Mortality of gallinaceous birds associated with 2% zinc phosphide baits for control of voles in alfalfa

Nontarget hazards to 52 ring-necked pheasants and 51 California quail were studied using 2·0% zinc phosphide (Zn3P2) steam-rolled-oat (SRO) baits to control gray-tailed voles in 12 (0·2-ha) enclosures planted in alfalfa. Pen raised birds were wing-clipped and 25–26 of each species were randomly assigned to three Zn3P2 baited or three control baited enclosures (eight or nine birds and 23–24 voles in each). Twenty-four birds of each species were also monitored twice daily using radiotelemetry. Following acclimation and pre-baiting (7 days), 2·0% Zn3P2 or control (0·0% Zn3P2) bait was broadcast by mechanical spreaders, and birds were observed or monitored for 14 days post-baiting. Zinc phosphide deaths, supported by necropsy results, occurred in 69% (18/26) of the pheasants, and none (0/26) of the quail. Other bird deaths were associated with six predator attacks, four accidents, and three escapes. Eight birds were missing at the completion of the study. These losses were not significantly different between species, baiting groups, and radiocollar status. The occurrence of all but one Zn3P2 death (17/18) within 24 h of exposure was highly significant (p < 0·00001) vs mortality observed pre-baiting and >1 day post-baiting. Sublethal toxic effects of Zn3P2 were observed in two pheasants. Postulated attributes of the efficacious bait (> 94% mortality for voles) that may have decreased nontarget exposure and environmental risks (particularly to quail) are discussed. Plans for research to assess the potential hazard of Zn3P2 to wild, free-ranging pheasants following a typical vole control program in alfalfa are underway.