Secondary poisoning of raptors is a risk that some of us have expressed concern about over the years. The only research that I've been able to find was veterinary research related to pets such as house cats. Indications from that research are that secondary poisoning should occur only if there are undissolved zinc phosphide pellets in the guts of the rodents being ingested.
It's easy to imagine that raptors "waddling from vole to vole" (as Lars expressed it) could scarf up voles that just gobbled up a few zinc phosphide pellets. So I wouldn't dismiss the concern that Mary Holbert raised.