The Tropical Majority

Have studies of temperate-zone birds warped our view?

Eugene Morton and Bridget Stutchbury don’t chase Alice down a rabbit hole when it’s time for fieldwork. But the pair of ornithologists does end up in territory where familiar patterns reverse and truisms turn inside out.

A blue-crowned motmot fits the stereotype that tropical birds are flashy dressers. That’s a myth though, contend ornithologists. In the tropics, just as in the temperate zone, most bird species show subdued colors. C. M. Milensky

In one of the tropics’ puzzles for scientists, clay-colored robin’s nestlings hatch during a tough season for finding baby food. M. Read

A royal flycatcher frequents dense undergrowth in the tropics, keeping its headfeathers folded back until it needs to make a point. C. M. Milensky

The breeding season of Hawaii’s nectar-sipping iiwi falls during an abundance of blooms. The timing of other birds’ breeding seasons, however, doesn’t follow such a simple pattern. J. Jeffrey/US FWS

Their wonderland lies in the tropics of the Western Hemisphere, where birds often fail to follow the rules, at least as temperate-zone scientists have explained them.

Morton, now at the Smithsonian Institution in Washington, D.C., recalls that his first hint of the ecological surprises waiting in the tropics came from his grad-student fieldwork in Panama. He began wondering whether clay-colored robins there had gone crazy. “They breed at the wrong time of the year,” he says.

In the temperate zone, the breeding season for most bird species is determined by baby food, baby food, and baby food. Chicks need protein in daunting amounts, and the spring frenzy of nesting typically sends the parents foraging right when food supplies peak.

Morton found that clay-colored robins, tropical cousins of the red-breasted migrant, breed with tight synchrony but in the dry season, when insects dwindle to their scarcest. Chicks often starve in their nests, and those that do survive typically start their independent life small and frail. When Morton brought chicks extra food himself, he boosted the percentage of chicks that successfully flew away from the nest.

As he puzzled over the birds, Morton began to wonder if some seasonal surge in attacks by coatis or other predators might have driven the birds to nest at an otherwise terrible time. Some observations support that explanation. Morton found that during the wet season, predators destroy about 85 percent of bird nests.

During the dry season, however, nest losses drop to 58 percent.

Since then, Morton’s come up with ideas for additional forces pushing clay-colored robins to breed at a strange time, and research on other species has revealed yet more possibilities for unexpected breeding-system dynamics. Such rule breaking in the tropics extends to other aspects of bird life, say Morton and Stutchbury, who are based at York University in Toronto. In their new treatise, Behavioral Ecology of Tropical Birds (Academic Press, London, 2001), they describe species that don’t fit the previously described pattern for territory defense, hormone surges, or outside-the-nest trysts.

Despite the surprises, the researchers refuse to say that bird life gets peculiar in the tropics. Such talk comes from a perspective they don’t endorse. “It is ironic that tropical birds are viewed as strange, and perhaps even bizarre, when they vastly outnumber temperate-zone species,” write Morton and Stutchbury. “Our premise is not ‘why tropical birds are so different’ but rather ‘why temperate-zone birds are so atypical.'”

Plain vanilla species

Temperate zone species seem like the plain vanilla of ornithology because they’re what people have studied most, Morton contends. By the mid-1990s, published studies in behavioral ecology had featured that midlatitude classic species, the red-winged blackbird, more often than all the tropical birds combined.

Morton predicts that anyone who bothers to count will find the same overwhelming abundance of studies on other temperate standbys, such as barn swallows and great tits.

“There’s no question that is the case,” says Robert Ridgely of the Academy of Natural Sciences in Philadelphia. “We do have this bias.”

However, Ridgely sees no deep psychology in the skew. Although he’s just finished some 20 years of work on a guide to the birds of Ecuador, most professional ornithologists stay closer to home. And home has mostly been in temperate countries.

Compared with the tropics, these milder climes seem bird poor. Panama covers land the size of South Carolina but has some 900 bird species–about the number in the whole of North America. The difference in variation shows up with particular drama in certain kinds of birds. For example, eastern Canada and the United States offer homes to one genus of hummingbirds and one of tanagers. In tropical Brazil, ornithologists have found 30 genera of each.

