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).
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 publication:
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.
Radio-telemetry is often used to track birds, and several investigators have examined the possible effects of radio-transmitters on birds. One approach to this question is to determine if transmitters induce physiological stress. Using hematological indicators of stress (heterophil-lymphocyte [H/L] ratios), studies of captive birds have revealed no evidence that radio-transmitters cause stress. However, studies in captivity may not reflect conditions faced by birds in the wild, especially during energetically demanding times, such as the overwintering period.We examined the possible effect of backpack-style transmitters on overwinteringHermit Thrushes (Catharus guttatus) in South Carolina by examining H/L ratios fromblood smears made before and after transmitter attachment. We captured and recaptured eight thrushes in an early winter period and 11 in a late winter period, and in both cases found that H/L ratios did not change after 1 mo of carrying transmitters. H/L ratios also did not differ significantly (in either trial) between thrushes with and without transmitters. Furthermore, thrushes that carried transmitters during both of our trials did not have higher final H/L ratios than those that carried transmitters during just one trial (the late winter). These results indicate that carrying backpack-style transmitters for 1 mo did not induce stress in overwintering Hermit Thrushes. These results are consistent with studies of captive birds, and provide support for the continued use of backpack radio-transmitters in this species, as long as weight recommendations are met.
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