Swifts are declining, and one of the reasons could be something happening on their migration or in winter quarters on top of the loss of nest-sites. The advent of light-sensitive geolocators has provided great potential to fill in the gaps of where they go, and where they linger when they leave our shores.
There is bound to be apprehension at fitting anything to a small bird, particularly a Swift with its extreme fitness requirements, however, return rates of birds in previous years are no worse than one would expect from normal mortality, especially for birds caught leaving the nest, rather than birds caught on the nest.
On 21st July, one Swift was caught leaving my camera box, and fitted with a geolocator by Chris Hewson and Phil Atkinson from the BTO. That night, and since, the bird returned to the nest-box. Here is some video:
I was impressed with the care taken that the fitted harness was perfect. The Swift seemed to be unaware of the geolocator on its back, and, as far as I can see it behaves normally. Not once have I seen it, or its partner or offspring pay any attention to the geolocator, even though they mutually preen vigorously and regularly. Further, the geolocator does not look as if it would have any negative aerodynamic effect, as has also been demonstrated with models in a wind-tunnel.