Wednesday, September 28, 2011

Connections: Birds and Crabs, Part VII

            Another predominant shorebird that visits the Delaware Bay is the ruddy turnstone.  This traveler is a short-legged bird about eight to ten inches in length.  It has a short, dark bill that is slightly upturned at the end and a harlequin pattern of black, white and rust-red in alternating patterns on its top along with a black patch on its chest (Go downtown and you’ll probably see a similar pattern on certain “ladies” standing on the street corner.).  Its belly is white and its short legs are coral-red.  In summer, its uppersides are a combination of white, rusty-red and black.  To many people (at least to this observer), the ruddy turnstone often looks like it has been playing in an artist’s palette - carrying away a panache of colors here and there on its body.
            The ruddy turnstone winters on three coasts from California to New Jersey to Argentina.  It breeds primarily along coastal tundras from western Alaska east to Greenland and can also be found in selected arctic regions of Europe and Asia.  In winter, it spends considerable time in a host of near-water habitats including mudflats, sandbars, sandy or muddy shores, beaches and rocky coasts.  It is a good swimmer and may spend hours bathing and preening in shallow water areas.
            When it searches for food, the ruddy turnstone has the unique ability of flipping over rocks as it looks for small animals, typically invertebrates.  This unique rock-flipping action gives it its name (Some folks call it the “seaweed bird” because it often feeds among the kelp at low tide.).  In addition, they have the ability to dig into the sand in pursuit of tiny crustaceans.  Its diet is diverse and eclectic – dining primarily on small mollusks, crustaceans, grasshoppers, insects, larvae, maggots, worms, and, of course, the eggs of horseshoe crabs.
            Males and females will pair off either before or after reaching their breeding ground.  They tend to build their nests in open grassy areas near the water.  After the female lays approximately four eggs in a grassy area both parents will trade off incubating the eggs.  Just like urban street gangs or a slightly inebriated construction worker guarding a certain barstool at the local bar, the male turnstones are extremely territorial.  They will often patrol the border of the nesting site and will aggressively chase away any potential intruders. About 22-24 days after the eggs are laid, the chicks will hatch – with both parents caring for the young.  However, the female will leave before the chicks are fully fledged (sounds like a recurring soap opera theme) while the male remains. 

Monday, September 19, 2011

Connections: Birds and Crabs, Part VI

Each long-distance journey poses significant physiological challenges for the red knot.  As a result, their bodies must make several adaptations in order to survive the trip (similar, I suppose, to the physical adaptations we all have to make when trying to cram ourselves into a much-too-narrow airplane seat for a cross-country flight).  Immediately prior to each journey, the birds’ flight muscle mass increases while their leg muscle mass decreases.  Their stomach and gizzard masses decrease, while fat mass increases by more than 50 percent.  Throughout their wintering range they feed primarily on small mussels and other mollusks, shell and all.  However, when traveling long distances they eschew those hard foods because of their shrunken gizzards.  It is primarily the soft eggs of the horseshoe crab they are able to ingest.  Since the red knot’s spring migration is synchronized with the release of horseshoe crab eggs, those eggs become the ideal food for a long-distance traveler.  As a result of the super-abundance of the eggs across Delaware Bay beaches, the birds save considerable energy in hunting for necessary food sources.

            When red knots arrive at the Delaware Bay they are exceedingly thin, almost to the point of emaciation.  As a result, they need to eat constantly in order to increase their fat mass sufficient to continue their journey.  It is not unusual for them to gain up to 10 percent of their body weight each day and double their body weight during their time along the Bay.  There is at least one estimate that each red knot must eat approximately 135,000 horseshoe crabs eggs (in a period of about two weeks) in order to double its overall weight.

