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.
"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.
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."