The anatomy of a shrike bite: Bill shape and bite performance in Loggerhead Shrikes

Abstract

We examined the relationship between bill morphology and bite performance in Loggerhead Shrikes (Lanius ludovicianus), small passerines with raptorial bills. Shrikes feed on arthropods and vertebrates, and our aim was to understand how upper bill shape and jaw performance are integrated to meet the demands of their phenotypically diverse prey. We used geometric morphometrics to quantify variation in the shape of the upper bill, which houses the hooked tip and tomial 'teeth,' and took measurements of bite force and pressure (force per unit bill contact area) of wild-caught Loggerhead Shrikes throughout California, USA. We expected that longer bill hooks and tomial teeth would covary positively with bite force, because both hooks and teeth, as well as force, ought to be important for capturing, subduing, and dispatching vertebrate prey. Shrikes, however, with relatively longer, narrower culmens and longer hook tips produced lower bite forces than those with relatively stouter culmens and shorter hook tips. There was no significant relationship, however, between upper bill shape and bite pressure. Our results suggest that shrikes may achieve functional equivalence in bite pressure (force per unit of bill contact area) through variation in the amount of force applied by bills of different shapes. In this way, bill shape may be decoupled from force production, but without any direct costs to overall bite performance. This decoupling, in turn, may provide shrikes with greater phenotypic flexibility to meet the varied demands of their arthropod and vertebrate prey. © 2014 The Linnean Society of London.