Measuring parental foraging effort
To do this, a group of about 50 breeding pairs is fenced off from the rest of the colony. The only way in or out of this area is over a weighbridge. Breeding penguins inside the fenced area each have a small PIT tag injected under the loose skin around the neck. These tiny devices, measuring 16 mm long and 2.5 mm wide, weigh only 0.2 g. PIT tags, which function like individual barcodes, are used in conjunction with electronic weighbridges so that as a bird crosses the weighbridge, sensors read the tag, weigh the bird and record the direction of travel (i.e. entering or leaving the colony). We are using this information to measure:
- How long birds spend at sea feeding
- How much food birds bring back to their chicks
- The frequency of feeding trips as the breeding season progresses
- Any sex differences in breeding and foraging behaviour
We are using instruments like time depth recorders, satellite tags, and transmitters to find out what birds are doing while they are at sea. The streamlined, battery-powered, transmitting devices can be temporarily attached to the feathers on a penguin's lower back with sticky tape. This minimises any adverse effect the tag may have (like drag when swimming), and also reduces the likelihood of the tag being dislodged. Studies have shown that tags have no significant effect on the duration of a foraging trip or on nesting success.
Txs give out a brief electronic pulse about 55 times every minute. These pulses can be picked up by a person holding an antenna and a receiver. The loudness of the beeps from the receiver indicates the direction towards and closeness of the transmitter — and of course the penguin to which it is attached! This can be a very useful way of finding a particular penguin in a colony of thousands. Signals from txs can also be used to determine:
- Behaviour at sea:
- no beeps (penguin is diving)
- beeps constant for 10 to 20 seconds (aerobic recovery between dives)
- beeps continuous (penguin probably resting on an ice-floe)
- intermittent beeps (penguin swimming or porpoising)
- Location at sea: This is done by telemetry or triangulation: 2 or 3 people up to 100 km apart attempt to get a fix on a penguin using their tx receivers. The information gained can be used to determine:
- size of a foraging area
- overlap of feeding areas of adjacent colonies
- distance to feeding grounds
Satellite tags automatically transmit data on a bird's location to orbiting satellites. The information recorded can then be downloaded from the satellite.
Time depth recorders (TDR)
These weigh 25.4 g and when taped to a penguin's lower back add about 1% to the bird's total cross-sectional area. TDRs are normally attached for one or two foraging trips. TDRs sample both light intensity and temperature every second, and can store more than 2 million sensor readings. The light sensor allows the duration and depth of a dive to be found – the deeper the dive, the darker the water!
Stomach flushing and chick feeding observations
To find out what birds are eating during the breeding season we collected diet samples.To do this we use a tube to gently add about half a litre of warm water into a penguin's stomach. Tipping the bird upside down causes the water and some of the stomach contents to pour into a collecting bucket.
Based on our stomach flushing we know the diet of the birds is restricted to a limited number of prey and we can now asses what the birds are feeding on by observing adults feeding their chicks.