Thursday, September 10, 2015

The Red-faced Warbler: Beauty and Mystery in the Mountains of Arizona


Guest post by Elizabeth Blaker. Photos by Dyer Lytle

This is a sample chapter from a book we are writing, called “Sing Mountains, Sing Deserts: The Curious Lives of Southwestern Birds” Please click on the following link to learn about our book project at Kickstarter Sing Mountains, Sing Deserts Kickstarter Link

A female red-faced warbler returns to her nest with a caterpillar
High in the shady mountain canyons of Arizona there lives an exquisite little bird, the Red-faced Warbler. The brilliant red feathers of its face and throat flash in the dappled sunshine filtering through the canopy of tall pines and shrubby oaks. In springtime the male Red-faced warbler sings from the branches and dances around his mate, displaying the alluring white patch at the base of his tail. What lady could resist?

Once a female Red-faced Warbler has accepted a mate, she tours potential nest sites on the ground beneath overhanging banks, or in the shadows of saplings or ferns in the leaf litter of the forest floor. She tries out different nest sites by sitting in them for a time. When one feels just right to her, she constructs a nest without the male’s help. During the nest-building, the male dances around her, inviting her to mate, or drives off other males wishing to mate with her during this most fertile phase of her reproductive cycle. Once the nest is complete, she will lay three or four pale eggs decorated with tiny light brown spots.

I encountered a pair of Red-faced Warblers one May morning as they flitted about the lower branches of an ash tree near the bank of the creek in Arizona’s spectacular Oak Creek Canyon. I was captivated by the beauty of these tiny red, black, and gray birds. As I watched them capture caterpillars and flies and visit their nest beneath a fern, I was intrigued to see that the female was very nearly as brightly colored as the male. I puzzled over why the female of a ground-nesting bird would be so colorful? One would think such brilliant colors would make her more visible to predators, which could pick her off her nest like a party-goer taking a canape from a tray, and then devour her eggs or young in the bargain.

Oak Creek Canyon from the rim above
It turns out that I’m not the only person who is curious about this bird. Since the 1980s, ornithologist Tom Martin has tramped the mountains of Arizona each spring and summer to study the Red-faced Warbler. He and a crew of students and field assistants camp for weeks in the forest along the Mogollon Rim, a dramatic two-hundred-mile-long escarpment forming the edge of the Colorado Plateau. The Rim rises as much as 3000 feet above the surrounding lands in places. Over millennia, melting winter snows atop the Colorado Plateau have run over the edge of the Mogollon Rim, carving drainages and canyons into it. The extra moisture in these places creates ideal micro-climates for the shady pine-oak forests that the Red-faced Warbler needs. Like miners in days of yore, Tom Martin and his team prospect these drainages along the Mogollon Rim, searching for the brilliant flash of crimson from a Red-faced Warbler rather than for gold – to an ornithologist, a wild bird, the particular species he or she seeks, is far more important than a hunk of yellow metal.

Like myself, Tom Martin was intrigued by the brilliant plumage of the Red-faced Warbler. To find out if brightly colored ground-nesting birds might be more vulnerable to predators than duller colored females, Martin had to break the question down into basic parts. First he had to actually test the assumption that nests built on the ground are more vulnerable to predators than those higher in shrubs and also in the tree canopy. Then separately he tested whether brightly-colored females and their nests more vulnerable to predators than duller birds.

Male Red-faced Warbler bringing food to his babies
Tom Martin and his team spent several nesting seasons searching the forests for the nests of Red-faced Warblers and a selection of other birds to round out the study. When a nest was located, they staked it out from inside a blind from the first light of dawn onward, writing down behaviors, taking video and audio recordings. Periodically they went to each nest to check for signs of predation, if possible– some nests were high in spindly trees that weren’t safe to climb. To accurately compare the brightness of the various birds in the study, Martin asked several people who were not involved in the study to look at pictures of the birds in a field guide and to score the color brightness of both the males and females of the various species. Then Martin combined the brightness scores with the nest predation data. What he learned was surprising.

Martin discovered that the nests on the ground were no more vulnerable to predators than those high in the treetops, but nests in shrubs were regularly raided by predators. His data also revealed that canopy-nesting birds and ground-nesting birds were equally bright, and in some of these, the females were nearly as brightly colored as the males; shrub-nesting birds had duller plumage with females that tended towards drab. Martin interpreted these results to mean that shrub-nesting birds cannot afford to have bright colors, especially the females, because they cannot risk attracting even more attention from predators. But, in a well-concealed nest among the plants and leaf litter of the forest floor, the brightly colored female Red-faced Warbler does not attract any more attention from predators than a bird that nests high in the canopy. I had my answer. Still, even though the female Red-faced Warbler could get away with her bright colors, why does she have them anyway? There must be a reason.

