Due to the challenges associated with studying birds that only return to land at night and nest out of sight in cavities, we know little about the extent and frequency of movements of breeding birds around the colony. Do birds fly directly to their nest-site on return to the colony? Or, do they range more widely throughout the colony? Storm-petrels typically undertake incubation shifts of 3-5 days’ duration and, it is assumed during this time, the off-duty partner remains far out at sea, foraging; however, might birds sometimes forage more locally and return to the colony on nights when change-overs do not occur? When incubating an egg or brooding a chick, does the bird ever leave the nest during each shift or does it sit tight for that whole time?
Radio-tags have been deployed on 6 adult birds in a trial project to help to start to answer some of these questions. Radio-tags consist of a small battery (10x4x3mm) with an attached antenna. Each tag is programmed to emit a radio pulse at a set frequency. By attaching tags of different frequencies to different birds, individuals can be detected and recorded using aerials mounted on posts within the colony and automated recording units. The units constantly search for the frequencies of the 6 tags that have been deployed; when one is detected, this is recorded, along with the time and date and signal strength, which is relative to the distance the bird is from the tracking system.
Radio-tags - the aerial on one has been cut down to a reduced length, ready for deployment on a stormie
The remote recording unit is attached to a power supply and an aerial (Biotrack Ltd)
Each tag weighs less than 0.6g and was mounted at the base of the four central tail feathers using Tesa© tape. A tag of such a small size will have minimal impact on the flight capability of a bird. The aerial was cut down to less than 7cm reducing any chance of snagging on rocks and limiting the detection range of a tag to less than 100m from a recording unit. Following repeated contact with salt water, the tape will gradually lose adhesion and detach over the course of a few weeks.
Attaching a radio-tag to the tail feathers (Hannah Watson)
Tags were deployed to birds whose nest locations are known, allowing us to monitor chick growth rate and subsequent breeding success to verify that the tags do not have any significant impact on birds. The current aims are to trial the equipment, determine how long tags last for and verify that there are no significant effects of the tags on birds. Next year, it is hoped that more tags will be deployed earlier in the season, during incubation, to obtain more detailed information about patterns of movement of breeding stormies within the colony.
Stormie with radio-tag attached (Hannah Watson)