To explain differences between the low-latitude crowd and its temperate relatives, Morton and Stutchbury turn to the basics of tropical habitats. Temperatures and day length tend to vary only a little during a year. Seasons change, but mostly from wet to dry, not from warm to freezing.

Without a temperate springtime, when’s a bird to nest? Some species reproduce year-round, but most show what Morton calls a surprising degree of seasonality. The season isn’t the same for all species, leaving ecologists with quite a complicated phenomenon to explain.

Peaks in food supply do dovetail nicely with nesting season for some species. Hawaii’s little nectar-sipping iiwi do most of their breeding when flowers bloom abundantly. The timing of southern Florida’s white-crowned pigeons’ breeding similarly reflects the abundance of fruit of the Florida poison tree.

However, there’s food and then there’s food, cautions Morton. Two species of manakins in Costa Rica eat mostly fruit, but they breed during a sorry time of year for collecting it. Morton proposes that such species feed insects to their young, since the nesting season does coincide with a buggy season.

The equivalent of teen-fueling pizza, however, may determine the nesting season for tropical house wrens. At first glance, these birds might seem out of sync with the food cycles in their environment. Their chick-feeding marathon ends before the insect abundance peaks.

However, the richest feast of insects comes when newly independent young are setting out to find territories of their own. To the new generation, food during this adolescent phase of wren life may prove even more critical than the supply of baby food. The young homeseekers face grim odds for finding territories of their own.

After years of musing about breeding seasons for clay-colored robins, Morton now suspects that a variety of forces push nesting into a tough time of year. Food for potential dads may matter. When males turn their attention to mating, small clusters sing at dawn before they feed. “It sounds like a temperate spring,” Morton says. Such a concert could allow females to assess a male’s condition. So, males may benefit from breeding in the season with plentiful supplies of the fruits that refresh them after a tough dawn battle of the bands.

Just how a tropical bird distinguishes the seasons remains a bit of a puzzle, though. Day lengths that shift by several hours tip off temperate species.

Ornithologists have been skeptical that birds near the equator pick up on the more subtle variations in day length there.

A study of tropical spotted antbirds, however, shows they do respond to small changes in day length. Birds in Panama experience about a 1-hour shift in day length throughout the year. When Michaela Hau and Martin Wikelski of Princeton University mimicked light changes for Panamanian antbirds in an aviary, as few as 17 minutes of extra light triggered gonad maturation. Light may provide the broad hints that a season’s going to change, but food may fine-tune a bird’s sense of timing. Hau found that adding crickets and other treats of the breeding season to an antbird’s diet prompted physiological readiness for breeding.

Territory defense

Breeding season in temperate latitudes typically is the period of all-out territory defense. Among perching birds, or passerines, in these zones, 90 percent defend territories during the breeding season but not the rest of the year. In Panama, however, only 13 percent of passerines claim territories this way. More commonly, territories last year-round.

Those two strategies bring different challenges. When the temperate spring competition starts, everybody hustles to grab a patch of land that has sat undefended since the past breeding season. In the tropics, however, a young bird of a species with year-round territories has fewer chances to find unoccupied space.

Pairs of dusky antbirds, for example, hold territories year-round. Both partners typically chirp as they go about their business in the dense growth, so Stutchbury and Morton wondered what would happen if they temporarily caged and moved one of the partners. In this intensely crowded real estate, how long would an antbird wait before advertising for a new partner? The answer they observed: Barely a minute.

Year-round homesteaders represent some two-thirds of the passerines in Panama. Many of these fall into splinter groups, and ornithologists have to figure out the rules for territorial games that birds don’t play up north.

For example, 11 species of Panamanian birds live as professional ant followers, gathering to feed on insects, spiders, and small lizards that are routed from their hiding places by streams of army ants. Birds defend territories, but armies march where they will. Sometimes the ants parade through one pair’s territory, sometimes through another’s.

Ornithologists have observed that a bird pair doesn’t try to chase away other ant followers that rush into the home territory when a column of the insects comes through. Insects abound in these circumstances, so perhaps territory holders don’t find it worthwhile to chase away intruders from such a cornucopia. Yet by following all the subtle interactions and positioning of birds while they feast, Edwin O. Willis of Universidae Estadual Paulista in Campinas, Brazil, concludes that territory holders occupy a dominant position in the crowd. They cede that position to the territory holders of the next domain when the ants move on.

