A lone Sand Martin twisting and turning over the lagoons on a misty morning in mid-March is often the first sign of summer at Rutland Water Nature Reserve. These diminutive birds winter in the same part of West Africa as the Rutland Ospreys and return at the same time too. Their arrival doesn’t make the same sort of headlines, but like the Ospreys, we have been able to gain a fascinating insight into their breeding behaviour in recent years at Rutland Water.
Sand Martins have always been seen at Rutland Water. The reservoir provides rich foraging grounds, particularly in early spring when insects are often hard to come by. It wasn’t until the early 2000’s though, that they became established as a breeding species at the nature reserve. Sand martins nest in exposed banks – often in quarries and river banks – by excavating a burrow. The ephemeral nature of these nest sites means that the birds often have to move around each year, and that was the case at Rutland Water. A few breeding pairs cropped up here and there, but there was no suitable habitat for a large colony to become established.
In 1999 we did something to try and change that. A large breeze block structure was built complete with 347 individual nesting chambers. Clay drainage pipes were used to provide entrance holes into each chamber; each of which was created by turning a hollow breeze block onto its side. Wooden ‘cat flaps’ at the rear of each chamber then allowed easy access into each nest site for monitoring. The artificial bank became an instant hit and now supports more than 200 breeding pairs, which, between them, raise more than 1000 chicks each year. What’s more, we are able to ring every youngster, enabling us to study site fidelity and movements of Rutland Water birds. Two years ago, thanks to funding from the Caterpillar Foundation, we built a second bank on lagoon 5 and that already supports more than 50 breeding pairs.
After several years of ad-hoc monitoring, we decided on a more coordinated approach to nest recording. Over the past three summers members of Rutland Water Ringing Group, led by Lloyd Park and Colin Hewitt, have used the standardized British Trust for Ornithology Nest Record Scheme (NRS) technique, which involves checking every nest once per week. Each nest check takes less than 10 seconds and enables us to keep track of which nests are occupied and whether they have eggs or chicks. This means we know exactly when chicks are ready to be ringed; usually when the youngsters are ages between 7 and 14 days of age . Full broods of chicks – anything from 1 to 6 chicks – are removed from the nests and the hole temporarily blocked up to ensure that the adults do not return to find an empty nest. Ringing takes a matter of minutes and then the full brood is returned to the nest and the blocker removed. This improved methodology meant that in 2014, 1027 chicks were ringed prior to them leaving the nest: a record.
In 2014 we installed a small high definition camera in the bank that was designed to be able to be moved between holes. This enabled us to follow the fortunes of five different sand martin families during the course of the summer.
So what have we learnt? There are three rows of holes on the bank and the top row always fills first each spring. The early occupancy means that three broods are often reared in these nests; although we cannot be certain that they are by the same adult birds. The Birds of the Western Palearctic states that Sand Martins only rear two broods each year; but our results appear to indicate otherwise. The colonial nature of the birds also means that it is not unusual for fledged juveniles to sit with a brood of much smaller chicks. For instance one night the camera revealed a nest of two adult birds, six large flightless chicks and two full-grown juveniles. Quite a nest full! It is also interesting how the size of chicks varies with a brood. We have recorded some chicks as heavy as 21g (14g is the norm) and the weights of chicks within a brood can vary by as much as 8g – more than half an adult Sand Martin’s body weight.
With such a high density of nests, the bank is always likely to be targeted by predators. Metal skirting fitted around the base of the bank usually provides sufficient protection, but on two occasions the defence has been breached. In 2014 thirty-two nests, containing 60 chicks, 64 eggs and 17 adults were lost to rats and in 2015 a female stoat was found breeding in the bank early in the season. 17 rings were recovered from her nest, suggesting that she had taken a large number of adult birds. Like the rats, the stoat was caught on camera, enabling us to stop any further damage. The stoat was captured and she and her five young, relocated.
Our results show that the majority of Sand Martins breeding in the two artificial banks are Rutland Water birds. Aside from the nest recording, each year we also catch adult birds that are breeding at the bank as part of the BTO’s Re-trapping Adults for Survival (RAS) project. This involves setting nets before it is light and then catching the adult birds as they leave their nest holes at dawn. This year we caught a total of 193 birds at the two sites over two early mornings in mid-June. 169 of the birds we caught (88%) were individuals that had been caught and ringed at Rutland Water, while the remaining 24 were unringed. This demonstrates how amazingly site faithful the birds are; despite crossing the Sahara twice a year most return to the same nest site each spring.
As you might expect we have had numerous recoveries of Sand Martins ringed at Rutland Water. Many are caught on the south coast each year, but we have also had recoveries from France and Spain. Most excitingly of all though, was a bird that was re-trapped in Djoudj National Park in northern Senegal. It is amazing to think that these tiny birds make such a remarkable journey twice a year. In 2014 one of the rat’s victims turned out to be six years old. It had been ringed as a nestling in the bank in June 2008, recaptured as a breeding adult four years later and then caught on migration in Spain in April 2013. Assuming that it wintered in Senegal this one individual, weighing just 14g, had made 12 flights across the Sahara, flying more than 36,000 miles in the process. They may not be as celebrated as the Ospreys, but in many ways, these diminutive Sand Martins are even more amazing.