656 BIRDS 



This correlation with accommodation still stands despite all the 

 evidence, experimental and otherwise, that the pecten has nothing to do 

 with accommodation. If the relation of the two is not causal, then we 

 must look for a third correlate which ties the first two together. This 

 appears to be furnished by the retinal metabolic rates of the birds : 



If, as the comparative s N d situation suggests, the retinae in actively 

 diurnal vertebrates, with relatively high visual acuities, consequent high 

 cone: rod ratios, and good accommodation, have higher metabolic re- 

 quirements than the rod-rich retinae of nocturnal, crude-visioned, poorly- 

 or non-accommodating forms, then of course we should expect the avian 

 pecten to 'go with' accommodation; but it is really going with diumality, 

 high visual acuity, and bustling activity. 



Reviewed with this thesis in mind, most birds do seem to have either 

 large and many-folded pectens, or small ones, depending upon their 

 behavior toward illumination and their general level of activity. In noc- 

 turnal birds, the length of the base of the pecten is decidedly less than 

 half the eyeball diameter. Among the palaeognaths* the ostrich and rhea 

 are bold, light-loving creatures and have up to 25 or 30 pecten vanes. 

 The cassowary is shy and crepuscular, spending most of its time in the 

 densest forests, and has a small pecten with only 4-5 folds. Moreover, 

 the cassowary pecten appears degenerate in that it has been invaded by 

 mesodermal connective tissue. The lizard-like pecten of the strongly 

 nocturnal Apteryx has no vanes at all, whereas those of some lizards 

 have three or four. 



It has been shown that the pecten of a large owl {Bubo bubo) is 

 smaller than that of an eagle (Aquila chrysa'etos) having the same retinal 

 area. Smaller owls compare in this same way with hawks; and the pecten 

 in all owls is incomplete in that it lacks a 'bridge'. The nocturnal frog- 

 mouths (Podargus spp.), close relatives of the owls, have bridgeless 

 pectens which are relatively even smaller, with but three or four folds. 

 The European goatsucker Caprimulgus europceus, another nocturnal 

 owl-relative, has three to five pecten folds. 



Among ducks and geese, which mostly have 10-16 folds, a conspic- 

 uous form with its six folds is the peculiar Cereopsis, a goose which 

 seldom leaves the ground (and, incidentally, has practically no ring- 

 wulst). The nightingale, Luscinia megarhyncha, has been claimed to 

 have only five folds; but this is an old and doubtful record. The parrots 



*Whose radiate pectens should probably be considered separately until we know more 

 about their relationship to the undulant pectens of other birds. 



