A Bird Bill Quiz

Bruce G. Marcot

The quiz question is:

What bird species of the world have bilaterally asymmetric bills? 
And why? 

... And the answer is actually quite interesting.  It's a list of birds that have undergone a degree of convergent evolution of their bill morphology and feeding adaptations.  What is really interesting is that this generally uncommon morphological condition covers spermivorous, insectivorous, and folivorous species!

These birds include:

1.  birds with crossed bills evolved for specialized feeding adaptations ("crossed" here means breaking the bilateral plane of symmetry):  

- Red Crossbill (Loxia curvirostra) complex (probably 5 or 6 sibling or cryptic species) in North America, which  feed on conifer cone seeds (that is, are spermivorous); also other Loxia species including White-winged Crossbill (L. leucoptera) in northern North America, Parrot Crossbill (L. pityopsittacus) in Scandinavia and western Russia, and Scottish Crossbill (L. scotica) in Scotland. 

- 'Akepa (Loxops coccineus) in Hawaii (Big Island); the finchlike bill is slightly crossed at the tip to aid in prying open places where insects can hide (insectivorous)

- 'Akeke'e (Loxops caeruleirostris), a close relative of the 'Akepa, on Kauai -- has crossed mandibles used to wedge apart scales of ohi'a leaf buds; feeds almost exclusively in ohi'a trees -- folivorous (also somewhat insectivorous?)

 [info on the Hawaiian birds from: personal observation; and Pratt, H. D. 1996. A pocket guide to Hawai'i's birds. Mutual Publishing, Honolulu, Hawaii. 112 pp.]

2.  another bird w/ an oddly asymmetric bill (but not a crossed bill):

 - Wrybill (Anarhynchus frontalis) of New Zealand, to feed on insects below rocks in streamside areas (insectivorous)

3.  Can you think of any others?  (birds that smash into a car's windscreen don't count...)

Symmetric but Diverse   

There are many other examples of specific adaptations of bill morphology, among species with symmetric bills, to feeding mode or diet (e.g., curlews, ibises, spoonbills, flamingos, storks, pelicans, etc.)

     Several salient examples of species complexes show adaptive radiation into a diversity of bill morphologies, feeding modes, and diets, such as the Hawaiian honeycreepers, the Galapagos finches, the Australian honeyeaters, Madagascar vangas, and others.  

Also consider the typically sympatric Downy and Hairy Woodpeckers of North America.  Not only are the bills different sizes because body sizes differ, they bills are different sizes in relative proportion to the head size, with Downys having bills lengths about the same width of their head, and Hairys having bill lengths greater than the width of their head.  They drill to different depths in bark to extract insects.  The differences in proportional as well as absolute bill sizes between these two species may help them differentiate foraging depths.

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