The Magic School Bus set me up for a lifetime of disappointment. In this excellent kids’ book and TV series, students set off on wild field trips in a bus that could transform into a spaceship or a submarine or . . . it could also shrink! In the real world, I can use binoculars to see birds a bit closer, but I wish I could be small enough to sit on a goldenrod flower and watch Swamp Sparrows pluck fluffy little seeds the size of a letter on this screen! How many of those seeds do they eat? How do they choose which seeds to take? Do they eat the fluffy part or remove it?
These are some of my many questions that may remain unanswered due to my tragic inability to shrink to the size of a bumble bee. More than anything, I want to know what it’s like to forage as a bird. Birds have to search for food constantly, and they seem to be finding it where I see nary a seed nor a bug. How and where are they finding all this food? Although we can’t peek over their shoulders while they forage, getting up close and personal with their habitats and physiology can reveal how birds are finding enough calories for long migrations and cold Maine winters.
With most flowers having gone to seed, there are fewer insects buzzing around these days, yet birds are still finding lots of juicy protein. These morsels are mostly larvae. Before the adult beetles, wasps, flies, and others ceased their buzzing, they laid eggs on or inside plants. When those eggs hatch, larvae start chowing down. There’s a Red Oak at Gilsland Farm that has low branches which offer flightless humans a chance to hunt for insects. Most leaves are in rough shape. The tree is a showroom for different larval diets and feeding behaviors. Some caterpillars (the larvae of butterflies and moths), have eaten around the margins of leaves, while others chewed lacy patterns between the veins. The larvae of many beetles, flies, sawflies, and some moths tunneled between the upper and lower surfaces of leaves, leaving trails or patches of dead tissue in their wake. These species are collectively known as leafminers.
Many birds, including Black-capped Chickadees, can learn to look for fresh leaf damage to find the insects that caused it. They check rolled leaves and leaves stuck together with silk, indications of pupating larvae. Recently, I spotted a Nashville Warbler probing a rolled oak leaf. It appeared to come up short, moving on with only a silk-covered bill to show for its efforts. Although not a sure sign of food, recognizing these visual clues is a good place to start.
Insects do their best to hide, blend in, and otherwise escape notice from predators, but birds are still very good at finding them. The first obvious-yet-perhaps-overlooked advantage is that birds are quite close to these insects. Scan plants with a magnifying glass, and you start noticing critters everywhere.
I turned over a dried goldenrod leaf the other day to reveal an orange rust (a powdery fungus), as well as the tiny larva of a rust-eating midge. The sparrows who haunt these meadows might come for the seeds and pick up a little extra protein while they’re here.
Birds also scan foliage for galls, abnormal plant growths often caused by insects. When gall-forming insects lay eggs in plant tissue, enzymes produced by the adult or in the saliva of feeding larvae cause plant cells to rapidly multiply around the young ones. These malformed cells provide both shelter and food in the form of nutritious plant tissue. A larva inside a gall will typically molt several times and then overwinter in their chamber or drop to the ground and pupate in the leaves or soil.
Some growths, like sunflower-shaped goldenrod bunch gall, are quite conspicuous to humans, but others, like the small swollen growths on goldenrod flowers caused by Schizomyia racemicola (pictured right), are quite subtle. Still, birds are able to find these abnormalities and the larva hiding within.
They owe this success to both their proximity and their incredible visual acuity. Acuity refers to sharpness and clarity. Depending on species, birds might have visual acuity up to eight times higher than that of humans. Raptors put this to good use spotting small prey from far away. It helps foraging songbirds spot the inconsistencies between a camouflaged caterpillar and its background or a seed in dirt. Birds have large eyes in relation to their bodies, which let in lots of light and have many more photoreceptor cells than ours. This results in a higher resolution image. They also have higher focusing power due to their ability to change the shape of not only the eye’s lens (we can do that too), but also their cornea (we can’t do that!)
Additionally, most birds have four types of cone cells in their eyes; we only have three. Cone cells are responsible for color vision. The fourth cone allows birds to see longer wavelengths of light than we can, specifically those on the ultraviolet spectrum. Conveniently, a lot of their food reflects UV light, which helps it stand out against non-UV-reflecting leaves and soil. Some fruits develop a powdery coating, known as “bloom,” when they’re ripe; you can see this on blueberries and plums (it’s what you might rub off an apple to make it shinier before taking a bite). The bloom scatters UV light, making it glow against green leaves. While there’s discussion around the bloom’s function for the plant, it can be a beacon for birds, telling them that fruit is ripe and ready.
The waxy coatings on many seeds also reflect UV light, as do many insects. Some researchers are finding that using UV flashlights to survey caterpillars is both easier and more fun than painstakingly combing through vegetation (who’s down for a UV Bug Night next summer?)
Unless it’s living near the equator, a bird’s habitat and diet often change significantly over the course of a year, and they adapt with “plastic” brains. Studies on migratory and nonmigratory birds show that the hippocampus, the region responsible for spatial memory, can grow in adulthood. Black-capped Chickadees grow new brain cells in the fall every time they cache a seed. All told, a chickadee’s hippocampus can grow by up to 30% to help it remember the location of tens of thousands of hidden food items over the winter. In the spring, when insects emerge, their hippocampus shrinks back to its original size. Only some birds have this brain elasticity.
Other birds show higher spatial memory related to migratory behavior. A 2006 study of White-crowned Sparrows found that a migratory subspecies had more hippocampal neurons than a nonmigratory subspecies. This difference was apparent in juveniles of both subspecies, but was more pronounced in adults. The increased capacity for spatial memory may help them navigate to the same territories year after year, locate important stopover sites along the way, and remember productive trees and bushes in both breeding and nonbreeding habitats. Oak trees support nearly one thousand species of caterpillars, so it behooves migrating songbirds to remember, for example, that big Red Oak at Gilsland Farm when they pass through in the spring.
Learning about birds can be quite humbling, but keep in mind that they’ve been evolving these remarkable adaptations for millions of years. Humans are just getting started a mere 300,000 years in. At least we got to the point of inventing binoculars and magnifying glasses. Maybe in a few hundred years we’ll get around to that shrinking school bus!
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