Something has happened to Bufflehead feeding ecology this winter, that I’ve never seen before, and it is certainly due to the benthic invertebrate community and the algae that sustains it. The change is apparent in the feeding behaviour of individuals and sex/age segregated groups. Whereas their main mode of feeding is by themselves, in autumn and early winter, they engage in co-operative feeding, by churning up kelp debris that accumulates in certain areas in the deeper waters of the mouth ( drakes and juvenile males), or along the lee southeast shore (juvenile females). I’m very familiar with this latter group as they are constantly in view beneath my house. Typically this young hen pack number ten to fifteen individuals, and they depend on the kelp windrow that follows the southeast curve of the beach, and this co-operative mode ceases by mid-winter. This season the group numbers thirty to forty, overseen by a single drake and his consort. ( Buffleheads have a complex society ).
Anyway, this group has been feeding intensively in a couple particular areas over the tide flats, one within sixty metres. Occasionally, large clumps of Wigeon also feed over the same area, picking up the Sea Lettuce that the Buffleheads churn to the surface. Although most of the Bufflehead feeding is at the bottom, they sometimes pick up organisms that comes to the surface attached to the kelp. Last evening, they were still feeding intensively at dusk and since the tide was low, I put on waders and grabbed an aquarium net to sample the feeding spot. The results were nothing short of spectacular, and demonstrated something new to me.
Previous gut samples of Buffleheads, obtained from Otter kills, indicated that the basis of Bufflehead feeding ecology in this detritivore community involved small amphipods (scuds), as compared to the Green Tanaid community that dominates individual diets over the mudflats. As documented in previous blogs, the large co-operative groups at the mouth of the bay that include other species ( Mergansers) revolve around the Broken-back Shrimp group, up to a couple centimetres in size.
I took two samples in the dark and it was apparent that the Buffleheads were feeding near the margins of a patch of Sea Lettuce. The differences are startling. The plate 1 shows the plankton haul through the focus of the feeding area whereas plate 2 shows results from its periphery.
Clearly, the young hen coo-operative is feasting on an abundance of very small specimens of Broken-back Shrimp (Heptacarpus, I think), as well as a few small scuds (left side) and mysids (top). By contrast, the bottom haul shows the dominance of the larger scuds which cling tenaciously to the kelp.
Even though the autumn was not especially stormy, with, presumably, less kelp detritus, the offshore drake co-operative remained strong into early December, indicating and abundance of the larger detritivores.
This is new to me in some thirty years of observation. I don’t know whether it has anything to do with the loss of the Mud Shrimp community, though I doubt it. As the sun sets this evening, the Gals are feeding intensely exactly where they were last night. Its hard to imagine how that small patch can continue to sustain such intensive predation.
Bufflehead gals won’tcha come out tonight, come out tonight…