The art of owl persuasion

In San Diego County, habitat loss has reduced the western burrowing owl’s historically wide range to a single breeding population in a section of the city of San Diego just north of the U.S.–Mexico border. 

Like the saying goes about putting all of your eggs in one basket, if something were to happen to this breeding node, the risk of local extinction would be great. 

As part of a long-term strategy to save the species in the county, San Diego Zoo Global (SDZG) researchers are working with a collaborative conservation team to create an additional breeding node at Rancho Jamul Ecological Reserve (RJER).

In 2020, it was the third year of burrowing owl releases to RJER, but the first year where all owls released were previously under managed care rather than being relocated directly from a wild setting.

Cohort 1 comprised six conservation-bred owls and two owls rehabilitated by Project Wildlife. The owls were unpaired singles and were released at the very start of the breeding season so they would be able to establish territories and find mates at the same time as resident owls at this site.

One month later, Cohort 2, consisting of ten owls from the conservation breeding program, was moved as breeding pairs to another site at RJER that was not yet inhabited by burrowing owls. Each pair of owls was placed in an acclimation aviary with artificial burrows, where they spent a month adjusting to their new surroundings (see SDZ Cribs: Burrowing Owl Edition)

But how do you persuade an owl to stay put after an experience that is, well, not unlike an alien abduction? 

When relocating animals, a major hurdle is preventing dispersal from the release site; therefore, we took the following steps to reduce the likelihood that the released owls would leave the site where we hoped they would settle, thrive and raise families: 

Capitalize on their biological clocks. Spring is the perfect time to release burrowing owls because they are motivated by the need to breed. By playing matchmaker with Cohort 2, we hoped that the pairs we formed would initiate nesting and egg-laying in their new burrows, thereby causing their parental instincts to anchor them to the site even after the acclimation aviaries were removed. Also, we hope that this strategy will be effective at maximizing the retention of juveniles hatched at RJER, as burrowing owls have a strong tendency to return to the area where they grew up.

Create the illusion of a bustling neighborhood. Burrowing owls are semi-colonial, so to convince the owls that they were surrounded by neighbors, we deployed recorded burrowing owl calls with speaker systems and applied artificial owl droppings to nearby burrow entrances. These things made RJER a more attractive place to settle down.

Provide free food! Burrowing owls are formidable hunters, but their size also makes them prey for larger carnivores. Thus, a burrowing owl’s efforts are split between hunting for food, and trying not to become food. We have been providing thawed mice for the released owls throughout the breeding season, as supplemental feeding has been shown to dramatically increase survival and it gives them more time for predator surveillance. 

Our efforts have paid off, and at the time of writing we have 12 active nests with chicks and/or eggs. We are happy to report that we continue to see success of our translocation and conservation-bred release program through settlement of released owls, recruitment of juvenile and wild owls, and breeding on-site.