Movements and Survival Rates of Hartlaub's Gulls Larus hartlaubii
The primary objectives of the project are to estimate the survival rates, movement patterns and age at first breeding of Hartlaub's Gull Larus hartlaubii. These objectives will be achieved by ringing gulls with individually engraved colour rings, and undertaking intensive follow-up observations of these birds. An ancillary objective is for SAFRING and the ADU to gain experience in undertaking such a project, so as to be able to provide leadership and guidance to other researchers planning to run similar projects.
Photo L.G. Underhill
Dates of project
Hartlaub's Gulls breed mostly in autumn, and most ringing will take place over the period April to June. Information on survival rates and age at first breeding accumulates over periods of 5–10 years. Information on movement patterns accumulates rapidly. The initial duration of the project is two years and three months, from 1 April 2000 to 30 June 2002, covering the initial period during which ringing will take place in 2000, and a two-year resighting period.
Background information and methods
Most citizens of Cape Town are surprised to learn that Hartlaub's Gull is about the 10th rarest of the world's roughly 50 gull species. Its range is largely restricted to the coast between Cape Agulhas and Swakopmund. About one half of the total population, currently estimated at about 30 000 birds, are within the Greater Cape Town area. On one hand it is a relatively rare species, endemic to the Atlantic Ocean coastline of South Africa and Namibia. On the other hand, it is widely regarded in Cape Town as a nuisance, if not a pest, through fowling buildings and water features. It has, at times, been a hazard to aircraft near airports. It is frequently the subject of complaints about the noise they make, which can continue throughout the night.
The main traditional breeding colony for the birds from this area is on Robben Island, and this is where we plan to do the ringing. If they breed elsewhere, the colony will need to be located, and ringing will take place there. The rings and the colour rings were tested at this colony during the 1999 breeding season. We aim to ring (and colour ring) 500 gulls, both adults and chicks, in autumn 2000. If the estimates of population size and the proportion based in Cape Town are correct, this means that about one gull in 30 will have a project ring. These gulls interact closely with people, and are common and visible at places frequented by large numbers of people such as the V&A Waterfront, Sea Point promenade, Hout Bay harbour and restaurant area, Fish Hoek beach and many other beaches, along the Liesbeek River opposite Hartleyvale, and at many sports fields, especially in periods of rainy weather.
Members of the bird clubs and other interested members of the public are encouraged to read the numbers on the engraved plastic rings, and report sightings to the Animal Demography Unit. Observations will be entered into a database. We hope to get 5000 sightings by June 2001.
Feedback to observers will consist of printouts of the resighting histories of the gulls they report. On first participation, observers will receive a welcoming letter, and a resighting history of the bird first reported. After that, it is envisaged that resighting histories will be posted to active participants at regular intervals, say every six months. This will need to be dependent on volumes of incoming data. This feedback to observers is a critical component of the project.
Noteworthy resightings will be submitted to popular journals and magazines: Promerops, the newsletter of the Cape Bird Club; Bird Numbers, the newsletter of the Animal Demography Unit; and Africa – Birds & Birding, the leading South African birding magazine.
Outputs from the project
- A database of ringing and resighting data; the database will be available for use at workshops and courses where capture-recapture methods are taught and demonstrated.
- Estimates on the survival rates and age at first breeding of Hartlaub's Gull, and an understanding of patterns of movements on a local scale.
- Popular articles aimed at raising awareness of bird ringing and seabird conservation issues.
- Scientific papers submitted to journals and presented at conferences.
- The results of this project will provide input into management plans for the Hartlaub's Gulls, especially if it becomes necessary to give consideration to controlling them near airports and tourist destinations.
The Animal Demography Unit works closely with Marine and Coastal Management, Department of Environmental Affairs and Tourism and with the Western Cape Nature Conservation Board on all projects related to the ringing of seabirds. Marine and Coastal Management has a broad mandate to undertake research on seabirds. The Robben Island Museum provides logistical support for the research on the island.