By Thomas Weston and Tony & Anne Taylor
Overview
Figure 1: An unringed Northern Wheatear. © Thomas Weston
Since the rat eradication on Lundy Island in the early 2000’s, the number of ground nesting birds and our seabird populations have increased as a result. One of the success species has been the Northern Wheatear (Oenanthe oenanthe). To understand how the population dynamics of the Wheatear changes each year, a British Trust for Ornithology (BTO) Retrapping Adults for Survival (RAS) project measuring the survival of the species was set up in 2013 by Tony and Ann Taylor and is still ongoing today, marking 2024 as the twelfth year the project has been undertaken.
The BTO RAS projects use amazing techniques to understand survival rates by looking at re-encounter rates of individuals within a population. In this writeup, the following BTO definitions are used to determine ‘survival rate’ which indicates the proportion of birds that survive and return to the site to breed each year; and ‘re-encounter rates’ that provide a measure of the probability of a bird’s presence being detected should it have survived and returned. In theory, the higher the re-encounter rate, the more precise the survival estimate should be.
This short writeup aims to provide an overview of the project and the 2024 results. To cover the main aspects of the Wheatear RAS study, sections have been split into the capture, resighting and nest recording of our Wheatears during the summer of 2024. A big thank you to Tony Taylor for providing the information and data for this writeup to be compiled, as well as his continued dedication to the Wheatear RAS study.
Methodology
Capturing unringed birds
The Lundy Island Wheatear RAS study area is defined as the coastal areas from the Castle, along the south and west coast, and as far as Halfway Wall. Since 2013, for three weeks in May and June, capturing and colour marking breeding adult Wheatears with individual colour rings has been an important task undertaken by the team. The Wheatears are caught using spring traps, a bird trap activated by the bird itself. To begin, the traps are set in areas of short grassland either on the plateau or the cliffs themselves, with a tasty meal worm treat to entice the birds to enter. The traps are watched from a suitable distance and once a bird has entered the spring trap, and activated it, a member of the team extracts the individual and puts it into a bag. The trap is reset, and the team move to a suitable ringing site nearby. Here the time and location are recorded before the bird is metal ringed with an individual BTO metal ring and a combination of colour rings. For each bird, three unique colour rings including a Lundy-specific colour ring are added to the birds’ legs. Two of these colour rings are single coloured. The third is a Lundy-specific one, unique to this RAS project, which is known as a wasp ring, consisting of a single colour banded with lines of another colour. Whilst in the hand, the birds’ biometrics are taken including age, sex, wing length, weight and maximum and minimum tarsus widths. The state of the brood patch (in females) or cloacal protuberance (in males) is also recorded, as confirmation that the bird is a Lundy breeder and not a late spring migrant passing through. The bird is released, and if the trap has not caught again it is moved.
Resighting colour ringed individuals
To determine the adult survival of the Wheatear population, recording the number of pairs within the RAS area between mid-May and mid-June, and recording any individuals with colour rings is essential. Colour ring reading is the first task undertaken by the team as the numbers of unringed birds and some nests are found using this technique. The colour rings can be read through binoculars, telescopes and through photographs. Through the team systematically making their way along the cliffs, colour rings are read and recorded alongside their location and whether they are paired with another colour ringed or unringed bird. By calculating the number of returning birds, the survival rate can be determined.
Nest recording
Due to the Wheatears being watched, either whilst resighting or when ringing, nests can be located with some ease. They are usually in rock crevices, old rabbit burrows or under boulders. The birds generally visit the nest area repeatedly, and before the nest is even located, based on the size of the food they are bringing back, the size of the chicks can be determined. Small pieces of food generally mean small chicks and large food pieces generally means larger, almost fledging chicks. Once a nest site is found, accessing it can be difficult if the burrow is too long or narrow, or in a dangerously steep area. However, every nest is recorded and if the adults are also colour ringed, then this provides even better data. Where nests are accessible, the chicks or eggs are recorded and an assessment of the chicks’ age is determined by the feather size, with chicks recorded as either in pin (IP), feathers small (FS), feathers medium (FM), or feathers large (FL). If the chicks are FS or FM, ringing can be safely undertaken by the team before the chicks are returned to the nest. Where possible, nests are recorded until fledging.
Results
Capturing unringed birds
In 2024, the Wheatear RAS team arrived on the 18th May and left on the 8th June. Using spring traps, a total of 61 newly colour-ringed birds were captured between Castle Hill and Halfway Wall on the west coast during this RAS study period. This included 24 breeding males and 35 breeding females. Two of the birds were captured with metal rings only. These birds had not lost any colour rings but were nestlings that had been ringed on the island in 2023 showing some great site fidelity.
Resighting colour ringed birds
Throughout the same period that the catching was being undertaken, a total of 63 adults that had been colour-ringed in previous years were resighted during the RAS period. These included 35 males and 28 females. Through looking at the life histories of the birds recorded, we know that the oldest of these birds were three females, ringed in 2016, 2019, and 2021. As they were already adult when originally ringed, this makes them at least 9yrs, 6yrs and 5yrs old respectively. It is amazing to think that the 2016 female had crossed the Sahara at least 18 times during her life so far. Additionally, with 63 ringed females, and at least six in the study area that were not able to be caught for colour ringing, there were an estimated 69 pairs in the RAS study site in 2024. This is the highest number of pairs ever recorded in the RAS study site. Through the resighting, we know the adult survival between 2023 and 2024 was 55% with the average annual survival since 2013 being 56%.
Nest recording
Through monitoring the pairs, a total of 5 broods, totalling 20 chicks, were found within accessible nests and metal-only ringed when they were of appropriate sizes. This is a small proportion of the total number of nests that were found in 2024, but compared to the previous years, this is a record for the RAS study. The inaccessible nests were in narrow rock crevices or deep burrows where the chicks could not be reached.
Conclusion
Figure 2: A photo taken in the 2024 Wheatear RAS study period. A colour ringed adult female Wheatear with food showing the Lundy banded colour ring but hiding the other colour rings. © Thomas Weston.
The results from the Wheatear RAS study site during 2024 were positive. There was a high number of colour ringed birds added to the population, and the recapture of the metal ringed birds originally ringed last year was a positive sign that Lundy born birds are returning to the island to breed. The resighting data was invaluable to the study so a big thank you to the team as well as visiting members of the public who also saw colour ringed birds on their walks around the island. The survival estimates for the Wheatears this year were similar to the average and demonstrates no extreme decreases in survival on the wintering grounds which is positive. We hope this continues, with good numbers of breeding adults returning next year. The nest recording resulted in the highest number of accessible nests accessed and ringed this year, and we hope to see these birds return next year when they can be colour marked as breeding adults.
How you can help!
Figure 3: A colour ringed adult male Wheatear. © Thomas Weston.
If you’re visiting Lundy and see a colour ringed Wheatear, please do let us know at our daily bird logs and by emailing the warden too. Thank you to everyone who has sent in a photo of a colour ringed bird, we really appreciated them, and every record is amazing! This year, we even had a couple of resightings of Lundy Wheatears on Portland in Dorset, photographed by visitors to Portland Bird Observatory so a big thank you for sending off-island records too.