Shrike!

Some time in the last 35 million years, deep within the heart of prehistoric tropical Africa, a taxonomic group of the superfamily Corvoidea diverged dramatically from their vireo-like ancestors. Their bills grew heavy and sharp, their jaw muscles strong, their eyes cunning and their hunger bloody. Soon, this new phylogenetic clade dispersed around the world, circumnavigating the Northern Hemisphere and speciating as they went. Today, science puts them in the tri-generic family that takes its name from the latin word for "butcher": Laniidae.

Shrikes are some of the most wicked-awesome passerines known to (wo)man. They sing like a songbird (sometimes), perch as daintily as any thrush or warbler, but hunt like a bird of prey. Swooping down from above, a shrike snaps its bill onto its quarry's neck, slicing and crushing the spinal cord. Once suitably immobilized, the shrike then they takes its meal back to its spiny larder: to impale it on a twig or thorn, so that it can be more easily torn apart. Shrikes will eagerly eat small songbirds, reptiles, rodents and insects, leaving their bloody bodies swinging in the wind while they screech their triumph from above!

And they're cute to boot.

All the shrikes 'round these parts are Loggerheaded Shrikes (Lanius ludovicianus)–in my humble opinion, the prettier of the two US/Canada shrikes. They can be distinguished from the Northern Shrike (Lanius excubitor) by their smaller, blunter bill, clean grey breasts and plumper demeanor.

One really cool study by Young et al. (2004) demonstrated the profound impact Loggerheads can have on their prey. As already mentioned, shrikes can only attack with their beak, their feet lacking the fierce talons of raptors; thus, they're forced to bring their faces dangerously close to their prey. Among their favorite foods in the Southwest are horned lizards (genus Phrynosoma), which possess a sharp crown of ossified spike ponting posteriorly–directly over their neck. Young et al. (2004) quantified the rates at which Flat-tailed Horned Lizards (P. mcalli) were predated by Loggerheads in reference to their horn size (this was done by measuring horns on the impaled skulls of lizards and comparing the horn size distribution to the naturally occurring distribution).

Their results are quite clear: Loggerheads find it much easier to catch and kill lizards with shorter parietal and squamosal horns. Given just how prevalent Loggerheads are in the Southwest (easily three times as common as Kestrels and Roadrunners, the other significant avian predators of lizards), it is likely that Shrikes provide some of the strongest selective pressure for longer-horned Phrynosoma.

Here's a figure from Young et al.'s paper:

That's all for today, class. Tune in next time to learn about why Burrowing Owls prefer cow poop to shag carpeting!


Reference:

Barker, KF, A Cibois, P Schikler, J Feinstein and J Cacraft. 2004. Phylogeny and diversification of the largest avian radiation. PNAS 101(30): 11040-11045.

Young, KV, ED Brodie Jr and ED Brodie III. 2004. How the horned lizard got its horns. Science 304(2): 65.