Thursday, September 8, 2011

Connections: Birds and Crabs, Part V

Although upwards of 30 different species of birds may crowd the beaches of Delaware Bay, there are four primary shorebirds that use the Bay as a stopover area during their northward migration.  Red Knots, Semi-palmated Sandpipers, Ruddy Turnstones, and Sanderlings make up approximately 97 percent of the birds that visit these shores.  Let’s take a look at each of these distinctive species.
            Red knots have an average wingspan of about twenty inches (this is slightly more than the distance from the tip of your middle finger to your elbow) and an overall length of approximately nine inches (a distance slightly longer than that from the tip of your middle finger to your wrist).  Their backs look like a psychedelic checkerboard – a crazy assembly of mottled buff, streaks of black, and bits of white in a seemingly haphazard pattern.  Their distinctive name comes partly from the fact that much of their head and all of their belly is robin-red.
            Red knots winter along the southern coasts of South America, primarily in Tierra del Fuego (Spanish for “land of fire,” and discovered by Ferdinand Magellan in 1520).  During its breeding season the Red Knot inhabits the mainland and islands scattered across the Arctic Circle.  Take a look at a globe and you will quickly appreciate the incredible distances it has to fly each year - 9,300 miles from south to north every spring and 9,300 miles from north to south every autumn (that’s an annual journey of 18,600 miles – a distance equivalent to three-and-a-half round trips, by car, between Los Angeles and Washington, DC).  As you might imagine, this bird is regarded as one of the longest-distance migrants in the animal kingdom.«
            Red knots migrate in enormous flocks – gatherings that are considerably larger than most other shorebirds.  Most migrating birds tend to cover enormous distances in one fell swoop (pardon the pun); however, red knots tend to segment their journeys into sections of about 1,500 miles at a time.  As a result, they tend to having “staging areas” – specific landing spots along the entire Atlantic coast.  They will use the spots regularly (just like you might travel to the same vacation destination year after year) – stopping in the same places at approximately the same times year after year.  These favorite stopping off points are familiar territory for the red knots and they can anticipate the available of food at each location.  On the downside, however, these sites make the bird susceptible to poaching, severe habitat change, and endemic diseases and toxins.  As an example, red knots were heavily hunted in the early 20th century, and have never fully recovered in parts of eastern Canada. 

« A red knot banded in May 1987 was identified again on Delaware Bay in May 2000. During the intervening thirteen years, it was estimated that this single bird flew approximately 242,350 miles, a distance greater than that between the earth and the moon (the average distance between these two celestial objects is 238,855 miles).

Sunday, September 4, 2011

Connections: Birds and Crabs, Part IV

As you might imagine, eggs that lay buried in the sand have the greatest likelihood for survival.  Eggs driven closer to the surface have the least likelihood for survival due to any number of environmental factors (lack of moisture, lack of adherence), but also because they become more readily available for shorebird consumption.  There’s also another factor at play here – the length of the shorebirds’ bills.  For example, red knots (whom you will meet shortly), one of the most prevalent of the migrating shorebirds, have some of the longest bird bills – on average 1.3 inches long.  Thus, they can forage in moist sand and extract any eggs within 1.3 inches of the surface.  Obviously, eggs buried deeper are beyond their reach and thus considerably safer and considerably more viable.
            You might think that the length of the red knot’s bill would deny it access to any of the deep-buried eggs.  Not so.  For, if nothing else, the red knot are opportunistic feeders – in short, they allow others to do all their work for them.  In this case the workers are the other horseshoe crabs who dig and scrape and mess around the nests of previous females – disturbing and excavating previously-laid eggs as they try to lay their own eggs.  Being the first to lay one’s eggs isn’t always a good thing – it may subject those eggs to the “digs” of females who follow.  Archeologists sometimes experience something similar  when they discover an ancient city that has been built on the ruins of a previous ancient city which has been built on the ruins of yet another ancient city.  A succession of spawning females – all nesting at approximately the same spot on the beach – has the potential to release thousands and thousands of eggs from each of the previous nests and onto the beach.  The more eggs released, the more eggs available for consumption – even by birds with short bills.