RAS: Renewing Acquaintances in Spring

For almost 20 years, the Retrapping Adults for Survival (RAS) scheme has used standardised bird ringing as a tool to monitor adult survival rates of species not frequently caught at Constant Effort Sites. The results are used to generate annual survival estimates which help us to understand more about the contribution changes in the probability of mortality make to population trends recorded by surveys such as the BTO/JNCC/RSPB Breeding Bird Survey (BBS). Birds are generally faithful to breeding sites between years, so RAS methodology aims to re-encounter as high a proportion of returning adults as possible each year; for some species, this task can be made significantly easier by fitting colour-marks, allowing birds to be individually identified without capture. 

The initial uptake for RAS was fantastic, with 75 datasets received in 1998. Since then, the number of projects has risen steadily and in 2015, a tremendous 190 datasets were received. We now have over 200 active projects studying 59 different species. 60% of projects focus on one of the 24 target species, as outlined in the Demographic Targeting Strategy, with a further 11% of projects targeting seabirds (which don’t yet feature in the target species list).

The most frequently studied species are still House Sparrow and Pied Flycatcher, which are the focus of 23 projects each. In third place is Sand Martin, which is studied by 15 RAS ringers, often at artificial banks such as the one at Rutland Water which enable breeding success to be monitored concurrently. Following closely behind is Starling (14 projects), a species that has become increasingly popular in recent years. Prior to 2013, there were only two RAS projects on this red-listed species so the additional data now being produced are very welcome! Not quite making double figures are Dipper and Reed Warbler, which are the species of choice for nine RAS ringers each. Perhaps surprisingly, there are still fewer RAS projects than we might expect on some generally well-ringed species, such as Swallow and Tree Sparrow (six projects each) – we would love to hear from anyone interested in taking up the challenge of a RAS on these species.

This colour-ringed Starling is part of a RAS population in Lancashire. Photo by Peter Alker.

The fruits of RAS ringers’ labours have just been published. The full suite of national RAS results for 2015 is now available and includes a trend for Tawny Owl, which we have been able to produce for the first time following the submission of some valuable historical data. RAS works particularly well for longer-lived species, such as owls and seabirds. A number of ringers with existing, long-term ringing projects have recently registered for RAS, instantly enabling us to produce survival trends for their studies.

A trend for Tawny Owl is available for the first time. Photo by Ruth Walker.

 

RAS survival trends for 12 species (Little Owl, Jackdaw, Sand Martin, Swallow, House Martin, Dipper, Pied Flycatcher, Stonechat, Wheatear, House Sparrow, Linnet and Siskin) are also included in the annually produced BirdTrends report, which provides a range of information about population trends and their potential drivers for over 100 breeding bird species.

We are very grateful to all our fabulous RAS ringers who put so much time and effort into generating this incredibly valuable data. Anybody considering starting a RAS or wondering whether a current project could be suitable for RAS is encouraged to contact the RAS organiser.

Wintering Sand Martins benefit from fewer conspecifics

The theme of this Blog (Demography - "The study of the characteristics of populations, such as size, growth, density, distribution, and vital statistics") certainly applies to David Norman’s long-running study of Sand Martins breeding in mid-Cheshire. In more than 200 mist-netting sessions over 23 years (1981-2003), he ringed 6,053 adult Sand Martins and recaptured 1,179 of them in a subsequent year, many of them multiple times for a total of over 10,000 handlings of adult birds.

Mist netting - David Norman

This long run of data with large sample sizes, and annual assessments of the breeding population from counting nest-holes, allowed a detailed analysis, jointly with Will Peach, which has recently been published in Ibis. The details are in the paper (click here for a copy or e-mail david@davidnorman.org.uk) but in outline the work shows …

1. The annual survival of adult Sand Martins averages around 35%, varying from as low as 10% to a high over 60%, mostly determined by the rainfall in their Sahel wintering quarters. There are fewer insects in the dry years and more birds starve. This is already well-known for several trans-Saharan migrants but this study also showed that the effect is non-linear: above a certain level of African rainfall the birds’ survival flattens off, limited by mortality elsewhere in the life cycle.

2. In this study, there was no effect of summer weather (temperature or rainfall) on adult survival.

3. The size of the breeding population is mostly determined by the survival and return of adults, and much less by recruitment of new birds (one-year-old first-time breeders and immigrants from elsewhere).

Sand Martin - Lawrence G Baxter

 4.  This is the first study to show that overwinter survival in the Sahelian winter quarters is density-dependent. Thus, if the population is high, there is more competition for insect food and more Sand Martins die; if the population is low, even in a dry year there is more food to go round and more martins survive and return to the breeding grounds. 'Population' here means the winter population of all western European Sand Martins, which mix in the Sahel during winter.

5. The recruitment rate of first-time breeding adults was also density-dependent. More un ringed ‘new’ adults were captured during summers when local colony size in Cheshire was relatively small, and vice versa. Competition amongst breeding pairs for nesting sites or food might have caused this pattern.

Sand Martin - John Harding

David and Will comment that such density dependence was suspected from previous studies on other species but required a vast amount of fieldwork and statistical analysis to prove it. Such competition for insect prey may apply to other insectivorous migrants that rely on the seasonal flooding of wetlands across the Sahel zone. Better rains in recent years have allowed breeding populations of several Sahel-dependent species to increase in recent years, but the threat of drought continues to hang over the people and the wildlife of the region.

Thanks to David Norman for letting us know.

Sand Martin Shenanigans

Richard du Feu is currently taking part in a Retrapping Adults for Survival (RAS) study on Sand Martins at Whittington, Lancashire. Little did he expect that one of the breeding males L334091, ringed at the site on 08/07/2010, would be later re-caught by ringers, whilst wintering in Africa, at Parc National du Djoudj, Senegal on 13/12/2010 (4363km in 158 days).

Amazingly he retrapped this same bird, on Tuesday night, at the original breeding site, just 15 metres from where he ringed it last year.


Richard also had two French ringed Sand Martins so far this year and two British controls with several birds moving up to 15km along the river lune. Higher than average numbers of adults have returned to many colonies this summer so the prospects for a productive breeding season are looking good.

Thanks to Richard for letting us know and to Jez Blackburn for the photo.