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Q: If cardinals get their red feathers from berries and insects and other things they eat, what foods make bluebirds blue?

A: That’s a good question, and the answer lies in the realm of physics. Cardinals and other red birds (and yellow and brown ones, etc.) produce their feather color from pigments in the foods they eat. But birds can’t make blue from foods like blueberries, because blue pigments are destroyed in their digestive systems. Instead, the blue on bluebirds and blue jays and others is produced by the structure of their feathers. Microscopic air pockets and crystals in each feather scatter blue light and reflect it back to our eyes, so that’s what we see, while absorbing the other color wavelengths. What we’re seeing when we look at a blue bird is not a true color, in many respects, but might be called a trick of the light.

Passing on an illness?

Q: After seeing a very sick and slow house finch at my feeders I was just about to go out to take them down and sanitize them, when a Cooper’s hawk nailed the finch. Do you think the hawk might catch whatever the finch had?

A: From your description of the house finch (crusty eyes, drooping posture), it sounds like it was suffering from the conjunctivitis that frequently afflicts this species. I checked with wildlife veterinarian Leslie Reed at the Wildlife Rehabilitation Center, who replied that the disease seems to be fairly species specific, but in a rare case, it’s possible that the hawk might become infected.

Robins ‘wasting’ worms

Q: On rainy days worms come out of the soil in large numbers and are found on my sidewalks and driveway, yet I’ve never seen robins come in to pick them off. The dead ones I can understand, but the live worms seem like a good meal for little effort. What gives?

A: Worms emerge during a rain to migrate or reproduce because they can move easily across the surface while remaining moist. Robins are big fans of juicy worms, so, like you, I’ve often wondered why so many go to waste after a rain. Duluth robin expert Laura Erickson notes that robins are active very early in the morning and can take advantage of any worms they see at first light. “I’ve watched them at dawn pigging out on worms on sidewalks and roadsides, and also on lawns. Nightcrawlers and other worms tend to go underground as the sun goes up, so the optimal time for eating them is from the moment it’s barely light enough to see them through the time when the worms retreat. Robins don’t like to stay in one place for long, so they take some here and they take some there. You could think of them as like State Fairgoers, who munch on a cheese stick here and a deep-fried candy bar there as they wander.”

Conclusion:  Robins are enjoying the rained-out worms, but they can’t eat them all, so many go to waste.

Unique mallard?

Q: I was photographing mallards in the local pond recently and one of the males seemed to have a dark blue head, not the usual green. Is this due to the angle of the light or do some mallards have this coloring?

A: I’ve seen this same effect on male mallards from time to time and it is a trick of the light. What we’re both seeing is the effect of sunlight at a certain angle making those usual green-looking heads look purple or dark blue. Like hummingbird throat feathers, the mallard’s head feathers have iridescent properties caused by tiny structures within the feathers. In the sun, most of the light bounces off but the green (or dark blue, at some angles) remains.

Choosy about eggs?

Q: I have a wood duck house at the end of my property and have noticed a female wood duck and a female merganser coming and going from it in the spring. Then, when I go to clean the box out in the fall, there are always unhatched eggs at the bottom of the nest. Does the wood duck hen refuse to incubate the merganser eggs?

A: You’re encountering the phenomenon called “egg dumping,” in which several female ducks sometimes deposit their eggs in the same cavity such as a wood duck box. Two or more females may lay their eggs inside, but only one hen incubates them. The two species’ eggs look remarkably alike, and it’s pretty dark inside a cavity, so I don’t think the female wood duck is choosing to incubate some eggs and not some others. But in these kinds of situations, there may be up to 20 or more eggs laid by several different female ducks. This is too many for one hen to keep uniformly warm, and to turn several times a day, a vital step in creating viable eggs. The adequately warmed and turned eggs will hatch into ducklings, usually a few from both species, but the eggs that were neglected won’t hatch, and these are what you’re finding in the fall.

Natural foods for orioles

Q: I put out a jar lid with grape jelly and some orange halves the minute I heard an oriole in early May and it worked like a charm, a couple of them showed up within minutes. But what would they eat if not for my jelly and oranges?

A: Baltimore orioles are big insect eaters, especially in early spring and summer, when they comb the high branches and twigs of deciduous trees for caterpillars, adult insects and spiders. They’ll even tear apart the gauzy nests of forest tent caterpillars to eat the larvae inside. Orioles relish flower nectar in the wild, when they can find it, and trumpet vine flowers, with their big helpings of nectar, are popular with orioles. As summer advances, they enjoy mulberries, raspberries, grapes and other fruits. And they feed their youngsters almost entirely on insects while they’re in the nest.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at valwrites@comcast.net.