Wednesday, February 01, 2012

Road Salt

White-winged Crossbills often become
"intoxicated" on road salt
Not long ago a friend arrived late for a community supper. The reason she gave for being late was received by some in the group as far-fetched, but they acknowledged that it was a creative and original excuse. She said, “I was coming down Newfane Hill and there were a couple of White-winged Crossbills in the middle of the road. They must have been intoxicated with salt because they wouldn’t move, so I had to get out of the car and make them move.” When I came soberly to her defense, it was then suggested that perhaps I had the same problem as the birds.

Intoxicated is not precisely correct, but it is close enough. Crossbills have a fondness for salt, and (according to Forbush) they will eat almost anything that is well-salted. Along our winter roadways, they can get their salt along with a dietary essential - dirt and grit. But too much salt can produce listlessness leading to mortality. That is, the birds don’t feel like moving and get run over, unless the approaching driver has a greater concern for the birds than getting somewhere on time - and there don’t seem to be too many drivers like that on our roads.

This opens a couple of doors on bird biology. Birds lack teeth and have little sense of taste, and so tend to swallow their food whole. Their stomachs have two parts, one with digestive juices which act similar to the human stomach and a second, large stomach known as the gizzard. The tough hard muscular walls of the gizzard, aided by swallowed sand and dirt, serve the function performed by human teeth, grinding and pulverizing solid substances such as seeds. Those flocks of finches and juncos which we see along our roadways in the winter are eating dirt - they are ingesting the grit necessary to enable the gizzard to “chew” the seeds which they swallow whole.

Coincidentally, they are also getting a good bit of salt as well, and this can present a problem for the birds. Human kidneys are not very effective at flushing salt from our system; it requires lots of fresh water. Those old mariners set adrift on oceans of salt water died of thirst: “water, water everywhere, and not a drop to drink.” If they drank the salt water, their kidneys would dehydrate the rest of the body to flush the salt.

The "tubenose" of pelagic birds, like Cory's Shearwater,
is visible evidence of the salt gland
which secrets salt from species which live on salt water

Bird kidneys are even less effective in dealing with excess salt than are human kidneys. However, sea birds will often spend months far removed from any fresh water and must drink sea water. How do they manage? They manage through a special gland located in the skull over and/or in front of the eyeholes and connected to the nasal opening by a duct. It is called the salt gland. The salt gland removes salt from the bloodstream and then secretes it in a highly concentrated form through the nostrils. The head shaking seen in some oceanic birds is done to expel this saline solution. Salt glands are larger and more developed among seabirds than land birds. Birds we are most likely to see with developed salt glands include gulls, terns, sea-going ducks (eiders), geese, coots, and rails.

Savannah Sparrow in a NJ salt marsh

The salt glands is present in songbirds (passerines) but is not functional unless the birds are regularly exposed to salt. The subspecies of the Savannah Sparrow which lives in salt marshes is able to secrete 2-3 times as much salt as other subspecies which live in salt-free habitats.

So why are the crossbills so fond of salt? I have no idea! The Birder’s Handbook simply notes under “Diet” that White-winged Crossbills are fond of salt, and then adds cyrptically: “Fondness for road salt produces occasional heavy mortality as listless birds are run over.” Too much salt is toxic, and so the birds are “intoxicated,” just as my friend said when explaining her tardiness. I guess we might say that salt is to crossbills what alcohol is to some humans; some individuals don’t know when they’ve had enough.

Good Birding!!

Link to additional information on "Birds and Salt"


Dan Huber said...

Chris, this is such a wonderful article. Great information you give on their biology and physiology. Loved it.


Laurence Butler said...

This is a great post, very informative. Your specifics with animal behavior and biology are fascinating. Thanks for sharing.

Jason Kessler said...

Fascinating. Who knew?

Many thanks.

Rohrerbot said...

Wow. This I did not know. And like usual, it's different for all the different groups of birds. Interesting.

Tammy said...

Very interesting info! Thanks for sharing!

Hilke Breder said...

Fascinating, Chris. This suggests an answer to the mystery a couple of years ago of finding a large number of dead and dying Gold Finches on our dirt road. I called the local vet but didn't get a satisfactory answer. Great post!


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