The presence of predatorial burrowing owls on the Farallon Islands deep into winter is harming the survival of adult ashy storm petrels, according to a study published by Point Blue Conservation Science. Owls arrive in the Farallones around mid-October, when the islands’ invasive house mouse population is at its peak. Several opportunistic owls stay to feast on one of the highest reported mouse densities for any island in the world. When the mouse population crashes in early winter due to storms and cold temperatures, the owls switch from a diet dominated by mice in December to one dominated by storm petrels in February. Removing the mice would reduce the abundance of owls, thus increasing the likelihood of petrel survival, the study found. “While in recent years we suspected that the burrowing owls were having a significant impact on the petrel population, this is the first time we’ve been able to demonstrate how serious those impacts are and how much the possibility of the petrel population’s recovery will be threatened if the situation is left unchecked,” said Dr. Nadav Nur, the lead author. Under current conditions, the authors expect petrel populations to decline by 63 percent in the next 20 years. But if the number of burrowing owls was halved, the petrel population would decline by just 26 percent. If owl abundance was reduced by 80 percent, the petrel population would stabilize or increase. The study bolsters an effort by the U.S. Fish and Wildlife Service to eradicate the islands of house mice, inadvertently brought by ship in the 19th century. By blanketing the islands with poisonous pellets dropped by helicopter twice over two weeks, the wildlife service believes it could rid the islands of mice. But after the plan received pushback from the California Coastal Commission, the service withdrew its consistency determination request and said it would resubmit it next spring. The Point Blue study does not evaluate the service’s mechanism for mouse eradication; it only explores eradication benefits. Nearly half of the world’s estimated 10,000 ashy storm petrels live on the Farallones; the elusive birds can live for 30 years, laying an egg a year. Although the service does not list the petrel as threatened or endangered, the species is designated as one of conservation concern in California. Because the Point Blue study uses population models and indexes, the authors acknowledge sources of uncertainty in their projections. “To what extent mouse eradication results in reduction of owl predation on storm petrels remains to be seen, but results from this study, and eradications on other islands, indicate the potential positive and significant population response by storm petrels,” they write. They also address the possibility that removing mice could lead owls to start hunting petrels sooner. But since the petrels are rare on the island in early winter, in the absence of mice, burrowing owls likely would be unable to sustain themselves until the petrels’ arrival in late January, they found.