26 April 2013

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.

22 April 2013

Tough times for sea birds

As you heard in a previous post, we have been receiving large numbers of reports of dead or dying Barn Owls. What we didn't mention is that we are also receiving reports of large numbers of dead seabirds on the east coast of Scotland and northern England but also as far south as East Anglia.

The majority of these birds have been found in the Firth of Forth, Scotland, and this seabird wreck has occurred at a time when these birds should be getting into prime condition for the breeding season, or even nesting now. High winds and 'uncomfortable' sea conditions are thought to be the prime cause, making finding food difficult.

Colour ringed Shag - Sarah Featherstone

The majority of reports of ringed birds have been Shag but have also included a wide variety of species, including Puffin, Razorbill, Guillemot, Kittiwake, Cormorant and even Little Auk. The graphs below show the 10 year average of reports (blue) against the number of reports so far this year.

You can clearly see the huge difference between early and late March. The number in early April is slightly less than late March and early indications show that late April will have many fewer reports of dead shags, as long as the trend continues.

 We receive very few recoveries of dead Puffins generally during the winter period but this year in late March we can see evidence of the wreck.

Strangely, Guillemots have fared very well, considering the previous two species, with a lower than average report rate. Although there have been large numbers of Guillemots washed up in Cornwall and Devon from a synthetic chemical called polyisobutene. Surprisingly only nine dead and two live Guillemots (which are yet to show in the graph above) have been found ringed out of a minimum estimate of 1500 dead birds.

16 April 2013

What a whopper!

A Blackbird weighing 125g is not particularly unusual... in winter. Catching three females at this weight in the second week in April is unusual.

My husband Jeff and I have been studying the Blackbirds in our Thetford garden since we moved here in 1996. Our local breeders are joined by their continental cousins in mid-November each year and they co-exist.  In the mid-winter, particularly when there is snow, their weights peak as they put on fat to survive the cold. In January of this year we caught 52 Blackbirds, seven were over 125g
in weight, indeed one weighed a massive 150g. As Spring approaches, so weights drop.  For the local breeders weights continue to fall – typically a breeding Blackbird will be under 100g with some getting below 90g. For the Continental birds it is a different story – having dropped to close to 100g they will start to increase again, this time putting on fat to fuel their migration. We have had two Norwegian ringed birds in our garden giving a good indication of where many of these birds are off to. Typically we think birds depart our garden at weights of 110g to 120g.

Male Blackbird - Allison Kew

Like most people we look forward to the Spring.  For us one of the features of Spring is the departure of the Continental Blackbirds, allowing us to focus on colour-ringing and re-sighting our breeding birds for our RAS project. We know when the Continentals have gone as the Blackbirds behave differently and we stop catching heavy birds. The date varies from year to year but is normally in the last two weeks in March.

Female Blackbird - Allison Kew

This year we’re still waiting – departure for many of our Blackbirds is at least a couple of weeks later than ever before, the strong, cold, easterly winds not providing good migration conditions.  Over the weekend of 6 and 7 April we had more birds over 117g than in all of the other Aprils combined and one was still here on the 13 April.  Indeed we’ve never had females at 125g in April before – this year there have been three.  All signs of delayed migration. We’re expecting that, with the more favourable weather conditions, all the Continentals will have gone by the coming weekend.

Meanwhile, our local breeders are also slow to get going - by now we’d expect most females to be feeding young and have some doing that, but nowhere near as many as normal.
Just goes to show how useful it is to collect measurements of birds. Without recording wing-length and weight we’d not understand what the Blackbirds were up to as migrants and residents can’t be distinguished by eye. 

Thanks particularly to Jo Lashwood who has done a lot of the recent hard work on this project and actually caught the 150g Blackbird.

Allison Kew
CES & RAS Organiser

10 April 2013

Dutch-ringed Yellow-browed Warbler

Way back in April 2008 we received details of a 'Willow Warbler' found dead outside Richard Lander School in Truro, Cornwall. This is perhaps a slightly early date for a returning migrant, but the interesting bit was that it was wearing a ring from the Dutch Ringing Scheme.

Following issues tracing the ring in The Netherlands, we have only just received the ringing details and these were rather surprising. Ring Y11467 was actually just the second foreign-ringed Yellow-browed Warbler to be found in the UK! It had been originally ringed on 3rd October 2007 on Schiermonnikoog (in red on the map below), an island off the north coast of the country, and had possibly spent the winter at one of Cornwall's many sewage works.

View Yellow-browed Warblers in a larger map

These works regularly attract wintering Yellow-broweds, especially so in recent years, with no fewer than five seen on one day at Carnon Downs Sewage Works in February 2012. Below is one of two birds present at Gwennap Sewage Works, Cornwall, in January 2013.

The only previous record of a foreign-ringed Yellow-browed was a Norwegian bird ringed in September 1990 and recaught on Fair Isle five days later (blue on the map). The only British-ringed bird to be found abroad was one ringed at Portland Bird Observatory in October 1988 and recaught the next day on Guernsey (green on the map).

09 April 2013

Cold spring hits Barn Owls

The past couple of winters have seen some severe cold weather, making life difficult for many bird species. The Barn Owl is one species that can be heavily affected, and we have seen reporting rates of dead ringed birds rise. This usually happens earlier in the year but this year the reporting rate for late March has shown an exceptional increase, just when these birds are supposed to be starting to breed!

Barn Owl - Les Foster

At this time of year we normally receive reports of birds that have been hit by vehicles on the roads but this year a high proportion have been found emaciated close to peoples houses and allotments. This is a sure sign that things are not well in the countryside and food resources are very low.

The graph below shows just how extraordinary late March 2013 has been. The figure is sure to rise for early April this year but we hope it doesn't compare to late March.

Deep snow, wind and flooding has hindered the ability for Barn Owls to find food. This will also be having an effect on other birds of prey that feed on rodents and small mammals.