Laura Erickson's For the Birds

Friday, July 10, 2020

The Heart of the Matter

A couple of times in the 1980s, on my wedding anniversary, I talked about bird hearts. Today isn’t my anniversary or anything, but I came across this old transcript from 1988 and thought it was worth a second look.  

Hearts and birds somehow seem to go together.  Robinson Jeffers said to give your heart to Hawks.  Othello preferred to wear his heart on his sleeve for daws to peck at, jackdaws being a species of European crow.  Poe begged his raven to "Take thy beak from out my heart." Christina Rosetti was certainly having a better day when she said her heart was like a singing bird.  

Bird hearts are every bit as interesting as human hearts. The avian heart is one of the most powerful pumps in the world. As the ornithologist poet, Joel Peters, wrote to a hummingbird: 

Darting, hovering helicopter  
Fueling at a flower
Tell me how your engine-heart 
Generates such power! 
Like mammals, birds have a four-chambered heart which completely separates the circulation routes for arterial and venous blood.  But a mammal heart is puny and weak compared to a comparably-sized bird's heart.  A robin's heart beats 570 times every minute.  A sleeping chickadee's heart beats 522 times a minute, but during activity, ornithologists presume it beats more like 1000 beats a minute—chickadees don’t allow us to hook them up to little EKG machines while they’re going about their business. An active Blue-throated Hummingbird's heart races at 1,260--our own Ruby-throated Hummingbird, while resting in a dark chamber, has a rate of 614.  While incubating eggs, the heart rate jumps, especially on cool nights.  A House Wren's heart beats 455 times a minute at rest normally, but goes up to 701 while sitting on a nest.  

Not only do birds have rapid heartbeats--they also have powerful blood pressure.  The systolic pressure for a starling is 180 mm, and for a canary is as high as 220.  In comparison, a healthy person's systolic pressure should not exceed 150.  (Mine was 195 in the ER while I was having my last myocardial infarction.) A bird's high pressure pushes the blood through the body very quickly--it takes only about six seconds for blood to make a complete circuit through the body of an adult chicken.  

In selectively breeding domestic turkeys for rapid growth and heavy meat production, breeders unwittingly also selected strains that have high blood pressures and weak arteries.  As a result, some turkey flocks have suffered high mortalities from aortic rupture followed by internal hemorrhaging.  To prevent this, a tranquilizing drug is often added to feed mixtures to reduce nervous tension and lower blood pressure.  

High blood pressure pushes even a wild, natural bird's heart just about to the limits of mechanical safety.  Many banders have caught birds in their nets only to have them die of heart attacks in their hands.  One cardinal, who died after an exhausting territorial squabble, was examined carefully but no outer wounds were present.  A post-mortem revealed a 7 mm wound in the ventricle, probably caused by the great pressure built during the excitement.  Although post-mortems are not done often on birds, the deaths of a Field Sparrow with a ruptured aorta and a Bald Eagle with a ruptured right auricle were also apparently victims of excitement.  

Since I wrote that original transcript, the famous David Sibley, while banding a chickadee in 2012, placed a microphone on its back to record its beating heart. (Here's his blogpost about it.) When I played his recording, I looked at the spectrograph and could count the beats—118 over the 8-second recording, so I calculated out that that tiny heart was beating 885 beats per minute! Imagine that. Well, you don’t have to imagine—just listen to it