Monday, December 14, 2009

Chickadees and the Shrike

This time of year, the weather provides an easy excuse for me to stay indoors.   I have the time and motivation to pay closer attention to the more common birds - the chickadees, nuthatches, house finches, tree sparrows and juncos outside my window.  

The chickadees caught my attention the other day.   I couldn't help but wonder:  How do those little creatures survive when the temperature drops way below freezing?

To find an answer, I did a google "scholar" search and re-read my copy of  the Birds of North America (BNA) monograph number 39 - Black-capped Chickadees by Susan Smith.

Tipping the scale at a third of an ounce (the weight of 2-25¢ coins), chickadees need to eat all day long this time of year.   During mild winters, they need a caloric input comparable to about 150 sunflower seeds a day.  When the thermometer drops below zero, that number goes up to approximately 250 seeds. 

Despite what I see at my feeders, chickadees eat more than sunflower, suet and peanuts.  Scientists who’ve studied them say the Black-capped Chickadee’s winter diet is 50% animal and 50% plant material. 

This time of year, their wild” diet is comprised of seeds and insects they’ve cached in autumn, insect eggs and pupae, spider eggs, animal fat from carrion (dead deer, skunks and even fish), seeds (goldenrod, ragweed and hemlocks) and fruits (including poison ivy). 

Although chickadees depend primarily on natural food sources, birdfeeders provide an important supplement to their winter diet. According to a University of Wisconsin study, Black-capped Chickadees get only 14-29 percent of their daily energy requirement at backyard birdfeeders.   This may explain why chickadees are more likely to visit feeders at dusk than at dawn.

In addition to caching food and adding more plant materials to their diet, Black-capped Chickadees have other adaptations that help them get through winter.

   - Chickadees have very warm coats.  Their dense winter feathers are incredibly efficient insulation.  The difference between a chickadee’s body temperature (108-degrees Fahrenheit) and the ambient air an inch away can be more than 120 degrees!  

   - Chickadees have a remarkable ability to remember where they've cached food during the fall.  What happens inside their brain to facilitate this is simply amazing.  According to Colin Saldanha, assistant professor of biological sciences at Lehigh University, in the fall, the hippocampus, the part of the brain responsible for spatial organization and memory, expands in volume by approximately 30 percent by adding new nerve cells. And in the spring when food is more readily available, the chickadee's hippocampus shrinks back to its normal size.  

   - Chickadees have the ability to metabolize food quickly.  They can put on up to 8% of their body weight in a day (that would be 12 pounds a day for a 150 pound man).  That’s all the fuel they have to get them through the night - along with an amazing ability to turn down their internal thermostat 12 to 15 degrees.  This regulated hypothermia conserves energy.  If they make it through the night, chickadees get to face another day of eating - to replenish their fat stores.

And if dealing with the weather wasn’t daunting enough, chickadees have to be alert to predators.  At night, chickadees are prey for Screech and Northern Saw-whet Owls.  During the day, it’s Sharp-shinned Hawks and Northern Shrikes.

As I watched one of the chickadees bounce like a ping-pong ball from the sunflower feeder to the Blue Spruce and back, I marveled at its high energy. 

Then all of a sudden, all the birds bolted.   

I stood up to get a better look at what caused the "evacuation."  I couldn’t believe my eyes when I spotted it.  Just ten yards from my window, sitting on top of my double-arm pole, was a bird that looked like a mockingbird on steroids.   

It was a Northern Shrike - a.k.a. the butcher bird - a predatory songbird, a winter visitor from the tundra.  This amazing creature can make a meal of any of the birds that visit my feeding station - including birds as large as a Mourning Dove and Blue Jay.  

This was only the second shrike I'd seen at my feeders in the past 10 years.  

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