Whereas my sample of eye colors for mammals (179 species) is small, the sample for birds (4,918 species) represents about half of all bird species. The total group can be broken into two fairly equal halves: non-passerines and passerines. We will concentrate first only on the non-passerines. They, like mammals, cover the full range from very light-eyed families to very dark-eyed families. That allows the reader to compare the pattern seen in mammals to the pattern seen here in birds.
With birds, we have enough species to limit the following list to only families with 15 or more species. [Taxonomy used was “World checklist of species” (based on the work of Howard and Moore) as it appeared in Perrins, C.M. (1990) The Illustrated Encyclopedia of Birds. New York: Prentice Hall.] That will give us a total of 38 families to consider.
Families of non-passerine birds with 15 or more species in the sample arranged in order of average eye-darkness:
---------------------------------------------------------1.00-------------
.995 Hummingbirds (49 species)
.860 Swifts (41 species)
.838 Sandpipers (68 species)
.825 Nightjars (40 species) DARK-EYED
.819 Terns (36 species)
.758 Kingfishers (61 species)
.756 Shearwaters (41 species)
.736 Turacos (18 species)
.734 Pratincoles (16 species)
.681 Storm Petrels (18 species)
----------------------------------------------------------.667--------------
.658 Trogans (19species)
.627 Falcons (61 species)
.575 Plovers (50 species)
.572 Pheasants (132 species)
.566 Guans (34 species)
.540 Tinamous (37 species)
.514 Toucans (18 species)
.500 Barbets (56 species)
.476 Ducks (148 species) MID-RANGE
.467 Auks, Murres, Puffins (15 species)
.464 Cuckoos (84 species)
.458 Rails (83 species)
.452 Bee-eaters ( 21 species)
.446 Woodpeckers (184 species)
.439 Gulls (37 species)
.435 Parrots (324 species)
.419 Hornbills (34 species)
.383 Penguins (15 species)
.375 Ibises & Spoonbills (28 species)
----------------------------------------------------------.333----------------
.306 Storks (18 species)
.300 Cormorants (25 species )
.258 Pigeons (249 species)
.233 Grebes (15 species)
.209 Typical Owls (73 species) LIGHT-EYED
.183 Cranes (15 species)
.180 Hawks (214 species)
.175 Bustards (20 species)
.025 Herons (61 species)
----------------------------------------------------------.000-----------------
As I look at the above list I see the following:
1. This list almost covers the full range of possible average eye darkness measures with herons at .025 and hummingbirds at.995. Those two families illustrate well the extremes of deliberate vs. quick behavior.
2. As with mammals, the families of true predators tend to have average eye darkness that is very light or very dark.
3. Herons, hawks and owls are three of the largest light-eyed families and I believe they are the three families of the 38 most often described as using stealth and surprise to ambush prey. Note that finding families of ambush hunters in the lightest one-third of the scale is what was also found with families of mammals [and will be found when we look at families of amphibians and reptiles].
4. In the darkest one-third of the scale, one finds 10 families. Except for the turacos (poor fliers with a diet of fruit and insects), these are fast-flying birds that feed on animal matter or nectar. Hummingbirds are aerial feeders that take insects as well as nectar on-the-wing. Swifts and nightjars are aerial feeders that take insects in-the-open and on-the-wing. (A family of passerines, swallows, feed in the same way.) Note that this is similar behavior to that of insectivorous bats--also very dark-eyed. Terns, sometimes called “sea swallows,” are aerial feeders that take insects in-the-open and on-the-wing and also snatch small fish from the surface of water. Some kingfishers also snatch small fish from the surface of water, but whatever the prey, they usually hunt from a perch--similar to flycatchers and have a similar level of average eye-darkness. Shearwaters and storm petrels use their flying skills to seek and feed on animal matter far out over the oceans. Pratincoles feed mostly on animal matter, some of which is caught on-the-wing.
5. Falcons seem out of place in the mid-range category. Part of the explanation is that the average eye darkness is brought down by a subfamily of light-eyed “Forest Falcons” that hunt from cover and also by a subfamily of “Caracaras” that are scavengers. (I need to remember that not all falcons are Peregrine falcons--a fast-flying dark-eyed species that catches other birds in-the-open and on-the-wing.)
6. Among families within the midrange of eye-darkness, many are herbivores (e.g. parrots) or omnivores (e,g. gulls). Woodpeckers mostly feed on immobile animal matter such as larvae. Bee-eaters and cuckoos each specialize in eating a source of animal matter that is noxious to most other animals: stinging insects and hairy caterpillars. These ways of feeding do not require the quick reactions of the dark-eyed direct-pursuit predators nor the deliberate moves and patient waiting of the light-eyed ambush predators. (Bee-eaters catch prey on-the-wing and are the most aerial of these families. In that sense, their behavior is more like that of dark-eyed families that feed on-the-wing.)
7. Between average eye darkness levels .233 and .476, [which represents only 25% of the full range of this list] there are six families of birds, each from a different order. Grebes, cormorants, penguins, rails, auks and ducks find some of their food, animal or plant, by swimming underwater. Given that none of these families are closely related to each other, I would have to guess that some form of convergent evolution that includes eye-darkness is responsible.
8. What I notice most about the above list is how closely the pattern mirrors the pattern seen in mammals.
June 25, 2006
Large Families of Non-Passerine Birds
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