Wednesday, April 10, 2013

Bird a Day - Cardinal

What: We've had cardinals successfully - and unsuccessfully - fledge young in our backyard. This male has been courting a couple of females, so hopefully this will be a "successful" year. Females have the same body shape, but are a waxy yellowish color with dustings of red on the sides. Their songs (and both males and females sing) are simple, but variable. They have that same sweetness to them that the titmouse has, but cardinals tend to sound more like a toy laser gun. Birdie Birdie Birdie Pew Pew Pew Pew.

Where: The silver maple!

Other notes: Their nests, which certainly aren't the tidiest of affairs, are often placed in super shrubby thickets where they have access to lots of cover (which is partly why they like honeysuckle so much). They always use strips of grapevine bark in the outer layers of their nests. Not sure why, perhaps to help conceal their nests from predators. In Medicine Quest Mark Plotkin describes eagles that select medicinal twigs to line their nests with (supported by research from Ohio Wesleyan). I looked up medicinal uses of grapevine bark and couldn't find any references, maybe the cardinals know something we don't?

Birds can't produce red pigment on their own, so in order to display a red plumage a cardinal needs to ingest foods with red in them. The pink of a flamingo is variable with the the amount of brine shrimp and blue-green algae in their diet; the more algae relative to crustaceans, the deeper the pink color. Both of these food sources contain carotenoids, and a redder coat may advertise to potential mates a healthier individual.

Male cardinals need to ingest foods with red pigments to get that brilliant color of theirs. Similarly, the better they are at procuring food the redder they'll be. Cardinals molt in the fall and so their spring coloration is due to what their diet consists of in the fall when they're replacing their feathers. All those fall berries (dogwood, barberry, grape, honeysuckle, etc.) provide the pigmentation the males need to say, "Yup, I'm fit and healthy and don't have a darn bit of trouble feeding myself." Males will also feed females as part of their courtship, as if to say, "And I can feed you to."

Other questions that come up for me:
  • Why risk being red when you can just sing? 
  • If you're already red to advertise your fitness, then why also sing? 
  • If they have similar diets, how do females avoid turning red?
  • Given their different colors, what's the difference in predation rates on males vs females?
As I was writing this I thought of an obvious possible explanation for why red is a common pigment among cardinals (and related species in the finch family Fringillidae and grosbeaks, which includes Emberizidae). Members of these two families, as evidenced by their bills, are granivores/frugivores (seed and fruit eaters). Plants advertise when their fruits are ready (which coincides with when the plant is done growing the seed and everything it will need to germinate and begin a new life) by changing colors. Most start green and turn a red, purple, or other color to signify to animals that they can eat the fruits now. The pigments used to signal this are, you guessed it, carotenoids.  Plants readily produce carotenoids to a) protect exposed surfaces from the sun, b) aid in photosynthesis, and c) advertising when fruits are edible. So the birds rely on the plants to tell them when to eat them and the pigments that tell them that to give them their coloration!

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