August 2nd, 2011
Seeing beneath the sediment
IT IS NOT UNCOMMON for naturalists to believe that the selective forces in nature are obstacles and difficulties which are heroically overcome by individuals which pass their superior genes onto their kind, driving the patterns of diversity and abundance on earth in a process we call “evolution”. However, I don’t think it always works quite like that. I think that the struggle is at once a little more optimistic and elegant: a little more perhaps like water flowing downhill. The way I see it is that nature is an expansive thing, with organisms responding to the pressures of their environment by taking the path of least resistance – of not necessarily troubling over obstacles, but rather constantly and optimistically dashing towards new or favourable opportunities. By taking advantage of physical and environmental opportunities, adopting behaviours which play to their strengths, organisms carve out a passage of survival, living to pass on their genes which, if favourably adapted to their behaviour and surroundings, become extant throughout the population. It’s more like that John Lennon song where he sings “there’s no problems, only solutions”.
Sandpipers demonstrate this wonderfully. When we examine the incredible diversity of bill shapes and lengths and combinations thereof with leg lengths and neck lengths we can see that this principle of exploiting ecological niches through taking advantage of openings and opportunities has been played out in a manner which is at once extreme and very obvious. These combinations of characters among sandpipers indicate massively rampant opportunism which has become available through a wonderful “invention” called “tactile foraging”.
Getting in touch with your prey
Unless it possessed a highly sensitive bill, this Bar-tailed Godwit would be undertaking a rather pointless exercise!
TACTILE FORAGING has freed sandpipers up from relying on merely spotting prey on top of the mud or sand and chasing it down. Tactile foraging has opened up to sandpipers the opportunity to exploit an amazing diversity and abundance of organisms which dwell beneath the mud for all or part of their days or lives. It’s about accessing a world where the game of finding food, shelter, territory and breeding opportunities has been going on in similar extremes of rampant opportunism for many hundreds of millions of years. Such is the diversity and abundance of organisms which dwell beneath the intertidal sand and mudflats of the world that a bird can specialise on one species or even one species in a particular stage of its life cycle as the mainstay of it food resources. For sandpipers, tactile foraging is vastly more than something like you or I sticking our fingers in the mud and sand to feel what is down there. The highly sensitive bills of shorebirds are the result of a process which has taken the sense of touch to extremes which, for many species, amount to virtually seeing beneath the sediment upon which they are foraging.
Red Knots master the muddy medium
A Red Knot in winter plumage on Boat Harbour, NSW.
TO UNDERSTAND this process, let’s examine the bill structures of one of the masters of tactile foraging, the Red Knot. The Red Knot is a medium-large sandpiper which, like all migratory shorebirds, assiduously avoids winters. Therefore, it enjoys the warmth of summer in the northern hemisphere, where it breeds on the tundra of far north Siberia and the island of New Siberia. When it cools down in the north, the Red Knot migrates south, to enjoy the mudflats of bays, estuaries and beaches in an Australian and New Zealand summer. Red Knots are found on other flyways too, but Australia has two races of Red Knot, Calidris canutus rogersi and C. canutus piersmai: the former visiting New Zealand and eastern Australia and the latter occurring in Western Australia. Through mastering tactile foraging, Red Knots have managed to practically specialise on a diet of small bivalves, around the size of the average little fingernail.
Magnified view of a Red Knot bill tip revealing the pits which contain Herbst Corpuscles. Image after "Shorebirds an illustrated behavioural ecology": Theunis Piersma, Bruno Ens, Leo Zwarts ISBN: 9789050111928
The bills of birds are bony structures attached to their skulls and are in many respects analogous to the upper and lower mandibles in a number of other vertebrate species. The bill is covered with a thin but tough layer of keratin (the same structure which comprises our finger nails). This keratin sheath serves enough purposes for birds to require several blog posts! Beneath the keratin sheath on the tip of the bill of the Red Knot, and many other sandpiper species, lies a number of rows of pits. These pits are lined with many tiny highly pressure sensitive cells called Herbst corpuscles. So sensitive are these cells that they can detect the surrounding pressure of the water between the grains of mud and sand when they probe with their bill and displace that sand and water. Here’s how it works . . .
Displaced interstitial water pressure responding to resistance by an object (in this case a small bivalve) in the sediment.
Most of us have walked along the wet sand of a beach and noted how the pressure of our footfall forces the water in the sand to retreat. The water, trapped between each grain of sand – known as “interstitial water” – retreats from the weight of our footfall due to the displacement it causes in the sand. A similar event takes place when a bird presses its bill into the sand: it displaces sand and water. The retreat of the water will take place in a regular fashion if the sediment is even and undisturbed and thus the pressure will be constant. However, if there is an object beneath the sand which impedes the flow of the interstitial water, the pressure will be uneven and the highly sensitive bill of the Red Knot is capable of detecting this. When they determine that the nearby object is a small bivalve, they snap it up whole and swallow it. The shell is crushed in their powerful gizzards and the shellfish moves into the bird’s proventriculus (sort of like our stomach) for digestion.
Seeing beneath the sediment
AND ADULT Red Knot becomes so skilled in the art of tactile foraging that it appears that they are capable of knowing what lies in the sediment up to 5 cms from the tip of their bills. In effect, this amounts to being able to “see” beneath the sediment upon which the bird is foraging. Research has demonstrated that it takes as many as three years for growing Red Knots to master the skills of using these extraordinary tools, but once that mastery is achieved, the Red Knots have a niche for themselves which very few sandpipers are capable of exploiting. They have evolved so that their bills are no longer or shorter than is required to access a particular population demographic of bivalves to suit the body size that they have also evolved. They have evolved the precise tools to ensure that they have a niche which they can so dominate, that other species cannot access it. Their cousin, the Great Knot, has similar tools and capacities, but a slightly longer bill that enables it reach down to different populations of bivalves and other invertebrate groups as well, which further illustrates the principle of “resource partitioning”, which will be a topic for a future post.
Great Knots "sewing" on Cable Beach, Broome WA as the tide falls.
All this explains the foraging patterns which many Calidrid Sandpipers adopt when foraging called “sewing”: walking along the sand or mud systematically probing in a side-to-side manner. This is often partaken of in a somewhat communal fashion, possibly indicating that foraging parties may respond to the success of various individuals in hitting upon clusters of prey (much as pigeons do). It certainly opens up the opportunity for humans to enjoy one of the more engaging sights on those mudflats which are frequented by this species.
It’s magic!
AGAIN WE find that when we peel away the layers of what is taking place around us we are surrounded by an almost never ending complexity. For the nature enthusiast this presents a whole fascinating contemplation of this amazing biosphere which is the outcome of billions of individuals over billions of years simply making the most of what they have right now, of a place where resources and the ability to tap into those resources present both limitations and opportunities, where success multiplies and failure dwindles. It’s a story where the more we know, the more wonderful the contemplation becomes and when we run out of understanding or the capacity to grasp its enormity and complexity we become once again a child and in that child’s ecstasy declare “it’s magic”!