During the month of June, Darwin’s diary was relatively quiet with only 6 short entries all month. Not surprisingly, almost all of these entries had something to say about collecting specimens of birds, reptiles and mammals. I guess the only real surprise is that (although he mentions insects briefly) he has very little to say in his diary or his Zoological Notebook about the many invertebrates that he is typically so fond of. The month starts with:
“The weather generally has been boisterous, so that very little work has been done with the Schooner as it is impossible to heave down to get to her bottom without quite calm weather. The delay is to me agreeable, although not serviceable, as there is not much more to be collected: — Birds insects & reptiles have been my chief game.” (June 1-7)
One of the bird species that he encounters throughout the summer and into the fall is what Darwin calls the “scissor beak”, because of its unusual (and unique) beak structure. We now know call this species a a black skimmer – Rynchops niger. Since he mentions them over a period of several months, today I’ll share the synthesis that we later wrote in Voyage of the Beagle. First, a brief description:
“I here saw a very extraordinary bird, called the Scissor-beak (Rhynchops nigra). It has short legs, web feet, extremely long-pointed wings, and is of about the size of a tern. The beak is flattened laterally, that is, in a plane at right angles to that of a spoonbill or duck. It is as flat and elastic as an ivory paper-cutter, and the lower mandible, differently from every other bird, is an inch and a half longer than the upper.” (Voyage of the Beagle)
Darwin’s sketch of Rynchops from Voyage (notice Darwin spells the genus name a little differently):
Black skimmers are closely related to gulls, terms, murres, skuas, auks, oystercatchers, and waders – many of your typical “shorebirds” – all of which are part of the Order Charadriiformes. There are actually three species of skimmer – Black, African and Indian. Black skimmers are found along the Atlantic coast of North Americas and in much of South America east of the Andes. They are geographically divided into three subspecies, of which Darwin encountered the southernmost – Rynchops niger intercedens.
Black Skimmer (from Wikipedia Commons):
He goes on to artistically describe in some detail the fishing habits of these unique birds:
“In a lake near Maldonado, from which the water had been nearly drained, and which, in consequence, swarmed with small fry, I saw several of these birds, generally in small flocks, flying rapidly backwards and forwards close to the surface of the lake. They kept their bills wide open, and the lower mandible half buried in the water. Thus skimming the surface, they ploughed it in their course: the water was quite smooth, and it formed a most curious spectacle to behold a flock, each bird leaving its narrow wake on the mirror-like surface. In their flight they frequently twist about with extreme quickness, and dexterously manage with their projecting lower mandible to plough up small fish, which are secured by the upper and shorter half of their scissor-like bills. This fact I repeatedly saw, as, like swallows, they continued to fly backwards and forwards close before me. Occasionally when leaving the surface of the water their flight was wild, irregular, and rapid; they then uttered loud harsh cries. When these birds are fishing, the advantage of the long primary feathers of their wings, in keeping them dry, is very evident. When thus employed, their forms resemble the symbol by which many artists represent marine birds. Their tails are much used in steering their irregular course.” (Voyage of the Beagle)
Black Skimmer fishing (from Wikipedia Commons):
As is frequently (though not always) the case, Darwin observations are right on. Rynchops uses its unique beak structure to skim the surface of the water and scoop up food – typically small fish, but also insects and small crustateans.
The skimmers are the only birds to have such a noticeably longer lower beak. And the black skimmers have another unique characteristic among birds – a vertical pupil, similar to that of a cat. It is suggested that this is an adaptation that allows them to focus their binocular vision on their beak while skimming the water – making them very effective “fishermen”.
Footage of black skimmers from YouTube – about halfway through is great footage of the birds fishing (documentary source is not cited):
Darwin ends this section of the book with a description from a few months later in the trip (yeah, I’m breaking the rules a little, but I’m sure I’ll forget to come back to it later):
“These birds are common far inland along the course of the Rio Parana; it is said that they remain here during the whole year, and breed in the marshes. During the day they rest in flocks on the grassy plains, at some distance from the water. Being at anchor, as I have said, in one of the deep creeks between the islands of the Parana, as the evening drew to a close, one of these scissor-beaks suddenly appeared. The water was quite still, and many little fish were rising. The bird continued for a long time to skim the surface, flying in its wild and irregular manner up and down the narrow canal, now dark with the growing night and the shadows of the overhanging trees. At Monte Video, I observed that some large flocks during the day remained on the mud-banks at the head of the harbour, in the same manner as on the grassy plains near the Parana; and every evening they took flight seaward. From these facts I suspect that the Rhynchops generally fishes by night, at which time many of the lower animals come most abundantly to the surface. M. Lesson states that he has seen these birds opening the shells of the mactræ buried in the sand-banks on the coast of Chile: from their weak bills, with the lower mandible so much projecting, their short legs and long wings, it is very improbable that this can be a general habit.” (Voyage of the Beagle)
Although black skimmers fish throughout the day, Darwin is correct in his observation that they tend to be most active at dusk/dawn and at night. Good binocular vision aside, their beak structure allows the to fish “by touch” – snapping their beak shut the instant it comes in contact with food.
It is always a pleasure to see the wheels turning in Darwin’s head – drawing conclusions, asking questions, thinking of the big picture, etc. Every species he describes seems to bring out a little bit of Darwin himself. In many ways, it is similar to how the lifework of an author paints a picture of his or her life. It is one of the things that gives Darwin so much depth. (RJV)
PS – Just to complete the picture of Rynchops – visit the Cornell Lab of Ornithology to hear the sounds made by the black skimmer.