Retention, Effect, and Utility of Tail-Mounted Satellite-Tracked Transmitters on Golden Eagles

Abstract

Studies deploying Platform Transmitter Terminals (PTTs) or Global System for Mobile Communications-GPS (GSM) packages on Golden Eagles have typically used backpack harnesses for attachment despite evidence indicating potential significant negative effects on reproduction and survival. Retention, safety, and utility of tail-mounted PTTs were tested on a sample of Golden Eagles in southwestern Montana. Argos satellite-tracked PTTs of two configurations were attached dorsally or ventrally to the central rectrices of 27 Golden Eagles to study survival. Sixteen packages were known to have been molted or removed (i.e., shed) by the eagle and 13 recovered. Of recovered tail-mounts, six (46%) were forcibly removed by eagles; five by males and one by a female. All packages that were forcibly removed were 32-g ventral mounts. Females tended to retain tail-mounts longer than males and dorsally mounted PTTs tended to be retained longer than those mounted ventrally. Eagles tagged in winter retained PTTs the longest. The duration of tail-mounted PIT retention was adequate for analysis of survival and yielded an adult annual survival rate (86%) consistent with recent U.S. Fish and Wildlife Service estimates (87%). Eight of 13 (62%) territorial adults tagged with PTTs were known to attempt breeding the year they were tracked and six (46%) produced young, rates that did not differ (P > 0.23) from those of a larger sample of the population surveyed during the same period. Logistic regression analysis of cumulative range size by monitoring duration of four territorial adult eagles with tail-mounted GPS indicated that 99% of total range was recorded within 140 d of tracking.