Looking for the ephemeral bonanzas of fruit can lead to an almost parallel system. Some 14 percent of Panama’s passerines defend year-round territories that the birds leave on foraging jaunts to wherever fruit is good that day. These forays often take them into another pair’s home territory, but birds crowding around fruit don’t try to defend a particular share of it.

Morton reports that clay-colored robins don’t object to visitors flocking to a tree in their territory–as long as the intruders don’t sing.

For birds in the temperate zone, ornithologists have presumed that territorial defense requires testosterone. The hormone typically surges in males during the breeding season, especially early on when they claim their territories and advertise for mates by singing. In one study, giving males surgical implants containing the hormone increased their territory size. Even for males without implants, a recording of an intruder sent a territorial male’s hormones skyward.

“Just half a minute, and he’s buzzed,” says Morton.

Just what happens in the tropics, though, isn’t clear. Wikelski and Hau have found that a male spotted antbird’s concentrations of testosterone in the blood hardly vary throughout the year.

Regardless of the season, the concentrations stay low compared with temperate zone birds’. Wikelski and Hau find that temperate birds typically reach 4 to 6 nanograms of testestoserone per milliliter of blood. They reported last year that testosterone spiked at only 1.5 ng/ml in male spotted antbirds, and that was only after researchers played recordings of an intruder for up to 90 minutes.

Yet tropical birds don’t settle their differences with polite little chirps, Morton says. They can chase and scrap with the feistiest of creatures. Moreover, if testosterone is such a big deal for territorial defense and singing, why do so many testosterone-poor females in the tropics defend territories and sing? “I think it’s a very exciting area,” says Wikelski.

Morton proposes an answer to the testosterone question, and it involves another facet of bird life that he suspects will turn out to differ between the temperate zone and the tropics: unfaithfulness.

Birds often pair up during the breeding season, but the advent of DNA fingerprinting shattered ornithological illusions about fidelity. A recent tally of the birds genetically studied by 1997 found that the majority of supposedly monogamous species had extra-pair encounters.

However, 90 percent of those studies focused on temperate zone birds, Morton protests. Paternity analysis in tropical birds has lagged. He and Stutchbury did check for evidence of extra-pair fertilizations in dusky antbirds in Panama. Although the birds hold territories year-round and breed over a long season, the researchers found no evidence for off-nest adventuring.

Morton now wonders whether extra-pair shenanigans arise primarily in birds that breed in tight synchrony, such as most temperate species. If so, he speculates, might testosterone surges correlate with those extra-pair encounters rather than with territory defense itself? Temperate-species males experiencing surges of hormones flit away from their nests readily and mate elsewhere.

As a first step to testing the new notion, Morton and Stutchbury looked to the clay-colored robin, known for its synchronous onset of romance despite its tropical habitat. Unlike the less synchronous dusky antbird, clay-colored robins produced young with an unexpected biological parent.

The matter of testosterone in the tropics raises even more questions for another long-time student of tropical birds, Stephen T. Emlen of Cornell University. He points out that Morton and Stutchbury focus mostly on the New World tropics. What about Africa? he asks.

Emlen heartily endorses the idea that understudied tropical birds will upset comfortable ornithology rules of thumb derived from the temperate zone. However, he predicts that African tropical species will differ even from their New World counterparts.

In Eastern Hemisphere tropics, broad regions present a less predictable world than most Western tropics offer. In Africa and even Australia, rains can disappear for a year or two and then suddenly drown the land.

Birds adapting to such uncertainty must take advantage of sudden bonanzas. For example, Emlen deduces that many male birds in tropical Africa stay in a state of reproductive near-readiness for months at a time. Stockpiling hormone precursors may help these birds’ immune systems handle the strain that extended hormone peaks would pose.

These hormone questions highlight Morton’s worry that too much attention to temperate birds has cramped certain research efforts. Studying birds from a very different environment may change the way researchers answer some basic questions.

Bird studies wield a lot of influence in basic understanding of ecology, Morton reminded his colleagues at the annual meeting of the American Ornithological Union in Seattle last August. “We have to get our act together,” he says.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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