SDZ Cribs: Burrowing Owl Edition
San Diego County is home to the western burrowing owl (Athene cunciularia hypugaea), a spunky, pint-sized species sadly in decline because of habitat fragmentation, loss and degradation. As a 2016 Applied Animal Ecology summer fellow at the San Diego Zoo Institute for Conservation Research, I was lucky enough to work with these owls. One of the projects I worked on investigated the differences in microclimate buffering abilities across multiple artificial burrow designs. This type of study is valuable because artificial burrows are such an important, widespread tool in burrowing owl recovery....and this is why today I'll be taking you on the grand tour of some of our artificial owl burrows, on this week's edition of [cue theme music]: SDZ Cribs, the Burrowing Owl Edition!
Like its name suggests, burrowing owls breed and roost in underground burrows. The only problem is, western burrowing owls are not capable of digging their own burrows. So where do they find their homes? Well, they have to do a little house hunting amongst burrows that have already been built by other animals. In San Diego County, burrowing owls often occupy California ground squirrel burrows.
Unfortunately, a lot of people see ground squirrels as pests, and have eradicated them from many potential burrowing owl habitats. By eliminating these 'construction workers,' we have greatly reduced the owls' options for burrows. Now, we often find burrowing owls utilizing urban landscapes instead of natural grasslands because natural burrows are no longer available. Unfortunately, urban environments come with unique risks not present in natural landscapes.
Luckily, there are a lot of people who care about the conservation and management of these owls. One solution is to install artificial burrows for the owls in protected areas that are suitable for wildlife conservation. While artificial burrows require maintenance and are not as self-sustaining as squirrel burrows, they have proven very useful in burrowing owl management, particularly as an initial, short-term solution.
Although artificial burrows are effective, they are not perfect. Data suggest that owls living in some artificial burrows have lower reproductive success than owls living in natural burrows. We have observed that natural burrows are better at buffering against outside conditions than artificial burrows, and since temperature and humidity can affect hatching success and chick survival, these microclimate differences may contribute to variations in reproductive success.
This finding inspired us to implement changes to the most commonly utilized artificial burrow design in the hopes of discovering a design that more closely mimics the microclimate of natural burrows. By creating artificial burrows that are as similar to natural burrows as possible, we may be able to improve the reproductive success of owls that live in artificial burrows, making such burrows an even more powerful tool in burrowing owl recovery!
Now, it's time to take a look at the “owl-mazing” burrows we installed this summer!
The Front Yard
Before we step on in, I just wanted to point out the lovely surrounding area (Figure 1). You'll notice that the landscaping aesthetic is: grassland. Burrowing owls tend to prefer grasslands with short vegetation, making it easier for them to hunt and move around on the ground.
The Grand Entrance
Welcome to the burrow! Here we have the entrance to the burrow tunnel, made from only the rarest, most exotic of materials: rocks, dirt and irrigation pipe. Our owls care a lot about sustainability, and you'll see that we've reused a couple pieces of concrete rubble for this burrow. We heavily fortified the entrance with rocks and chicken wire to keep the pesky neighbors (cough cough, coyotes, cough) from digging up the burrow. We tried to build a burrow entrance that would last a while, but also blend in with the natural surroundings. Before we head in, let's take a quick look at the other burrow entrance. Many artificial burrow designs have two entrances into the chamber. Why, you ask? Remember those pesky neighbors I was talking about? Well, the double entrance allows the owls another escape route in case a predator, such as a snake or skunk, enters the burrow.
The Anti-Predator Patio
Okay, we are finally entering the burrow! The tunnel entrance is retrofitted with a foot worth of larger, six-inch irrigation tubing (Figure 3). Aside from impressing guests with a spacious entryway, the large tunnel opening provides a bit more space so that more owls can rush into the chamber in case a predator comes.
The Flood Insurance
As you pass through the entryway, you might notice the floor dipping slightly (Figure 4). The first couple of feet of the tunnels have a slight five to six inch-deep dip in the floor. Aside from being a personal home slide for the kiddos, this dip actually helps prevent the burrows from flooding. If water starts to seep into the tunnel following a rain event, this dip will help prevent water from running into the burrow chamber.
As we walk through the hallway, feel free to admire our black walls. All top of the line, perforated, corrugated irrigation pipe. Did someone just ask about the lack of lighting in this place? Yes, well we don't believe in installing lighting. Burrowing owls can see in the dark, and they like to be environmentally friendly by not using any electricity.
There are three different models in this planned community, all with different tunnel shapes (Figure 5). Plan 1 (standard) replicates the most-used design in California—a U-shaped burrow with the chamber in the middle of the U. Plan 2 (curvy) incorporates curves and turns to mimic the natural complexity of a squirrel burrow, in order to cut down on the direct flow of air from above-ground and take advantage of the insulating power of soil. Plan 3 (Y), is slightly simpler than the curvy design, but should also cut down on air flow into the burrow chamber with the single entrance to the chamber. Comparing measurements of burrow chamber temperature and humidity will show whether any of these designs will better mimic the microclimate inside natural burrow chambers.
Ahh, and now we enter the grand chamber (Figures 6-7). This is where all the magic happens! Yup, yup, all original dirt flooring, and a lovely carpet of plant material. The chamber is made from an irrigation valve box. Here is where the female owl will lay and incubate her eggs, and where the eggs will eventually hatch! The chicks will stay in the nesting chamber for about two weeks, and then finally emerge into the outside world. While the female burrowing owl is inside incubating, the male will bring her food such as rodents and insects....which explains the mouse carcass in the corner. It doesn't have air conditioning, but living under layers of soil helps stabilize the temperature and humidity, buffering the owls from really hot and really cold temperatures.
I almost forgot to show you our awesome skylight (Figure 8)! Our burrow chambers are designed with an access point for researchers. Two nested 5-gallon buckets attached to the top of the chamber can be removed, making it easier for researchers to peer in, monitor the nest and clean it out, if necessary.
Well, that about wraps up our tour of our artificial burrows. Hope you enjoyed getting to see one of the ways we are trying to improve burrowing owl recovery. Tune in next time, on SDZ Cribs, Burrowing Owl Edition!