On The Trails: A rescue; an odd association; bird flocks

An Alaskan hermit (Pagurus brandtii) crab without a protective shell. The crab would insert its long body into a suitably sized shell and protect the opening with its large claw. This species is often associated with those special sponges. (Photo by Aaron Baldwin)

By Mary F. Willson

My hummingbird feeder is an ordinary one, with four red plastic flowers marking the accessible nectar ports, and there has been a lot of activity this summer. One August day I looked out to check the amount of nectar remaining and noticed a dark blob floating the nectar. Dirt?? Mold?? But I had cleaned the feeder the previous day. So I took down the feeder, poured out the blob and put it on the deck railing. It wriggled! And it turned out to be a small wasp that had — for some reason — squeezed through one of the nectar ports and crawled in. No future there!

The wasp on the railing was not wriggling much. A visiting friend decided to rinse off the sticky nectar and gave the wasp a little shower of clean water. The wasp then began to clean itself, using its legs to wipe down the wings and body. After a good grooming session and some waving of wings, it took off — to whatever future it may find.

While on a kayak-camping trip in the Beardslee Islands in Glacier Bay, a friend found a rather unusual specimen in the intertidal kelp beds. A hermit crab had found a home in a snail shell (possibly a whelk), as they often do. But this specimen was covered by a sponge with the scientific name of Suberites domuncula. The color of this sponge varies greatly, but it is often orange. Here in Alaska, we have two species of hermit crabs that are found almost exclusively with these sponges, although elsewhere the association may not be so close. In a broad sense, the association can be called a commensalism, a word that refers to "eating at the same table." The sponge can grow on rocks, snail shells, and crab carapaces. If a crab already has a ‘home’ in a snail shell, the sponge can settle over the snail shell. Although there have been reports that the sponge may dissolve the shell, this is disputed. 

What might the hermits and the sponge gain from the non-obligatory commensal association? In this case, the commensal association can be somewhat symbiotic, with both participants gaining something. The sponge gets a site for settlement, some mobility, and may also feed on floating bits left over from crab feeding. The hermit crab gets a form of camouflage. If the sponge on the crab’s carapace becomes really big, it makes it impossible for the crab to fit into another snail shell, so the sponge itself then becomes a substitute living space. The sponge is said to bear anti-microbial compounds as well as highly toxic neurotoxins that can be lethal to fish and some other critters (but not hawksbill turtles). Those chemicals may protect the sponge — but are there consequences for the hermit crab?

An intertidal specimen of the sponge that often settles on shells and hermit crabs. (Photo by KM Hocker)

Hermit crabs are known to find "houses" not just in mollusk shells, but also in corals, bryozoans, and anemones. In fact, one group of hermits lives only in anemone "houses."  Clearly, this kind of association can be very complex, leaving me with more questions than answers.

On a recent stroll in the Dredge Lakes area, a friend and I came across a great fluttering in the alder trees. Small birds flitted to and fro, talking quietly. This group was comprised of chickadees, a nuthatch, and two or three kinds of warblers. Such mixed-species flocks often occur in fall and winter in the temperate zone.

In our region the flocks contain quite a variety of species. Chickadees are often considered to be ‘"nuclear" species, their calls, vigilance, and activities leading to flock formation and movement patterns. They may be joined not only by nuthatches and warblers, but also kinglets, creepers, and downy woodpeckers. Why do such flocks form? 

There are two principal, non-exclusive hypotheses. One idea is that birds in a flock provide many eyes to be on the lookout for predators (although the flock’s activities might also draw some predatory attention). That leaves more time and attention for foraging. This idea applies to any flock of the same or mixed species.

In a mixed-species flock, even when all of the participants are insectivores, there is less potential competition for prey than in a single-species flock, because the species have different foraging habits, at least to some degree. Also, sometimes the foraging of one species disturbs the prey, making it available to others. Or divergent foraging patterns or pathways in the foliage might lead to the discovery of new prey sources. Research has provided evidence for both of these ideas, at various times and places. 

Thanks to Aaron Baldwin for helpful consultation.

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent.