emperor penguins alternated dive bouts (4.8 ± 4.5 h) and rest periods on sea ice (2.5 ± 2.3 h). After recorder deployment and release near the colony, the birds spent 17.9 ± 8.4% of their time traveling until they reached the ice edge. Once at the ice edge, they stayed there more than 4 hours before the first dive. After the first dive, the mean proportions of time spent on the ice and in water were 30.8 ± 7.4% and 69.2 ± 7.4%, respectively. When in the water, they spent 67.9 ± 3.1% of time making dives deeper than 5 m. Dive activity had no typical diurnal pattern for individual birds. While in the water between dives, the birds had short resting periods (1.2 ± 1.7 min) and periods of swimming at depths shallower than 5 m (0.25 ± 0.38 min). When the birds were on the ice, they primarily used time for resting (90.3 ± 4.1% of time) and spent only 9.7 ± 4.1% of time traveling. Thus, it appears that, during foraging trips at sea, emperor penguins traveled during dives >5 m depth, and that sea ice was primarily used for resting. Sea ice probably provides refuge from natural predators such as leopard seals. We also suggest that 24 hours of sunlight and the cycling of dive bouts with short rest periods on sea ice allow emperor penguins to dive continuously throughout the day during foraging trips to sea.

Figure 1. Time distribution in 24 h cycles of dives (≥20 m) for emperor penguins at Cape Washington in the Ross Sea, Antarctica.

Figure 2. Percentages of traveling time in each hour for emperor penguins on the way from the colony to the ice edge at Cape Washington in the Ross Sea, Antarctica.

Each arrow shows when recording was started.

Figure 3. Frequency of the durations of rest periods (A) and swimming (B) at water surface (<5 m) between consecutive dives for emperor penguins.

Figure 4. An example of classification results for activities on sea ice (A) and in water (B) during ca. 4 days recording (C) on an emperor penguin (CW01).

Figure 5. Examples of results of k-means cluster analysis on the spectral components of body acceleration recorded on an emperor penguin.

Figure 6. A flowsheet to categorize nine activity types of emperor penguins by time (lower) and a diagram of the first step in the procedures (upper), using depth data and two components of acceleration data along with the longitudinal axis of the body recorded by a logger.

Pitch angle was estimated from the low-frequency component less than 0.5 Hz and traveling activity estimated from high-frequency component in the k-means cluster analysis.

Figure 7. Histogram of pitch angles estimated on an emperor penguin on sea ice.

When the bird was standing or walking, the pitch angle was greater than 50 degree.

Figure 8. Histogram of maximum dive depth for all dives (maximum depth ≥1); 40% of all dives were <5 m in depth.