Phalaropes by Rubega and Obst (1993):
If the proper balance were struck between cohesion of the drop and its adhesion to the bird's beak, progressive mandibular spreading would increase the amount of potential energy stored in the surface of the drop by increasing the free surface area of the drop. Because physical systems move to reduce potential energy (unless this tendency is opposed), the drop would move in the direction that was most likely to reduce free surface area. As long as drop positions more proximal to the gape result in smaller free surface areas and, hence, lower energy states, than those at the distal end of the bill, the drop should move toward the buccal cavity.
In other words, the droplet of water held in the bill sticks to both mandibles. As the jaws spread, the droplet is stretched out. This creates tension, which forces the droplet up the bill to reduce the distance between mandibles and the surface tension of the droplet. Rubega and Obst filmed phalaropes in the lab, using both living birds capturing prey, and specimen bills to test the mechanism and successfully eliminated tongue movement and suction as potential explanations: the dead bill still draws water drops with no tongue movement, and living birds demonstrated no throat pumping associated with sucking water up the bill.
Rubega and Obst’s laboratory studies have shown this mechanism can take less than a tenth of a second! This mechanism is discussed by both Rubega and Obst (1993) and Estrella et al. (2007) in terms of feeding efficiency, in that it allows shorebirds to feed on small prey items rapidly and with minimal energy expenditure in prey handling.
Estrella et al. (2007) extended the laboratory studies to the field, recording a total of six species in the wild (Little Stint, Dunlin, Sanderling, Curlew Sandpiper, Common Redshank, Black-winged Stilt; the study are was in Spain) adding to the four species shown in the lab to demonstrate STT (Red-necked Phalarope, Wilson’s Phalarope, Western Sandpiper, Least Sandpiper) to give a total of at least ten species in two families demonstrating this very interesting feeding method. It is very likely to be widespread in the shorebirds as a group.
This field study begans to illustrate the constraints of bill structure on the use of STT hypothesized by Rubega and Obst (1993). Estrella et al. (2007) observed Redshank and Stilt using head jerks to assist the motion up the bill. The bill must gradually widen towards the head for STT to work, and the authors note that Redshank and Stilt have fairly uniform narrow bill widths along their length.
Interestingly enough, their data shows variability by species in time per cycle (one opening movement of the bill, several cycles necessary to draw all the way up the bill) but the overall total transport time per prey item does not vary according to bill length. This indicates internal bill morphology dictates differences in STT function between species, not overall bill length.
This is a fascinating feeding mechanism that I was previously unaware of. With the demonstration by Estrella et al. (2007) of surface tension transport’s widespread use in the field, the door is open for many more novel and interesting studies on bill morphology and foraging bioenergetics with shorebirds.Finally, go here for some truly excellent photography of shorebirds demonstrating STT.
Margaret A. Rubega and Bryan S. Obst. 1993. Surface-tension feeding in phalaropes: discovery of a novel feeding mechanism. The Auk. 110(2): 169-178. April 1993. (PDF)