Foraging strategies and prey encounter rate of free-ranging Little Penguins

Brief description

Data loggers were fitted to four Little Penguins in August 2002 to reconstruct the time/activity
budget of free-ranging Little Penguins from Penguin
Island, Western Australia as they foraged in the shallow waters of Comet Bay, Western Australia.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Data-loggers were fitted to four Little Penguins in August 2002. All birds were nesting on the central part of Penguin Island, Rockingham (32 16'S, 115 21'E), Western Australia, in rectangular, wooden nest-boxes, allowing easy capture of the adults and checking of the chicks. Nautical dusk and dawn for the study period were taken to be 0550 h and 1848 h (local time), respectively, (http://lychnis.imcce.fr/cgi-bin/levcou.cgi). Birds were captured at their nest site, either at night or before departure for sea early in the morning. The sex (determined from the bill depth; Gales 1989), breeding status and the mass of both the adult and its chicks were noted. Loggers were then attached using waterproof (Tesa) tape (Wilson et al. 1997a) on the median line of the birds back, near the tail so as to minimize drag (Bannasch et al. 1994). The attachment of the logger was completed in <5 min and birds were released at the entrance of their nest-box.

Statement: Time-budgets and activity patterns of birds were recorded using miniaturized, cylindrical, four-channel data loggers (M190-D2GT, 12 bit resolution, 52·15 mm, 16g, Little Leonardo, Tokyo, Japan). The absolute accuracy for the depth sensor was 0.1m. The devices simultaneously monitored depth (1 Hz) and acceleration (16 Hz) along the longitudinal (surging) and dorso-ventral (heaving) axes of the birds. The units contained sensors capable of measuring both dynamic acceleration (e.g. vibration) and static acceleration (gravity). In the absence of movement, values of static acceleration ranged from +1 to -1 G. For instance, a vertically upright logger would correspond to values of 0 G on the heaving axis and -1 or +1 G on the surging axis, depending on whether the logger was head-up or head down, respectively, (see Yoda et al. 1999 for technical details).

Statement: Calibration sessions were conducted at the Perth Zoo, Western Australia. One Little Penguin was equipped with an accelerometer, attached using tape in same position as used on free-ranging individuals. This bird was then released in the pool where it swam with conspecifics. The behaviour of the bird was filmed using a digital video-camera (Handycam, Sony Ltd., 30 frames/s) while it fed on dead pilchards S. sagax. An exhibit window comprising one complete side of the pool allowed
us to track the bird for the whole period it was submerged. These video sessions were subsequently used to confirm the relationship between the signals recorded by the logger, the posture and activity of the birds.

Data were downloaded into a computer and analysed using IGOR Pro (Wavemetrics Inc., USA, 2000, Version 4.01). A dive started when birds departed from the water surface and ended when they returned to it. Only dives >1 m were analysed. The bottom phase of dives was considered to have started and ended the first and last times that the rate of change of depth did not exceed 0.25 m/s. All depth changes occurring within this bottom phase are subsequently termed undulations (see Wilson 1995; Simeone and Wilson 2003).

Since surging acceleration was recorded along the main body axis of the birds, the static component of this acceleration channel was the most sensitive to changes in body orientation and was consequently used to calculate dive angle. Body angle was defined using the method described by Watanuki et al. (2003). Briefly, we used a low-pass filter (Tanaka et al. 2001) to separate the component of the gravity acceleration along the surging axis from the high frequency component resulting from wing beat activity. Body angle (&) was then calculated using the following equation:

& = a sin(A/g) - alpha

where A is gravity acceleration along the surging axis, g is gravity and alpha is logger attachment angle. Attachment angle was calculated assuming that body angle was 0 when birds were at the sea surface between two dives.

The accurate start time of the foraging trip was inferred from body angle values since the birds' body angles differed between standing on land and swimming at sea, with distinctive acceleration values of ca. 0 G and ca. +0.4 G, respectively, on the surging axis. Flipper beats were apparent in the acceleration signals as an oscillating pattern present simultaneously on both axes, with each propulsive stroke recorded on the heaving axis resulting in a forward acceleration recorded on the surging axis. This oscillating pattern has been identified as limb beating in several studies using similar loggers on a variety of birds' species (Yoda et al. 2001; Sato et al. 2002; Watanuki et al. 2003; Ropert-Coudert et al. 2004b, c; Weimerskirch et al. 2005). All parts of the Little Penguin dives lacking these distinctive oscillating patterns were considered to be passive swimming phases where birds use buoyancy to ascend (cf. Sato et al. 2002). The amplitude and frequency of each wingbeat were analysed using the heaving acceleration signal (the most sensitive to undulation in the birds body resulting from flipper beats).

Hourly irradiation data measured by a solar panel(with 0 degrees of inclination and a reflectivity of 0.1) were downloaded from the web page of the Australian National University(http://solar.anu.edu.au/Sun/Irrad/Irradiation.html) for Perth, Western Australia during August.

Flipper beat frequencies were calculated using a Power Spectrum Density analysis (Igor Procedure Version 1.1, LH 971028, Wavemetrics Inc., USA, 2000, Version 4.01).

Notes

Credit
Japan Society for Promotion of Science, Tokyo, Japan and Murdoch University, Perth, Australia.

Purpose
To help in the conservation and management of Little Penguins