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Retention Rates and Impacts of Spear-Suture and Harness-Backpack Transmitters on Wintering Northern Pintails

Joseph P. Fleskes, David S. Gilmer, U.S. Geological Survey, Biological Resources Division, California Science Center, Dixon Field Station, 6924 Tremont Road, Dixon, CA 95620 USA, and Robert L. Jarvis, Department of Fisheries and Wildlife, Oregon State University, Nash Hall 104, Corvallis, OR 97331 USA

We used spear-suture (n = 82, 1993-94 only) and harness-backpack transmitters (n = 337) to study movements, survival and habitat use of female northern pintails (Anas acuta) in California, August -March, 1991-94. Spear-suture transmitters were retained for 29-154 days compared to a loss of <1% of harness-backpack transmitters. Transmitter type was related to the timing but not overall magnitude of mortalities. Compared to birds marked with spear-suture transmitters, pintails marked with harness transmitters died at a greater rate early but lower rate later during winter. Thus, survival curves in 1993-94 differed between transmitter types for both adult (log-rank test, X2 = 4.74, P = 0.03) and hatch-year (log-rank test, X2 = 3.91, P = 0.05) females. However, test power comparing point estimates was poor, especially late in winter after many females had shed spear-suture transmitters, and the estimated survival rate for birds equipped with harness and spear-suture transmitters did not differ at the end of the first hunting season (z ≤ 1.59, P ≥ 0.11), second hunting season (z ≤ 0.44, P ≥ 0.66), or winter (z ≤ 0.22, P ≥ 0.83). Proportional hazards modeling also indicated no relation between survival and transmitter type alone (P = 0.76) or after accounting for age-class and body condition (P = 0.80).

The major source (83%) of northern pintail mortality was hunting. Hunters reported similar flight behavior and social status for radio-tagged and other female pintails, but mean mass of radio-tagged pintails was about 15% less (P < 0.001) than other pintails shot during the same interval. Based on the relation between body mass at capture and hatch-year survival that we observed (3% increase in hazard for each 10 g mass reduction), radio-tagging may have reduced survival of hatch-year female pintails by 18%.

We recommend using harness-backpack rather than spear-suture transmitters for wintering pintail studies that last longer than one month because we found no clear survival advantage for birds with spear-suture transmitters and harness transmitters were easier to attach and were better retained. However, we recommend further research of attachment methods to improve retention rates and minimize impacts of radio-tagging.

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