Dawn Chorus 2: Ecology and Evolution

Background – what is the dawn chorus?

In forests around the world so many birds sing in the twilight before sunrise that there is a word for this event: the dawn chorus. Nowhere is the dawn chorus more evident and just plain cool than in a tropical rain forest. Haven’t been there? Visit a recent blog [Dawn Chorus I] and have a magical morning in the rain forest. You have to understand that bird diversity in lowland tropical forests is so high that over 200 different bird species can cross a single point in a given year. That’s crazy. And when those birds all start singing in the dawn chorus, they have to sort themselves out or no one gets heard. The point of singing, after all, is to attract mates and let your neighbors know where your turf is.

Science behind the dawn chorus.

Scientists have long tried to explain how birds “sort themselves out” in the dawn chorus. The time that any bird species sings is highly repeatable and predictable. If you have spent just a single morning listening to the dawn chorus at La Selva Biological Station in Costa Rica, for example, you just know who will sing first, and second, and at what time, and so on. Well, easy for you to say. You have a watch. So how have birds evolved to sing at a specific time and in a specific order every morning? Scientists have hypotheses (tentative explanations awaiting confirmation through testing) to explain why. Here are two of them.

Hypothesis 1. Acoustic transmission hypothesis. Birds sing to be heard, and the quietest time of the day is typically just before dawn. The air is still and transmits sound better at this time. If you have ever camped at a high mountain lake or meadow you know just what I mean.

Hypothesis 2. Inefficient foraging hypothesis. Birds are awake before dawn and they are hungry from a long night fasting, but when it is still too dark to look for food, they sing instead.

Turns out, science finally has an answer. Karl Berg, Robb Brumfield, and Victor Apanius recorded the dawn chorus for one month in the Amazon in Ecuador, by using remotely operated microphones stashed in the forest. They also recorded some other data that should effect how well birds in the forest can see. They observed the height in the forest that a bird sings and forages, because the when the sun starts to glow, that faint light starts up in the high canopy before it reaches the shady lower levels. They also measured the sizes of birds eyes (after correcting for body size), because birds with relatively larger eyes should be able to see better in lower light.

As scientists do, Berg and company made predictions based on their hypotheses.

Prediction 1. Birds that live higher in the forest will sing earlier (they get the sunlight earlier in the morning).

Prediction 2. Birds with larger eyes will sing earlier (they can see better to start foraging earlier, and they sing earlier as a consequence).

And that, friends, is exactly what they found. When night turns to twilight, the birds that see light first sing first: birds in the canopy sing earlier than those near the ground, and birds with larger eyes sing before birds with smaller eyes, all things being equal.

Why it matters.

Yawn. Crickets. “So what” you say? Everything humans do on the planet is affecting the dawn chorus of birds far and wide. Scientists have found that birds living near airports sing earlier than the same bird species living away from airports, because they are trying not to have their lovely voices drowned out by airplanes. Birds that live where there is artificial light sing earlier than birds that live where humans don’t have strong lights. The dawn chorus is a magnificent piece of music by the greatest composer on Earth: nature.   No one would conceive of having traffic noise or airplane noise drown out the music at Boston Symphony Hall or the Philharmonie de Paris. Every time we cut down a tract of lowland rain forest, it is like we are paving over another Sidney Opera House for the birds. Get this: Ecuador is selling off as much as 1/3 of their Amazonian rain forests to Chinese oil companies [LINK]. I’m no fortuneteller but this doesn’t have the look of an ecotourism scheme. If you have ever listened to the dawn chorus, you know that when these forests come down everyone loses.

References

The trail of science behind the dawn chorus is a curvy one indeed and I straightened it out a bit for the “sake of blog.”  For full details, you can read the original article by Berg and Team and look at the references that they list as well:

Berg, K. S., R. T. Brumfield, and V. Apanius.  2006.  Phylogenetic and ecological determinants of the Neotropical dawn chorus.  Proceedings of the Royal Society B 273:999-1005.  BergPROCB2006

Encore!

A few bios from some of the dawn chorus players follow.

Black-and-white Owl – Strix nigrolineata

Honduras
Black-and-white Owl Strix nigrolineata. Photo © James Adams of the Lodge at Pico Bonito.

To introduce the dawn chorus, why not start with an owl?   When night  starts to glow with the twilight of dawn, owls are on their way to bed.  Their calls often overlap with the first songs of the dawn chorus.

Short-billed Pigeon – Patagioenas nigrirostris

Short-billed Pigeon (Patagioenas nigrirostris). Photo © Roy Toft Photo Safaris.
Short-billed Pigeon (Patagioenas nigrirostris). Photo © Roy Toft Photo Safaris.

A true canopy lover, the Short-billed Pigeon can be found from Mexico to the Brazil’s central Amazon. Their bouncy call is fun to hear.

Black-mandibled Toucan – Ramphastos ambiguus

Chestnut-mandibled Toucan Ramphastos ambiguus. Photo © Nick Athanas.
Black-mandibled Toucan Ramphastos ambiguus. Photo © Nick Athanas.

Big toucans are split into two groups based on their calls: the yelpers and the croackers. The Black-mandibled Toucan is all yelper.

Bright-rumped Attila – Attila spadiceus

Bright-rumped Attila Attila spadiceus. Photo © Nick Athenas.
Bright-rumped Attila Attila spadiceus. Photo © Nick Athenas.

Want happy? THIS is happy. The bird sings like a lunatic. The Attila is a type of flycatcher. It’s hooked bill helps it catch larger prey like small lizards.  Yeah, a flycatcher that can eat lizards when it wants to is cool.

Montezuma Oropendola – Psarocolius montezuma

Honduras
Montezuma Oropendola Psarocolius montezuma. Photo © James Adams of the Lodge at Pico Bonito

Looks ridiculous and sounds ridiculous.  Related to our orioles, the males have bright blue and pink skin patches on the face with which to conquer the dames, and if that doesn’t do it, their song sounds like bubbling volcanic mud bogs, which they sing while hanging upside down.

Slate-colored Solitaire – Myadestes unicolor

Slate-colored Solitaire, Myadestes unicolor. Photo © Nick Athanas.
Slate-colored Solitaire, Myadestes unicolor. Photo © Nick Athanas.

This is a case of saving the best for last. Slate-colored Solitaires range from southern Mexico through Nicaragua. They live in high elevation cloud forests that are literally giant sponges in the clouds, soaking up water for everyone living down slope. The lovely song of of the Slate-colored Solitaire is proof that magic lives in wild forests.

Parting Shot

I would like to share some canopy music unlike anything you have ever heard before or will likely ever hear again.  Click here.

Leave a Reply

Your email address will not be published. Required fields are marked *