Mid-May, and golden-crowned sparrows are everywhere, dodging about in the brush, and feeding on their northward migration. I’ve seen them pecking at seeds, and there’s a report from Ketchikan of over two hundred and fifty of them pecking away in one small patch of backyard. Then a friend told me that someone in Tenakee had watched their young broccoli and cauliflower plants being demolished by these sparrows; lettuce and carrots seem to be less popular. I’ve seen them gobble up the spore-producing ‘cones’ of horsetails too—would that they‘d eat more of ‘em!
I think of sparrows as eating chiefly insects during the breeding season and mostly seeds in the off-season. So it came as a surprise (to me) that, in addition to eating some insects and seeds, golden-crowns might be serious leaf-eaters. However, the literature is full of decades-old reports of intensive leaf-eating by golden-crowns. They love newly sprouted lawn grass, weed seedlings, and—gardeners beware—leaves of annual flowers such as primulas and pansies, and young plants of the cabbage family (broccoli, cauliflower, and so on), beets, and peas. Evidently they have a very different diet than that of their closest relatives, such as the white-crowned sparrow.
Leaves are generally a difficult food for vertebrates to process, compared to, say, meat. The cell walls of plants are composed of complex molecules such as cellulose and lignin, but vertebrates lack the enzymes needed to digest them well. Therefore, much of the material ingested by an herbivore is not readily accessible to the consumer, which then relies on the contents of the cells. Yet there are many kinds of vertebrate herbivores—and they have solved the digestibility problem in a variety of ways. Many of them have symbiotic bacteria and protozoans, commonly housed in special chambers in the digestive tract. The microbes can break down the complex molecules, and may also synthesize certain vitamins and amino acids. Some mammals house the helpful microbes in complex stomachs (for example, cows, sheep, kangaroos, deer) and typically regurgitate a cud of partially digested material for a second chewing. Others house the microbes in pouches (called ceca) attached to the gut or in the large intestine itself (for example, horses, elephants, howler monkeys, beavers). Still others rely simply on eating large quantities of plant material, being quite selective of high-quality forage when possible, and passing it through the digestive tract quickly, thus making room for more (for example, the giant panda).
A similar diversity of adaptations is found among birds. Grouse and ptarmigan have large microbe-inhabited ceca and, in addition, their intestines elongate in winter, when their diet includes quantities of difficult-to-digest plant fiber. Geese have smaller ceca, but ingest great amounts of plant material, passing it through the gut fairly quickly; when feasible, they are also quite selective of high-nitrogen plants. In the late 1900s, it was discovered that a peculiar South American bird known as the hoatzin has a modified crop and esophagus in which symbiotic microbes help break down plant tissues (this is the only known bird with such modifications at the front end of its digestive system, in contrast to the many mammals with a similar arrangement).
For a long time, it was thought that only relatively large birds could be plant-eaters, because they commonly need to process so much material every day. However, in South America there are three species of leaf-eating birds called plantcutters, which are quite small, weighing roughly forty-two grams (about an ounce and a half). They don’t seem to have special symbiotic microbes in their system at all. How do they manage?? Recent studies show that, like many other avian herbivores, plantcutters tend to select high-protein leaves and pass the material quickly through the gut. In addition, they chew their leaves, using a serrated bill, which may help break down the plant tissue. Perhaps most importantly, they have unusually high levels of enzyme activity in the intestine, so they can extract the soluble nutrients very effectively.
That brings us to back to golden-crowned sparrows, which weigh only about twenty-nine grams (about one ounce). They are highly herbivorous, especially outside of the nesting season, but how do they manage to do this? There doesn’t seem to be any information in the literature on this subject! There is a report that they chop up fresh greens between the edges of their bills, in effect chewing their food a bit. But that’s all we seem to know about how this small leaf-eater manages its difficult diet.