Effects of backpack radiotags on female northern pintails wintering in California

To test whether backpack radiotags impacted the wintering biology of northern pintails (Anas acuta), I attached spear-suture (SSU, n = 82 in 1993) or harness (HAR, n = 337 in 1991-1993) backpack radiotags to female Hatch-Year (HY) and After-Hatch-Year (AHY) pintails after their autumn arrival in California. I evaluated impacts of radiotags on 1) wintering population distribution; 2) flock status, flock size, and body mass at harvest; and 3) August-March survival. I also compared retention of SSU and HAR following attachment. Distribution, flock status, and flock size at harvest of HAR, SSU, and unmarked (UMK) pintails were similar. However, harvest mass of HAR pintails averaged 133 g (SE = 25 g) less than UMK pintails; loss tended to be greatest for heavier HY females in 1993. Daily survival rates during 1993-1994 of HAR vs. SSU pintails were similar for both HY (0.9979 vs. 0.9974) and AHY (0.9988 vs. 0.9986) female pintails. Retention ranged from 30-158 days ((x) over bar = 81 d, SE = 5 d) for the 37 SSU that I confirmed as being shed; all other SSU failed <158 days or were on pintails that died <158 days. Two HAR were shed during the 202-205-day annual study over 3 years. Both backpack radiotag types are appropriate for study of some aspects of pintail wintering ecology, but I recommend against SSU radiotags for >1-month studies due to poor retention past a month.