To track small birds you need to catch them; twice…
LINKED PAPER
Cyprus Wheatears Oenanthe cypriaca likely reach sub-Saharan African wintering grounds in a single migratory flight. Xenophontos, M., Blackburn, E. & Cresswell, W. 2017. Journal of Avian Biology. DOI: 10.1111/jav.01119 VIEW
The success of any geolocator study rests on the ability to recatch the tagged bird. If you can’t get your tag back with all its location data for you to download, then there are no data. And to do that you need to recatch the same individual you tagged, one year or so later. That’s obvious, yet the fascination with the whole process of tagging seems to revolve around the tag design, its weight, its attachment to the bird, its technical capacity and so on, yet the real craft and skill of the study – catching and re-catching the same bird – is often neglected.
Figure 1 Pink white pink white. An adult female Cyprus Wheatear we first caught in May 2014 in a spring trap; we fitted a geolocator and she returned the next year. But we couldn’t catch her again that year and her nest failed before we found it. She returned the next year in 2016, but again we failed to catch her. At last in May 2017 we recaught her – very easily – with a single mist net in front of her nest hole among some rocks. But the geolocator had been lost! A good thing that they do drop off if a bird is not recaptured (the elastic on the harnesses will perish eventually) but a shame that we didn’t eventually recover the geolocator data. We fitted her with a new geolocator so hopefully next year we will finally find out about her migration and wintering strategy © Will Cresswell
We have been tagging Cyprus Wheatears, Oenanthe cypriaca, in the Troodos Mountains of Cyprus: a pilot study in 2014-15 to get proof of principle and now a much larger intensive study involving hundreds of geolocators fitted to adult and juvenile birds, with our primary aim to look at the development of migration with age. So we need to catch and recatch hundreds of birds. And when we track a juvenile again as an adult, then we will need to catch that individual at least three separate times. It’s a daunting task, particularly because the limiting factor in the pilot study was our ability to recatch the returning birds, not their survival or lack of return.
Figure 2 Pink black pink black just after being recaptured by its nest in May 2015 and just prior to removing its geolocator, fitted the previous spring. This bird wintered in a single site in northern Ethiopia, which it reached in a single direct flight from Cyprus over 2,600 km away. It was still alive in May 2017 returning to exactly the same breeding territory as all years. When we find this year’s nest we will catch it again to put another geolocator on it to see whether the same applies to the wintering territory
© Will Cresswell
Of 24 birds tagged, 14 returned (58%) – a great survival rate for a small trans-Saharan migrant and more or less exactly the same as our control group of 81 colour-ringed only birds. But then we were only able to recatch 6 of them. A success in terms of finding out the wintering range of the species (South Sudan to Eritrea), their winter movements (or not) and their ability to get from Cyprus to their wintering ground in a single flight. But a failure in terms of a sample size of 6 only – when there were another 8 to be had, if we could only recapture them. This failure has focused us on the real challenge of geolocator studies – the recapture. And this is, of course, old school ornithology: the art of trapping.
Figure 3 Pink orange pink orange. A male tagged in May 2014 photographed in May 2016, in its second glorious year of not being recaptured. It would perch high above our playback and mist nets but we were never close to catching it again. Note the little tuft of grey on the back to show that it still had its geolocator © Will Cresswell
We catch Cyprus Wheatears predominantly with spring traps. It’s a very effective method. Playback a male singing, bait your trap with a maggot, place in a territory before nest building gets started and expect to catch the male within a few minutes and the female a few minutes later. But the problem is that once caught in a spring trap, they will never take the bait again.
Figure 4 A male Cyprus Wheatear with a newly fitted geolocator tag. The back feathers are soon preened over the tag apart from the light stalk which just protrudes above the feathers. The tags weigh < 4% of the bird’s mass and we have not been able to detect any negative effects on survival of fitting them to date © Will Cresswell
We have now spring trapped hundreds of Cyprus Wheatears over the 8 years of the study and we have only ever recaught two individuals in a spring trap. Some individuals in the study were trapped over 5 years ago now and they still remember how they were caught by us. A previously caught and now colour-ringed individual will happily come to look at a trap, approaching close to it, but it never goes for the maggot in the trap again (although it will happily take a free-ranging maggot). It’s admirable but so frustrating, and means we need another method to recatch the tagged bird. The obvious alternative is to mist-net, but in an often windy, montane environment covered with Berberis cretica (imagine Velcro for mistnets that will attach permanently with just one touch…), this is very difficult. Targeted mist-netting for birds visiting the nest has been our most successful approach so far, but then we need an even more basic ornithological skill – nest finding.
Figure 5 Tricks to catch a Cyprus Wheatear. It helps that males early on in the season cannot resist playback of another male’s song in their territory. After eventually disregarding the wheatear equivalent of R2D2, this male then turned its attention to the nearby maggot in a spring trap and joined our colour-ringed population © Will Cresswell
Hindsight is a wonderful thing when you do have the opportunity to continue a study. We are now catching our Cyprus Wheatears first with mist nets – we are less worried about who we catch then so we can site our nets to suit us. Then, the following year we have the easy option to recatch with spring traps – our best and most reliable method saved for the very high value returning birds with geolocators – and if we need to repeat tag a bird, we have the final fall-back option of catching the bird at the nest in the following year (very high effort but we will only have a small number of these very, high value birds).
Figure 6 Red yellow black white. A female Cyprus Wheatear colour-ringed in 2012 © Will Cresswell
So, tagging small birds sounds high-tech but actually its success is entirely to do with fundamental ornithological skills.
References
Xenophontos, M., Blackburn, E. & Cresswell, W. 2017. Cyprus Wheatears Oenanthe cypriaca likely reach sub-Saharan African wintering grounds in a single migratory flight. Journal of Avian Biology 48: 529-535. VIEW
Image credit
Featured image: Black white orange white – a male Cyprus Wheatear Oenanthe cypriaca © Will Cresswell
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