Let's cut straight to the chase: there's just one owl. It's great to know that it has survived for over a week in the Montrose Beach Dunes (a state-designated natural area) on the city's north side, despite many potential predators in the area and many eager birders trying to add it to their state lists. The last time a Burrowing Owl that showed up there--in 2008--it only survived for a morning before falling prey to a Cooper's Hawk. (For more information, see this link.)
The two photos below were taken by Jerry Goldner. The image on the left is from 6 October, the day the owl was first found. The photo on the right was taken over a week later, on 15 October. As I was examining the photos to determine how many birds were involved in these sightings, Field Museum post-doc Nick Block pointed out that a very distinctive white crescent was present under the bird's right eye in both photos. An assymmetrical plumage feature like this seems like a great individual identifier. Bingo.
The same day that Jerry took the right-hand photo, other observers watched the owl cough up a pellet--undigested bits from whatever it has been eating. This is a great way to determine a bird's diet and, thanks to Anne Kotowski, John Leonard, and Nick Crouch, I was able to get the pellet today and examine its contents (with a penny for size perspective).
It was clear when I started picking through the pellet that it contained the bones of a single rodent and many insect parts. Fortunately I was able to find the lower jaw bones and the upper incisors of the rodent, enough to identify it as Mus musculus, better known as house mouse. The notch in the upper incisor is distinctive (noted by the pink arrow below), as is the first molar (black arrow) being larger than the second and third molars combined (green arrow). The identification was confirmed by mammal curator Larry Heaney. Insect Division collection manager Jim Boone looked at the insect parts as well, and right off the bat identified some of the parts as coming from the beetle genus Pterostichus, in the family Carabidae (ground beetles). He's still working on identifying other insects in the pellet contents, and I'll post separately about the insects that the pellet contained later.
Thanks to everybody named above, as well as to Jason Weckstein and John Bates of the Field Museum's Bird Division. Visit Jerry Goldner's website at profilesofnature.com.