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The Problems and Rewards of Radio-Tagging Nestling Thrushes - The Design and Testing of Harnesses That Accommodate Growth

Ian F. Hill, Institute of Terrestrial Ecology, Furzebrook Research Station, Furzebrook Road, Wareham, Dorset BH20 5AS United Kingdom, Brian Cresswell, Biotrack, 52 Furzebrook Road, Wareham, Dorset BH20 5AX United Kingdom, and Robert E. Kenward, Institute of Terrestrial Ecology, Furzebrook Research Station, Furzebrook Road, Wareham, Dorset BH20 5AS United Kingdom

The young of many passerines leave the nest before they are fully-grown. They must be tagged several days before fledging to prevent explosion from the nest. This has caused such problems when fitting radio tags to nestlings that it is rarely attempted. We were forced to broach the problem for research into post-nestling dispersal and mortality in European blackbirds Turdus merula and song thrush T. philomelos. The objective of this study was to identify the possible role of post-nestling behavior in the long term decline in the British song thrush population.

We established that a backpack design was the only practical method of attachment for tags with a life of 100 days (weight 2.2 g). Five key criteria were considered fundamental to effective harness design: (i) accommodating growth by incorporating elastication, (ii) preventing entanglement by strictly limiting the amount of stretch available, (iii) ease of fitting - with a "one-size-fits-all" specification, (iv) comfort and the need to spread the contact area on the leading edge of the patagium, and (v) predictable detachment through the establishment of a weak link.

It became clear that structural simplicity was crucial to the success of any harness design adopted. The use of joints to link materials tends to add weight and bulk to the harness and their behavior may be difficult to forecast in the field. As such, "maverick joints" must be eliminated and the number of harness elements minimized.

A range of materials and designs were initially tested on birds brought into captivity in a controlled environment.  Subsequently, over 150 nestling Turdus spp. have been successfully tagged and monitored at a field site in Dorset, UK. Our solution uses controlled elastication in a wing-loop harness weighing approximately 0.1 g. It will be discussed primarily with regard to thrushes but should be equally applicable to many other passerines.

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