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Eared Grebes and Implant Radio-Transmitters

W. Sean Boyd and Saul D. Schneider, Canadian Wildlife Service, Environment Canada, RR 1 - 5421 Robertson Road, Delta, BC V4K 3N2 Canada

We captured and radio-marked adult eared grebes (Podiceps nigricollis; mostly males) at Riske Creek, British Columbia, over 2 years. In July 1995, 17 birds were surgically implanted with Holohil Ltd. RI-2C type transmitters with internal antennae (IA) (7 g, 6-month battery life). In July 1996, we captured and randomly selected 29 birds to be implanted with the same transmitters as above (except 12 g and 14 months) and another 17 birds to be implanted with RI-2C type transmitters with external antennae (EA) (10 g, 14 months). We ground-tracked the radio-marked birds on the Riske Creek study area until late August each year and then conducted aerial telemetry surveys on Mono Lake, CA, in fall (1x in October 1995; 3x from September-November 1996). Approximately 12 of the 17 IA transmitter types left the study area intact in 1995. Roughly 26 of the 29 IA types and 16 of the 17 EA types left the study area intact in 1996. On Mono Lake, we found the same proportion of IA types in 1995 and 1996 (42% or 5/12 and 46% or 12/26, respectively), suggesting that this marking protocol was consistent across years and that at least ca. 50% of the eared grebes breeding at Riske Creek stage at this important site in fall. In comparison, only 6% (1/16) of the EA type transmitters were tracked to Mono Lake in the fall of 1996. This large difference in occurrence between implant types is significant and suggests that the transmitters with external antennae had some unknown, negative effect on bird survival, or ? Although the IA type had severe range limitations (1-2 km from the air at 1000 m altitude), it may be the most suitable technique to describe the origin of and movement of eared grebes through Mono Lake. This is largely due to the relatively restricted size of the lake (ca. 180 km2) and the large number of grebes present (>1.5 million birds); with sufficient aerial coverage, all or most of these short-range transmitters can be located if present.

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