The photo below shows all of the remains I recovered from the nest, along with the book that Elizabeth Ries and I used to identify them. In all, we found 35 fish remains (skulls, skull fragments, opercular bones, jawbones, and an otolith), seven prairie dog skulls and one foot, one desert cottontail skull, one common muskrat skull, three western painted turtle shells, and some unknown vertebrae. Even in a relatively dry environment, the eagles show a clear preference for fish. As you can see from the opercular bones and skulls, some of the fish were pretty large!
|Top to bottom: desert cottontail, muskrat, black-footed prairie dog|
I started by thinking about the animals I had seen around the nest. How about a rabbit? Elizabeth and I began with looking at what kind of rabbits lived in the area. After a lot of comparison, we settled on a desert cottontail. The skull has a relatively round, large braincase, the eye sockets are similar, with a posterior extension that is "long, broad, and often fused to the braincase" (this helped us rule out brush rabbit), and a broad and high intraorbital region, or top of the skull.
Elizabeth suggested we take a look at the ventral, or underside, of the skull. Good idea!
|Desert cottontail, ventral view|
Since the ventral view was so helpful in identifying mystery skull #2, we decided to start with the underside in skull #3. Wow - look at those distinctive teeth! This should be easy!
|It was a muskrat, not a pocket gopher!|
Elizabeth and I found the skulls quite fascinating. We discussed the distinctive teeth of the muskrat, which is the only mammal of the three to eat underwater. Its short tooth 'plate' allows it to grind tough fibers behind closed lips, which keeps water from running down its throat. We noted the high, rounded skull and facial tilt of the rabbit skull, which may help it to leap and bound - something neither prairie dogs nor muskrats do - by keeping its nose down and out from its line of sight. And we talked about the high post-orbital processes of the prairie dog, which also occur in some squirrels and marmots. These bony barbs help anchor ligaments, pulleys, and other connective tissue, but why do prairie dogs have them when muskrats don't? Were they inherited from a distant ancestor, or do they reflect a more recent evolutionary development. Even though the animals were long gone, their skulls told us a lot about the way they had lived.
We'll look at fish and turtles in the next blog!
Why so many prairie dogs versus other mammals? There is an active colony very close to the nest, just below some large power poles with great cross braces. The eagles can perch right above the colony and wait for an unwary prairie dog to get a little too far from its hole. Prairie dogs are also relatively large and meaty - a nice size dinner for the amount of energy expended catching it!
Interested in ID'ing the skulls of North American animals? Check out Animal Skulls, a Guide to North American Species, by Mark Elbroch. We would not have been able to make ID's without it!
Did you know that prairie dogs are believed to have a rudimentary language? Given Ma and Pa's presence, I assume they have a call for 'eagle': http://www.npr.org/2011/01/20/132650631/new-language-discovered-prairiedogese