Radio Transmitters No Problem for Hermit Thrushes

January 1, 2008 by Gregory Gough

Scientists often want to track birds to find out exactly where they go. But this is usually difficult, since birds are rather mobile and often fly away or hide in dense vegetation when scientists get too close.

Since they can't track them on foot, scientists can attach radio-transmitters to birds so they monitor the bird's movements 24/7 without unduly disturbing them. Although modern transmitters are amazingly small and light (some weighing as little as 0.35 grams), some worry that the cumulative effect of carrying the extra weight might hurt the birds.

Given the body size of the birds involved, the weight of the transmitter is about the same as a 200-pound man carrying a 5-pound backpack.

To see what the effect of carrying a radio-transmitter might be, Smithsonian Migratory Bird Center scientists studied hermit thrushes (Catharus guttatus) in South Carolina in the winter of 2006/2007.

In the pictures below, you can see a radio transmitter on the back of a hermit thrush (left) and the antenna sticking out beyond the tail (right).

bird with transmitter
bird with antenna

To measure the birds stress levels, scientists drew blood both from birds with transmitters and from those without for a period of 1 month. By comparing the heterphil to lymphocyte ratios, the scientists determined that birds with transmitters were no more stressed than birds without.

This article summarizes the information in this scientific paper:

Davis, A. K., Diggs, N. E., Cooper, R. J. and Marra, Peter P. 2008. Hematological Stress Indices Reveal no Effect of Radio-Transmitters on Wintering Hermit Thrushes. Journal of Field Ornithology, 79(3): 293-297.

Download scientific paper