A visit to Portscatho isn’t complete without a quick check of the birds using the beach and rocks. A range of waders can often be seen and in recent weeks some fairly scarce visitors to the site, such as grey plover and purple sandpiper, have been present. The most numerous wader though, is usually the turnstone and this species may well be the most charismatic. With patience they often allow close views, and whilst watching them feed busily just a few metres away it’s amazing to think that in the spring they leave the area and travel vast distances to remote breeding grounds in high Arctic tundra.
Much of our knowledge of wader migration has come from bird ringing. A metal ring bearing a unique number will identify the individual bird for the rest of its life. In order to gain information of the migration of a ringed bird, it would normally have to be re-trapped at another location so the ring can be re-read. An alternative method that has proved to be highly successful with waders is attaching a combination of coloured rings or ‘flags’ to the bird’s legs. The individual can then be re-sighted by anyone at any point on its migration route without the need to re-trap it. The usefulness of this method is demonstrated by the example of the black-tailed godwit. Before a colour-ringing programme was introduced on the species, only 2.5% of ringed birds were ever recovered, but following the scheme more than 80% of marked birds were subsequently re-sighted. This method still has limitations, with a major drawback being that re-sightings will obviously be limited to areas where there are observers. As migratory wader species are likely to spend a lot of time in remote areas (when breeding, wintering or on passage), many important locations they use will escape detection.
A real breakthrough in this aspect of migration studies has occurred as a result of the use of satellite transmitters. These give regular readings of the bird’s location as it moves, providing incredibly useful information on migration routes (in both spring and autumn), the location and length of stays at stopover sites as well as the location of both breeding and wintering areas. The transmitters are usually attached to the bird with a harness (which will biodegrade and fall off over time), rather like a small backpack. One current limitation with this method is that the size and weight of the transmitter restricts its use to larger-bodied birds. However, this will change in future, as more miniature versions of the technology develop. Even in the twenty years or so that these transmitters have been used on birds, their dimensions have decreased to such a degree that the lightest models now weigh around 5 grams and can be used on relatively small species such as hobby and bar-tailed godwit.
Even so, this technology is still too large and heavy for a species the size of turnstone, but a sensor known as a geolocator can provide similar information. Geolocators store information on light levels at set times, allowing estimates of sunrise and sunset to be made, which can be converted into latitude and longitude readings, enabling the geographical position of the bird to be calculated. The main drawback is that in order to retrieve the information from the geolocater, the bird must be recaptured. However, as the technology develops it is likely that the data could be accessed remotely in future (and the tags will become lighter and smaller).
Geolocators were recently used on turnstones wintering in south-east Australia and provided fascinating information on their migration routes and timings to and from their breeding grounds in eastern Siberia. Six birds were fitted with geolocators (attached to leg flags rather than backpack harnesses to enable the birds to gain weight more freely in the build up to their migration) and the sensors (weighing just 1 gram) were subsequently retrieved from four birds in the following winter. All four turnstones were found to start their migration with a massive non-stop northward flight of 7,600 km to Taiwan. Understandably, they then spent time feeding up, both in this area and in parts of northern China, before flying a further 5,000 km to their Siberian breeding grounds. In the autumn, returning birds were found to take varying routes, mostly through Asia, although one individual took a completely different and unexpected flight path across the Pacific Ocean. This involved a non-stop flight southwards from the Aleutian Islands covering 7,800 km to Kiribati, before a further 5,000km flight to Australia. In the following winter, a geolocator was used on the same turnstone again and it took a similarly spectacular route across the Pacific as it returned from its breeding grounds (though this time it stopped off at a different set of remote islands to refuel).
The turnstones wintering in south-west Britain have not yet been subject to such a detailed study, but bird ringing has provided information on their breeding areas and stopover sites. This suggests that most turnstones, especially those wintering in western Britain, fly to Iceland in the spring where they stay and feed up before crossing southern Greenland to reach their breeding grounds in northern Greenland and north-east Canada. It may be that some birds wintering in south-west Britain first move through passage sites in the north of the country before crossing to Iceland and it’s even possible that some individuals could skip that stage altogether, making the direct flight of around 4,000 km straight to their breeding sites.
Interestingly, it has been found that turnstones that spend the winter together will not necessarily breed at the same site. Birds breeding on Ellesmere Island in Canada were found to have wintered in France, Portugal, Wales and The Netherlands. Wherever their breeding sites may be, it has been shown through colour-ringing studies that turnstones tend to be highly faithful to their wintering sites. Individuals will typically occupy the same areas of shoreline in flocks of stable membership for their whole life (about nine years on average). Fingers crossed that they will continue to be a common sight on the Portscatho shoreline for winters to come, and hopefully, technological developments will reveal further details about their amazing migrations.