In females during the nesting season, I often see what appears to be tiny spiders floating around in the liquid in their crops (the skin and esophageal wall are extremely thin and so close to transparent that a well-filled crop looks like a goth snow globe). The habitat along the San Pedro River is rich in other invertebrates, including gnats and leafhoppers that also show up in crop contents, so are the females hunting spiders preferentially, and if so, why? Maybe to build smarter, more competitive offspring. Hummingbirds have pretty large brains anyway, but feeding them a diet rich in the amino acid taurine, of which spiders are an excellent source, may be a way that mothers build brain quality into their young.
I have yet to find a way to sample what the birds I band are eating without putting their safety at risk, so I’m not certain of either the identification of the critters in their crops or the proportion of one prey item to another, but the article seems to support this casual observation and provide a possible reason behind it.
And speaking of large brains, I have to thank GrrlScientist for leading me to the article cited above through a link to an article on brain size and evolution in her regular Birds in the News feature.
Large brains have long been hypothesized to favor species diversification by giving their possessors the behavioral flexibility needed to exploit new ecological opportunities more rapidly and efficiently than smaller-brained competitors, and now there’s evidence to support this “behavioral drive” hypothesis. Ecologist Daniel Sol of CREAF-Autonomous University of Barcelona and evolutionary biologist Trevor Price of the University of Chicago analyzed measurements of 7,209 bird species and found that those bird families that show the greatest diversity in body size tend to have proportionally large brains.
The use of encephalization as a measure of intelligence is somewhat controversial, but one study found that the brains of hummingbirds averaged 2.5 times as large as those of gallinaceous birds (which include chickens, turkeys, quail, and pheasants). Though we’re still working out how hummingbirds brains are organized and how much space is devoted to the thinking/learning parts of the brain versus those that control the birds’ complex motor and sensory functions, we know that hummingbirds can learn songs, keep track of what flowers they’ve visited and how long they take to refill, and remember feeders and flower patches along their migration routes from year to year.
The natural limits on how large or small a hummingbird can be make the size range of living hummingbird species a poor measure of evolutionary diversification, and diversity of plumage, behavior, and ecology are harder to quantify for comparison to brain size, but these factors plus the large size of the Trochilidae (approximately 330 species, one of the largest bird families in the world) and their long life spans suggest that hummingbirds have seen big-brain evolutionary benefits comparable to those Sol and Price attribute to corvids (ravens, crows, jays, magpies) and parrots.