PIERSMA T, MACCURDY R.B, GABRIELSON R.M. CLUDERAY J, DEKINGA A, SPAULDINGE.L, OUDMAN T, ONRUST J, GILS J.A. VAN, WINKLER D.W & BIJLEVELD A.I (2014) Fine-scale measurements of individual movements within bird flocks: the principles and three applications of TOA tracking. LIMOSA 87 (2): 156-167.

We review the design principles of the 'Time Of Arrival' (TOA) tracking system, developed at the Laboratory of Ornithology at Cornell University, as well as the joint attempts by NIOZ and Cornell University to turn this proven idea into a field-ready system for simultaneously tracking large numbers of individual shorebirds. Instead of conventional GPS, in which positions are determined by the time a digital signal takes to travel between satellites and a mobile receiver, TOA determines the position of a mobile transmitter tag by the time its digital signal takes to travel to a fixed land-based network of receiver stations. The system used here included nine receiver stations and a current maximum of 48 transmitters, each emitting a digital signal every second. If in range of at least three receiver stations, each tag transmission can be used to compute a position estimate (Fig. 1). Currently, only the TOA system permits collecting positioning data with high spatial and temporal resolution using many small (ca. 4-6 g) and affordable tags, with the majority of the financial investment being in the receiving stations.
      The system was employed (1) on Red Knots Calidris canutus islandica on intertidal mudflats in the western Dutch Wadden Sea in August-September 2011, (2) on Eurasian Golden Plovers Pluvialis apricaria in agricultural grasslands in Friesland in September-October 2012, and (3) on Red Knots C. c. canutus on intertidal mudflats at the Banc d'Arguin, Mauritania in January-March 2013. We were able to show that Red Knots in the Wadden Sea focussed their foraging on large banks of Edible Cockles Cerastoderma edule. Interestingly, knots avoided those areas with highest prey densities, probably because at these densities intraspecific competition between individual prey reduced their quality (Fig. 4). On the Banc d'Arguin, Red Knot movement routines showed large variation between individuals, with different individuals restricting their space-use to specific areas across the tidal cycle (Fig. 6). Routines of the Golden Plovers were structured by day and night. In daytime, the plovers flocked together and to some degree moved around within the study area, but at dusk they transitioned into mobile flocks leading to highly dispersive and individually distinctive movements, partially outside the study area. The nights were spent with limited movement on particular meadows, that were left at dawn (Fig. 5). The impetus to make the TOA system work was our wish to achieve a greater understanding of the ways in which individuals within flocks associate with and learn from each other. The unmasking of these stories awaits the further processing of millions of arrival times and individual position fixes. Nevertheless, we can conclude that at this stage of the progress in tracking technology, we could not have made the present advances in our biological understanding of shorebirds without the TOA system. Whether the current TOA prototype will ever see a more general or even commercial use remains to be seen.

The dusk ballet of Golden Plovers


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