Young red-faced warblers in their nest on the ground
Could it be that female Red-faced Warblers compete with each other for males? After all, researchers have long thought that in many bird species the male’s bright plumage is due to the female’s predilection for mating with bright males– duller males don’t get to pass on their dull genes. Maybe in the Red-faced Warbler, mate choice is a two-way street? Tom Martin and his crew noticed that every spring there were several male Red-faced Warblers that were singletons, yet all the females had mates. If there are more than enough male Red-faced Warblers to go around, it seems that there should not be competition among the females for mates. But here’s the rub: if some males are much more desirable as mates than others, females will compete for the best males, and if the young of the best males tend to survive more often than those of lesser males, traits (such as bright feather colors) of both the winning male and the female that mated with him would become more common in a species.

Researcher Patricia Barber, working with Tom Martin, studied the relationships between male and female Red-faced Warblers. She saw that while male Red-faced Warblers don’t bring food to their mates while they are on the nest, they do capture food for their babies. During Barber’s vigil in the bird blind in the mountains of Arizona, she saw that male Red-faced Warblers drive off other males that might try to have a tryst with his mate. Female Red-faced Warblers defend their territories against other females. It seems there would be little cost to a female if her mate cheats, as long as he doesn’t spend time feeding and caring for the interloper’s offspring, so why would the female Red-faced Warbler defend her territory from another female?

Patricia Barber (nee Moore) also examined proteins in Red-faced Warbler eggs while working on her master’s degree, and found that one of the Red-faced Warbler eggs did not belong to the female that owned the nest. This extra egg is an intriguing clue to the reason for territorial defense among female Red-faced Warblers, and in the mystery of bright females. Here’s why. There is a limit to the number of offspring that can be successfully reared by a pair of birds, otherwise the size of broods would be much greater than the three or four eggs laid by Red-faced Warblers. Extra eggs dumped into the nest lower the survival odds of the nest owner’s eggs and babies because they are additional hungry, hungry mouths to feed. A brightly colored female Red-faced Warbler might be more intimidating to other females than a duller bird would be, and thus better able to prevent others from laying eggs in her nest. The offspring of bright females would then have a survival advantage over those of duller females that are burdened with raising extra babies.

Along the west fork of the Oak Creek in early autumn
Another possible clue to the brightness of female Red-faced Warblers can be found on their wintering grounds in the highlands of southern Mexico and Guatemala. There, in the pine-oak woodlands similar to their summer homes, they join mixed-species flocks of Townsend’s Warblers, Slate-throated Redstarts, Golden-cheeked Warblers, Greater Pewees, Tufted Flycatchers, Crescent-chested Warblers, and even Mountain Trogons. One species of bird they do NOT want to flock with is another Red-faced Warbler.

David King, a Forest Service biologist from Massachusetts, and John Rappole, an ornithologist from the Smithsonian Conservation Biology Institute (now retired) followed mixed-species flocks in the highlands of Guatemala and Honduras over three winters, documenting how each species captured insects, whether they hunted on the ground, in outer or inner branches, high or low in trees and bushes, whether they flew while hunting or gleaned insects from leaves. All this was to learn which species flock together and if the members of the flock compete with each other for food. What they found was that each species tends to use different insect catching strategies or locations in the tree branches, shrubs, or in the leaf litter. The species structure of a bird flock reduces the direct competition for food for each of the various species in the flock. This may be why Red-faced Warblers don’t want to flock with their own species – what could be more direct competition? Flocking with other species allows the birds safety in numbers, with more birds to spot danger and sound the alarm, or to confuse predators, but without the direct competition for food that would occur in a single-species flock. It is possible that brightly colored Red-faced Warblers, either male or female, are better able to drive off competitors of their own species, reducing competition for food, and increasing their own chances of surviving the winter to breed the next year. If the survival advantage was just enough, over time it could result in bright females and bright males.

Darwin would have liked the Red-faced Warbler, had he ever visited the mountains of the Southwest or those south of the border. The beauty of the Red-faced Warbler is so much deeper than the brilliant colors of their feathers. There is elegance in the strategies they use for survival, the merry-go-round dance of the species as a whole and individuals that comprise it, the pine-oak woodlands and the birds it supports, evolving together through the ages. And a bit of lively mystery as well.

This is a sample chapter from a book we are writing, called “Sing Mountains, Sing Deserts: The Curious Lives of Southwestern Birds” Please click on the following link to learn about our book project at Kickstarter Sing Mountains, Sing Deserts Kickstarter Link

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