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Hematological Stress Indices Reveal No Effect of Radio-Transmitters on Wintering Hermit Thrushes

Andrew K. Davis, Nora E. Diggs, Robert J. Cooper and Peter P. Marra
Journal of Field Ornithology
Vol. 79, No. 3 (Sep., 2008), pp. 293-297
Published by: Wiley on behalf of Association of Field Ornithologists
Stable URL: http://www.jstor.org/stable/27715276
Page Count: 5
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Hematological Stress Indices Reveal No Effect of Radio-Transmitters on Wintering Hermit Thrushes
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Abstract

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 overwintering Hermit Thrushes (Catharus guttatus) in South Carolina by examining H/L ratios from blood 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 I 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. /// La radio telemetría es comúnmente usada para el seguimiento de las aves y algunos investigadores han examinado los posibles efectos de los radiotransmisores sobre las aves. Una manera de investigar esta pregunta es determinar si los transmisores causan estrés fisiológico. Cuando se usan los indicadores hematológicos del estrés (tasas de heterofilos-linfocitos [H/L]), los estudios sobre las aves en cautiverio han revelado que no existe evidencia que los radiotransmisores causan el estrés. Sin embargo, los estudios realizados en cautiverio posiblemente no reflejan las condiciones en las cuales se encuentran las aves silvestres, especialmente durante tiempos energéticamente costosos como es la época de invierno. Examinamos el posible efecto de los transmisores de tipo mochila sobre los individuos de Catharus guttatus en Carolina del Sur mediante un examen de la tasa H/L basado en muestras de sangre tomadas antes y después de la colocación de radiotransmisores. Capturamos y recapturamos ocho individuos a comienzos del invierno y 11 individuos a finales del invierno. En los dos casos encontramos que las tasas de H/L no cambiaron después de un mes durante la cual las aves llevaron radiotransmisores. Las tasas de H/L tampoco variaron significativamente en las dos pruebas entre individuos con radiotransmisores e individuos sin radiotransmisores. Adicionalmente, los individuos que portaban transmisores en las dos pruebas no tuvieron tasas de H/L elevados al final de la prueba en comparación con los individuos que portaban un transmisor durante solo una prueba (la de finales del invierno). Estos resultados indican que portar radiotransmisores de tipo mochila por un mes no causa el estrés en individuos de Catharus guttatus durante el invierno. Estos resultados son consistentes con los estudios hechos sobre aves en cautiverio y proveen apoyo al uso de los radiotransmisores de tipo mochila en esta especie, siempre y cuando los estandares de peso sean tenidos en cuenta.

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