For the past two years, our burrowing owl project has been focused on how to effectively relocate California ground squirrels to help re-engineer nonnative grasslands and make them more amenable to burrowing owls (see Burrowing Owls: Closer than You Think and Digging into Burrowing Owl Recovery). But this year, we get to take a closer look at the owls themselves. Don’t get me wrong, I love the squirrels (more than I ever thought I would), but I’m a bird biologist, so I’m really excited to start working directly with the burrowing owls!
Burrowing owls range widely across the western US and make use of a variety of “grassland” habitats, from open prairie to empty suburban lots to airports. But their populations are declining, mostly due to loss of habitat and eradication of the fossorial (digging) mammals that they depend on to build burrows. One solution is the installation of artificial burrows. However, artificial burrows are not self-sustaining like natural squirrel burrows and, although we know the owls use them, we don’t know how they compare to natural burrows.
This year, one of our main objectives is to compare reproductive output, food provisioning, and predation at natural versus artificial burrows, using camera traps and banding the birds to accomplish this. The camera traps allow us to see what is going on at the burrow while we aren’t there, and the banding allows us to identify each individual (see Bling with a Purpose).
At this point, the breeding season is in full swing. We are monitoring almost 30 nest burrows (both natural and artificial); this includes placing camera traps at about 20 of the burrows. We check on each burrow about once a week (we don’t want to visit too often and risk disturbing the birds) and do any camera trap maintenance needed, such as changing batteries and switching out the memory cards that contain our priceless data in the form of photographs. We also watch the birds from a distance to figure out what stage of the breeding season they are in—for me, this is the best part!
Over the last two months, we have been inventorying burrows and following their progression through the breeding season. On any given day, we head out to the field in the morning and work our way through our route for the day checking on each burrow as we go. When we arrive at a burrow, we observe from the truck (which acts as our blind) from a safe distance to see what is going on at the burrow. Early in the breeding season, we might see both parent birds or just the male standing guard at a burrow. In general, the males are lighter in color than the females, because they spend more time outside so the sun bleaches their feathers. As the breeding season progresses, the difference in plumage becomes more marked, as the males get more and more bleached. By the end of the summer, though, it can be hard to tell the males and females apart as both get bleached by the sun.
Once the pair has chosen their nest burrow, we usually only see the male of the pair; he is often standing watch over the burrow from nearby (often at the entrance of a satellite burrow where he spends much of his time—we call it the “man cave”). At this point, the female is spending most of her time in the burrow incubating the eggs. After about a month, the eggs hatch, and two weeks after that, the young start to come out to the burrow entrance. We usually do a quick examination of the photos in the field to help us determine if there are chicks present, but we also get good clues from the female’s behavior. If she is very protective of the burrow or stays very close to the burrow when we approach, it’s a safe bet that there are babies in the burrow.
Currently, we have nests in all different stages of breeding—some have pretty large chicks, some still have eggs, and some still seem to be deciding if they are even going to breed. In the coming weeks, we will band all of the owls from burrows that have camera traps, and over the next several months, we will pour over the hundreds of thousands of camera trap photos to catalogue how often prey was delivered to the burrow, what type of prey was brought, what types of predators come to the burrow, and other pertinent information. This is a huge undertaking, since we have almost 40 camera traps set up that can take over 30,000 pictures in one week alone! Any volunteers? Seriously, if you’re interested in helping, visit our volunteer page and sign up! Who wouldn’t want to spend their time looking at pictures of these adorable and comical little birds?!
Colleen Wisinski is a research associate for the San Diego Zoo Institute for Conservation Research.