Shorebird migration is well underway now. Most of these migrants we are seeing in Maine belong to species that nest on the arctic tundra. These birds only have time to produce one clutch of young in the brief arctic summer. The birds depart their breeding grounds as soon as possible after nesting. Interestingly, the adults leave before their young can even fly. Obviously, the migratory routes must be hard-wired in the brains of the young birds.
One of these species is a favorite of mine, the Semipalmated Sandpiper. I have written in a previous column about the arduous fall migration of these birds. Most Semipalmated Sandpipers wend their way to the upper Bay of Fundy in July and August. The birds fatten for a couple of weeks and then depart on a southeasterly track en route to Suriname and adjoining South American countries. This flight must be conducted non-stop over the ocean and requires between 48 and 96 hours of sustained flight. The trade winds do provide significant help for the birds once they go south of the 30th parallel of latitude.
In today’s column, I want to discuss some research that I did in Nova Scotia over the course of two summers. In particular, I will describe the intriguing relationship between the Semipalmated Sandpipers and their major prey in the upper Bay of Fundy mudflats.
The prey species is a small, shrimp-like crustacean called Corophium volutator. Reaching a length of 3/8 of an inch (10 millimeters), Corophium live in U-shaped burrows in the intertidal flats of the upper Bay of Fundy. During the summer, the densities of these crustaceans may exceed 100,000 per square meter.
Corophium have two generations each year. One generation is born in May and grows to reproductive size by the middle of July. These adults reproduce in July and August. Their offspring will grow during the fall and reproduce the following spring in May.
The females brood their eggs in a special brood pouch and ultimately release juveniles that make their own burrows as soon as they are released. The young are only about 1 mm long when they leave mom’s brood pouch to strike out on their own. Once reproduction has occurred, adults will live for a few weeks longer at the most.
The annual population cycle of Corophium therefore involves two generations that scarcely overlap: a three-month summer generation and a nine-month overwintering generation.
Migrating Semipalmated Sandpipers rely almost exclusively on Corophium while they are in the upper Bay of Fundy. The sandpipers are adept at detecting the Corophium in the sediment with their sensitive bills and extracting them. A successful peck by a Semipalmated Sandpiper is easily determined because the Corophium struggles in the bill of the sandpiper. By analyzing videotapes of feeding sandpipers, I was able to quantify the number of Corophium that a sandpiper takes per minute.
I did my research on the mudflat at Avonport in the Minas Basin of the Bay of Fundy. The tidal range there is huge: 35 to 50 feet between high and low tide depending on the stage of the moon. At low tide, a tremendous area of intertidal mud is exposed, giving the Semipalmated Sandpipers access to lots of Corophium.
Around 40,000 Semipalmated Sandpipers foraged on the Avonport flats. I found that each sandpiper ate an average of 17 Corophium per minute! The birds feed both during the day and night when the tide is out. Each sandpiper eats well over 10,000 Corophium per day. That is how the sandpipers can double their weight in just two weeks as they store the necessary fat to fuel their migration to South America.
We also know that Semipalmated Sandpipers choose the large Corophium to consume. As it turns out, I was able to show that Semipalmated Sandpipers are effectively managing their prey population.
When the Semipalmated Sandpipers arrive in July and August, the Corophium born in May are fully grown and are reproducing. Those are the prey the sandpipers go after. Because the adults live for a few weeks after reproducing, they compete with their offspring for food and space in the mudflat. By removing the large Corophium, the sandpipers improve the survivorship of the newly born Corophium. The large Corophium have mostly finished reproducing so their removal by the sandpipers does not affect the population to a great degree.
So, by preying on the large Corophium, the sandpipers increase the number of Corophium that will overwinter. Those Corophium will produce the summer generation in May that will provide food for the gluttonous sandpipers the following July and August.
[First published August 22, 2009]