People often talk about the delicate balance between different species of animals.  For example, if you poison off lowly insects with pesticides, fish that feed on the insects will die -- and soon you'll have fewer birds that eat the fish.

Nowhere is this cascading effect more dramatic than with the case of horseshoe crabs and red knot birds.  As fishermen in the 1990's grabbed increasing numbers of these prehistoric-looking crabs off of the beaches of Delaware Bay to use as bait, the number of red knot birds plummeted.  Why?  Because these amazing birds -- which make one of the longest migrations of any creature, from the tip of South America to the frozen landscape of northern Canada -- rely on the horeshoe crab eggs for food.  Without the food on the Delaware beaches, the birds starve on their grueling flights. 

This Sunday, Feb. 10, at 7 p.m., you can tune in to Maryland Public Television for an excellent documentary on this problem. It's called  "Crash: A Tale of Two Species."  I watched a preview DVD of the program sent to reporters in advance, and I give it five stars.  The photography is stunning, the science is compelling, the story is moving and I learned a lot -- for example, about the extraordinary properties of horseshoe crab blood, which happens to be blue.  As it turns out, these ancient creatures are in high demand not only as bait for eel trappers -- but also because their blue blood is used in medicine.  But as this demand rises, the number of red knot birds falls.  It's not as simple as: leave the crabs alone to save the birds. What about the watermen?  What about the medical patients?  How do we balance these conflicting demands?  Check out the debate this Sunday. It's part of the PBS Nature series.

 Horseshoe crabs