April 11, 2019

Curve-billed Thrasher with multiple prey items

Curve-billed Thrasher with prey items

Curve-billed Thrasher with Soldier Beetle

Curve-billed Thrasher with Soldier Beetle

I suspect, for many birders, the signature sound of the Sonoran Desert is the soft, mournful cooing of the Greater Roadrunner or the raspy, incessant ratcheting of the Cactus Wren.  For me, however, it’s the sharp, exclamatory call of the Curve-billed Thrasher.  Whew-WHEET!  The “wolf whistle.”  Like the other two it is often heard before the singer is seen, though all three are typically broadcast from a high point in the habitat.  Unlike the other two, the thrasher’s call cleaves the desert air, awakening all within earshot to the possibilities of the coming day.

Thus it was on a past March morning that I heard paired Curve-billeds calling (both male and female use the wolf whistle) and followed them visually from separate Saguaros downward to the same Chain Fruit Cholla.  Knowing this is one of the species’ favorite nest sites and suspecting housekeeping, I hurried over, kept a respectful distance, and waited.

Nest disturbance aside, I always keep a respectful distance from this lethal cactus.  Chain Fruit Chollas are known as Jumping Chollas for a reason.  I once inadvertently brushed one and had a large segment attach itself to the inside of my entire forearm, from wrist to elbow.  I can’t describe the pain or how long it took to get it detached, but my entire arm was black and blue for a week.  This is also the reason, of course, that Curve-billeds prefer it for nesting.

It didn’t take long to determine that this pair was, indeed, nesting in this cholla, and when one of them left bearing a fecal sac, I knew they were feeding nestlings.  Curve-billeds are, of course, by far the most common of Arizona’s five breeding thrashers, the consequence being they are often seen but not often truly observed because of their “low value” to listers and even casual birders.  On the spot I decided it was time to rectify this situation in my own case, and I set up tripod and camera hoping to see and record things I didn’t know about one of our stereotypical Sonoran Desert species.

What do Curve-billeds actually eat?  The literature indicates this species is primarily a bugs and berries specialist.  We’ve had a pair feed in our yard for years, but they have never nested with us.  We’ve seen them pick up Russian Olives off the ground, and we frequently see them on our suet feeder.  We also see evidence that they dig up the dirt in the cracks of our driveway and walkways because we have to sweep up after them, but we’ve never seen them extract anything in their digging.  Exactly what insects are they taking?

After three mornings observing an average of seventy trips in four hours, and several hundred digital images, I can only say I still can’t tell you for sure because I’m not a trained entomologist.  I can tell you a lot of things you probably didn’t know about Curve-billed Thrashers, but much of the stuff they were bringing to the nest, the insect material to feed nestlings, was beyond my bug expertise to identify.

Identification was complicated by two things:  thrasher bills are large and strong whereas insects are small and fragile; thrashers capture multiple items on many of their foraging trips.  The first of these meant that many of the prey items were mangled beyond recognition.  The second meant that rather than the camera capturing individual items, what I often saw on the computer screen was simply a mass of protein looking like it had been run through a blender.  Better for the kids no doubt, but not so good for identification purposes outside a biology lab.

Looking through the viewfinder as a parent returned with food, I was often reminded of puffin photos in which the bird has multiple small fish lined up perpendicularly to its bill.  How do they do that?  Try it at home.  See if you can pick up a food item with your lips or teeth without dropping a food item already in your lips or teeth.  Puffins are able to do this because they have flexible, hinged beaks, strong, rough tongues, and backward facing spines inside the upper mandible.  If thrashers have any of these adaptations, I could not find evidence in the literature.

I did get two images with cactus fruit, two with grasshoppers, one with a moth, and one with a soldier beetle I was able to identify as Podabrus pruinosus.  Many times a parent returned, like a little brown puffin, with caterpillars, worms, and grubs lined up perpendicularly.  The majority of times when the returnee brought items intact I could only guess—springtails, earwings—things that live in the soil or under rocks.  There were several trips with flat, waxy objects, perhaps scale insects, and YES, one lizard.

I can tell you foraging begins at sun up, picks up as the sun warms up the insects, peaks at mid-morning, then drops off before noon.  Male and female Curve-billeds are the same size, both feed the nestlings, but the male seems to lose interest first as the day progresses.  Though I was able to distinguish between my pair because one had a small aggregation of darker spots on one side of its breast, I must admit I can only guess “she” was the female because the other bird seemed to come less often as the morning became warmer, beginning to spend more time retiring to “his” singing post to proclaim his territory and family.

Seldom did the pair coincide at the nest with prey.  Often they would head out in opposite directions, but just as often they went the same way.  Sometimes one or the other or both would go out to the same area several times in a row and return with the same type of item. Every now and then the female would just drop to the ground under the nest cholla and forage almost to my feet.  At other times they would foray out 100 yards or more from the nest. 

Did I mention the nest was in Papago Park in east Phoenix, which is my “home patch.”  On two occasions (out of 20 I witnessed in three days), the parent removing a fecal sac flew directly from the nest to a trash barrel at a nearby picnic ramada (forty yards) and dropped it in!  Coincidence?  From behind me to the south I sometimes heard another wolf whistle, and twice a third Curve-billed strayed into the territory.  Both male and female immediately accosted the interloper and drove it away.  Once I saw the female thrasher harass and drive to the ground a Round-tailed Ground Squirrel that had climbed up into the branches of a nearby Rabbitbrush bush.  Presumably Ground Squirrels could be nest predators.

My mornings at the Curved-billed nest were part of my ongoing attempt to learn more about the common species all around us that we take for granted, and I’d like to convince other birders to do the same.  There are fascinating things to be observed, even in the lives of our most common avian neighbors, and fascination is what got us all hooked in the beginning.

Curve-billed Thrasher with lizard

Curve-billed Thrasher with lizard

Curve-billed Thrasher with moth

Curve-billed Thrasher with moth