Let me refine that last statement. I have, since then, thought a little bit about complementariness - and I think there's a bit of a paradox here. I'm going to present it and even number some of it's components - to make it appear rigorous - and present a few hypotheses to explain it. Maybe a real bird person will laugh me off my island but here goes:
Fact 1: Kittiwakes are pale surface-pickers that live with dark diving murres on steep cliffs.
Fact 2: Gloopishes are pale-ish surface-pickers that live with dark diving puffins in steep grassy cliffs.
Fact 3: Seagulls are pale part-time surface-pickers that, among many possible habitats, hang out on similar boulders right on the water as dark diving guillemots.
Assumption 1: There is enough food in the sea for everyone. As evidence I cite the phenomenal amount of animal life on this island.
Assumption 2: Habitat is a limiting factor for bird populations in the breeding season. Evidence, every single possible nook and cranny on this island has a bird in it.
Summary: Six species of bird, three habitats, two foraging strategies
Paradox: Too direct a competition for a niche often leads to one animal squeezing the other out. But if food is not a limiting factor, why can't two divers be cliff-dwelling neighbors? If space is a limiting factor, doesn't it seem that each nesting habitat should support only one species?
Half-baked Explanation 1: Evolution. I have a hunch the gloopish, seagull and kittiwake are more closely related, evolutionarily, and the murre, guillemot and puffin are more closely related. This is based on coloration (white and grey versus black), posture (horizontal, squatting versus vertical, standing), flying style (gliding versus furious flapping - feet tucked away versus feet splayed), foraging strategy (surface skimming versus diving), beak and head shape. Speciation was perhaps driven by filling habitats - as if there were three cozy pockets to slip into - the cliff-dwelling pocket, the grassy-cliff pocket, and the rock sitting pocket (can this really be cozy?), and in each pocket there was not room for the ancestral species to produce more than one animal.
Half-baked Explanation 2: Habitat Nuance. Perhaps the habitats are only superficially similar. Perhaps the murres need wider ledges and cannot use the little things that kittiwakes build their nests on. Puffins, being burrowers are certainly pickier than gloopishes, who can nest on any pile of dirt they find - though maybe they need a more rocky fundament? Seagulls and guillemots are both weak links in any case. Seagulls nest on rocks right on the seagull rookery. Some nest in the open on the grass in the hills. Some nest on loamy ledges in the landslide area. As for guillemots, I honestly have no idea where they nest. I just see them spend their days squeaking on the rocks and talking fashion.
I don't know. I think this is interesting. I wonder if there are other examples or if this has already been dealt with by the bird people? How do people go about "testing" evolutionary hypotheses anyhow?
July 16, 2004
