One Species or Two? The Grebes of the Sacramento National Wildlife Refuge

One of these is not like the others. Can you tell which one is which?

The concept of a species has apparently always been a problem in biology. The definition I was taught, the idea that a species is a group of organisms that can produce viable offspring, is a real problem in my own field of geology and paleontology. How can we know if a species was capable of successfully reproducing from fossil evidence? We depend on the morphology a great deal, but it often produces problems. Consider what future paleontologists will think about fossils of all the dog breeds in existence...would a Chihuahua be considered the same species as a German Shepherd? There are debates about whether the large number of species of ceratopsian dinosaurs might actually be growth stages of a single species.

As we learn more, the idea of boundaries between species becomes even more uncertain. There are some species that are more closely related than others, and hybridization is a common phenomenon. Such hybrids may be a powerful driver of evolutionary change. Which brings us to these unique birds in the pictures today. They include a single picture of some Clark's Grebes (Aechmophorus clarkii), and some Western Grebes (Aechmophorus occidentalis). We saw them in the ponds of the Sacramento National Wildlife Refuge near Willows, California, on the first day of our recent journey through British Columbia and northern Washington state. What criteria did you use to distinguish between the two?

Biologists considered them to be the same species until 1985, when they split them because they didn't tend to interbreed, had different calls, occupied slightly different niches in their wetlands environments, and on major differences in their DNA sequences. The Western Grebes are more numerous (~110,000 vs ~11,000), but both are threatened by loss of their wetland environments. The most visible differences are the eyes, which are surrounded by black in the Western Grebe and white in the Clark's Grebe. The beak of the Clark's Grebe is brighter yellow while the Western's is more olive in color.

The Sacramento National Wildlife Refuge is one of our favorite birding spots. It has spectacular seasons in fall and winter when the migrant ducks, geese, and cranes arrive from the Arctic. The birds gather in flocks 10,000 strong, and few sights are as memorable as seeing all of the Snow or Ross's Geese taking off at once. Summer is much quieter, but is a good time to view the year-round species and the raptors. I noted 26 different species on our late June visit.

Western Grebes at Clear Lake in Northern California

I'm only now getting through all of the pictures I got during my Christmas journeys. It started with the Tundra Swans that we saw near Williams. We were preparing to cross through the Coast Ranges to get to Highway 101, and Clear Lake offered the most interesting way, bird-wise and geology-wise. Clear Lake is partially dammed by lava flows, and the Mt. Konocti volcano looms over the waters. It's only a few thousand years old and magma chambers in the region are still simmering deep underground.

There is a pier and boat landing at Lucerne that is a favorite stop. We've almost always seen something interesting from there and we weren't disappointed. There were Bonaparte's Gulls by the hundreds, and lots of Pied-billed Grebes and the ever-present Mallards. And lurking close to the pier were several Western Grebes (Aechmophorus occeidentalis). I was able to get a few nice shots.

I always have to remind myself which grebe I'm looking at. The Western Grebe and the Clarke's Grebe were once thought to be the same species, but there were enough differences that they were officially separated. The main tell in the field is that a Clarke's Grebe has white around the striking red eye, while the Western Grebe has black.

Western Grebe at Clear Lake (and some neat ripples)

I wasn't actually out birding or anything like that today, I was just taking a short break from a long drive. But we stopped for the break at Clear Lake, an interesting natural lake in the Coast Ranges north of the Bay Area. It is the largest natural lake entirely in California (Tahoe is larger, but partly in Nevada; Salton Sea is larger, but was an accident). According to Wikipedia, the lake covers 68 square miles and averages 27 feet in depth (greatest depth is 60 feet). It is in an area of active volcanism, with magma-related geothermal developments nearby, and some volcanoes that are only 10,000 years old  (Mt. Konockti, see picture below). The volcanism is kind of a surprise, since there are no subduction zones or divergent boundaries nearby.

The Western Grebes (Aechmophorus occidentalis) and the closely related Clarke's Grebe are pretty much purely aquatic birds (their legs are set so far back that they are extremely clumsy on land). They live and breed in inland freshwater lakes in summer, and spend winters at sea along the Pacific coast. The two species were once thought to be one, but there are physical and genetic differences, and they very rarely interbreed.

I was photographing the birds for their own sake, of course, but later I noticed the incredibly symmetrical ripples around the bird in the first two pictures. I don't know how I missed them at the time I was snapping the pictures.

I did take a moment to photograph the volcano. It was a pretty sight today (previous trips had been in bad weather). Clear Lake has a few problems, but it is an underrated treasure in the natural landscape of California.