Posted by: Rob Viens | June 10, 2012

The Trembling Naturalist and the Hummingbirds

June 10th – Today’s entry is short, but oh so sweet. It’s Sunday in 1832, and Darwin simply writes:

“Like a schoolboy in his holidays, I tremble as I perceive another week completed.” (June 10)

The raw excitement in that one line is tangible. You can almost feel it as he “trembles” with pure joy. Speaking of “trembling”, let’s talk about hummingbirds…

Yesterday, while climbing Pedra da Gávea, Darwin encountered hummingbirds. “Yeah”, your thinking, “hummingbirds are pretty cool, but to a naturalist they are not that big of a deal”.

But consider that hummingbirds are a New World species. That means that yesterday was Darwin’s first  recorded encounter with these amazing little birds. If you have had the pleasure of ever seeming one, think back to that first time you came face to face with a hummingbird.  I’m willing to bet it was pretty magical.  They are almost like something out of a fairytale the way they stop in mid-air, hover, and quickly zig-zag around a bush of flowers. This was Darwin’s first time.

I know I included this yesterday, but it is worth repeating Darwin’s description in this context:

“As we passed along, we were amused by watching the humming birds.— I counted four species—the smallest at but a short distance precisely resembles in its habits & appearance a Sphinx.— The wings moved so rapidly, that they were scarcely visible, & so remaining stationary the little bird darted its beak into the wild flowers.— making an extraordinary buzzing noise at the same time, with its wings.— Those that I have met with, frequent shaded & retired forests & may there be seen chasing away the rival butterfly.” (June 9)

The hummingbirds make up the family Trochilidae which consists of over 300 different species.  As noted, they are a New World species, with the greatest diversity found in South America (which suggests that is where they evolved).  Most of the hummingbirds in the US and Canada migrate south in the winter (including our local species in Washington – the Rufous Hummingbird).

Of course, what makes hummingbirds so amazing is their flight. These little birds flap their wings about 25-100 times per second, can hover in mid-air, and can even fly backwards! All this work requires a lot of energy, so hummingbirds have an incredibly high metabolic rate (and some have a heartbeat of over 1000 beats per minute while in flight). Because flight is so energy intensive, they spend a lot of time perched, and can (when it gets cold or food is in short supply) go into a tupor. This is a sort of “hibernation” where the hummingbird metabolism slows down dramatically (including dropping their heart rate to below 100 beats a minute).

Hummingbirds filmed around Serra dos Tucanos, Brazil (posted to YouTube by redshouldervids):

All this energy requires a lot of food, so hummingbirds spend most of their “flight time” searching for and eating nectar (they are nectarivores), as well as insects and spiders (as their protein source).  They lap up the nectar like a dog or cat drinks water, but with the added benefit of a grooved tongue that can help channel the nectar into the hummingbird’s mouth.

Most hummingbirds specialize in feeding off of the nectar of long, tube-shaped flowers. In fact, many have co-evolved with these flowers, such that the hummingbird’s beak and the flower’s shape compliment each other perfectly. The benefit of this is that no other animals will be able to get at the nectar of that particular flower (meaning there is more available for the birds).  And, at the same time, the flower can be assured that when the hummingbirds picks up some pollen, that it will be carried to another flower of the same species. The tradeoff is that these species have become so specialized that if one becomes threatened or extinct, the other with quickly follow.

Hummingbird from Itatiaia National Park near Rio (from National Geographic):

Hummingbird

Darwin’s only point of comparison is the sphinx moth which (when found in the New World) are sometimes mistaken for hummingbirds. They also have the ability to “hover” and quickly move from side-to-side like a hummingbird, and the fastest members of their family (Sphingidae) have been clocked moving at 50 kph (about 30 mph). Nowadays, we consider this an example of convergent evolution – where two groups of organisms, evolving under the same environmental pressures, develop a similar adaptation.

White-lined Sphinx Moth (from the US Forest Service)
White-lined Sphinx Moth
(Some additional nice photos of a White-lined Sphinx Moth can be found at The Firefly Forest blog.)

Clear-Winged Hummingbird Sphinx Moth (posted to YouTube by NaturesFairy)

And to think, along with the rest of the Aves, hummingbirds are a direct descendent of theropod dinosaurs.  What would T-Rex think of that? (RJV)

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