April 14, 2010 -- A female purple-throated carib isn't just interested in the plumage of a male hummingbird. She also wants to know what he owns. Good looks and a nice personality are secondary.
In what is believed to be a unique relationship in the world of birds, a male purple-throated carib doesn't just protect his own territory. He also holds a significant portion of his kingdom in reserve for females only, thus attracting many potential mates that he can observe and either select or shoo away.
"I don't know of anything else like it," John Kress, a botanist with the Smithsonian's National Museum of Natural History, said in a telephone interview. Kress and a colleague, Ethan Temeles, an ornithologist and biology professor at Amherst College in Massachusetts, disclosed their discovery in the online edition of The Proceedings of the Royal Society B.
Hummingbirds, which easily rank among the most alluring and surprising creatures on the planet, survive on nectar from flowers, in addition to insects and spiders that provide the necessary protein. It is common for a male hummingbird to stake out his territory in hopes of luring females into his lair. But this is the first reported case of males "parceling" off part of their fiefdom for females.
Birds Defend Territory to Lure Females
"What sets this apart is the birds are actually defending parts of territories not for their own energy needs at all, but simply for nectar for the females. We call it nectar farming," Kress said.
Kress and Temeles have been working the same territory for decades, and 10 years ago their paths finally crossed. "I heard about some work he was doing in the Caribbean," Kress said. "I had never been there," although he had been studying the same flowering plants as Temeles, called "heliconia," and hummingbirds for 30 years.
"He's a hummingbird ecologist and I'm a botanist, so we put our heads together and our eyes together and our backgrounds together and have been studying this system since 2001," Kress added.
The purple-throated hummers are native to the mountainous islands of the Eastern Caribbean, and it is on the island of Dominica that the researchers made some of their most important discoveries.
The genders differ in that the males are considerably larger, apparently because of all that combat needed to protect their territory, and the females have longer, more curved bills.
Flowers and Birds 'Co-Evolved,' Researchers Say
That is particularly significant because in the highlands of the islands one form of the heliconia has flowers that are useful only to the female. The male's bill isn't long enough to reach the nectar in the bottom of the long, tubular flowers.
The researchers believe both the flowers and the birds have "co-evolved" to serve each other's needs. The birds spread the heliconia's pollen, and the birds and the flowers have evolved in such a way as to ensure that the females will always have access to some food, because the males can't get it. That is precisely what Charles Darwin predicted in his seminal writings.
"This study indicates that the larger size of male purple-throated carib hummingbirds is a result, in part, of their aggressive interactions with one another," Temeles said. "In other words, it is the larger male who will win a fight with a smaller male, and thus win the right to reproduce, which is exactly what Darwin hypothesized would happen in his theory of sexual selection."
Males Protect More Nectar to Win Females' Affection
Kress added that it's quite clear in the field that this is a case of male hummingbirds protecting more nectar than they need for themselves for the sole purpose of winning the affection, or at least the cooperation, of a female.
The female usually comes in and circles around the male, he said. If she doesn't pass muster, the male will chase her off.
"Often he will let a female feed once or twice, and then interact with her by buzzing around her," Kress said. "And then, if he's really interested, he will let her feed a number of times, and then they go into this mating dance."
Unlike the Ruby-throated hummingbirds that famously migrate back and forth across the Gulf of Mexico in a marathon performance, the purple-throated carib stays put.
Over the past few years, Kress and Temeles have put leg bands on some of the hummers to nail down who's who. Surprisingly, they found the same male defending the same territory over a six year period.
Other researchers have found that migratory hummingbirds follow the same course year after year. In numerous cases, banded birds have been found in the same place on the same day in different years.
Hummingbirds' Social Life More Complex Than Initially Thought
It's not known exactly how they know where to go, because their guidance system is still unclear, but it is apparently hereditary. Young birds follow the same courses as their ancestors, and they travel alone, so no one is teaching them. How they do that is a great mystery.
What is known, however, is they are remarkable flyers. Other birds flap their wings to fly. Hummers whip their wings in figure eight patterns, so they can back up, hover, and even fly upside down.
And they do so with gusto, as anyone with a feeder knows. Warfare between males is a fierce spectacle, but rarely results in serious injury. It's more like a "loud squabble" that beak-to-beak combat, Kress said.
And now, it turns out that even their social life is a bit more complex than most birders had thought. Out in the Caribbean, the males know how to take care of the ladies.
Temeles said that watching females flock to males that have the most nectar was "amazing." It was the most fun he's ever had, he added, at least in a rain forest.