An eastern bluebird brings a pine needle to use in nesting box last week in Freeport. AP Photo/Robert F. Bukaty

It still feels early, but we are quickly heading into the nesting season and some early birds have already gotten a start on their first clutch. Raptors like owls and eagles start nesting as early as January because it takes so long to raise their young, but even some songbirds are getting started now with the hopes that they can sneak in multiple broods this year.

Eastern bluebirds are in the category of early nesters, with some of the most ambitious ones already sitting on eggs. This leads to a question sent in by Deb Hart, who noticed that her bluebirds were laying white eggs rather than their traditional blue ones.

My favorite thing about birding is that there is always something to learn, and getting into the details of intraspecific egg variation was definitely a new one for me. But thankfully the research has been done and I’ll give a big tip of the cap to Lynn Siefferman at Auburn University who has published some studies that I’ll share highlights from here. First, it is quite rare for Eastern bluebirds to lay white eggs. Their eggs, typically 3-6 in a clutch, are typically a pale blue or blue-green color, and Siefferman found that less than 2% of eggs (in her Alabama-based study) were white, and less than 1% were pink.

With some speculation as to why there would be color differences, Siefferman did a series of studies to see if there were any benefits, particularly in helping detect brood parasites. Would making eggs a different color help them somehow? It is pretty common for brown-headed cowbirds, European starlings and house sparrows to parasitize a bluebird’s nest (laying one of their eggs in the nest and attempting to trick the bluebird into raising it). The species, especially the latter two of those, will often take over the whole nesting site for themselves. In the cases of eggs just being dropped in, there was no indication that color played a role in detecting those unwanted eggs. (In many cases, they can be detected though we don’t know exactly how, and the bluebird will build a new nest over the top of the other one.)

The answer, as Siefferman found, lay in the health and age of the female eastern bluebirds. The study found that as body condition increased, so did the amount of blue-green in the eggs. Determining the age of a bluebird can be difficult after 2 years of age, but this study found that young (1 year old or “second year”) females laid less-pigmented eggs than older females.

The last fun fact: bluebirds don’t produce any blue pigment in their feathers. The color we see as blue in their plumage is a “structural color” in that blue gets refracted while the other colors are absorbed. In the eggs, however, the bluebirds produce biliverdin, a blue pigment in their shell gland. Interesting that they can produce this “actual” color in their eggs and that “structural” color in their plumage.

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TIPS FOR ATTRACTING THE BIRDS YOU WANT

One perennial topic for people who attract birds to their yards with feeders is how to attract only the ones they want. This question comes in every year, especially in the spring (and often again in the fall) when blackbirds are moving in large flocks and can descend on feeders by the dozens. It doesn’t take long for a mixed flock of 100 blackbirds, cowbirds and grackles to clean out every seed you’ve put out, which can get expensive. So here are some tips on ways to hopefully narrow down the list of birds you’re serving.

I’ll give my preferred answer first: feed them all! You’ve put out bird feeders to attract birds to your yard, but now you don’t want birds? Kidding aside, I think many people are overlooking the beauty and complexity of some of our more “common” or familiar species. One of the most frequently misidentified birds is a female red-winged blackbird. They’re smaller than the males, brown and streaky, and superficially look a lot more like a sparrow than a blackbird. And common grackles are absolutely stunning. Following up on the structural color we talked about above with bluebirds, check out the iridescence on these grackles.

If you’re still not sold on “all birds,” diversifying your feeders and seeds is your next step. Finding feeders that are specific to certain species will help. Hummingbird feeders are the obvious example, but switching to a feeder with smaller openings, like a tube feeder, and using thistle (a tiny seed) will attract birds like finches and will not be appealing to grackles. Always keep in mind that a bird’s beak is basically its utensils, so finding feeders that exclude that size beak will help keep those birds away. You can also selectively narrow the size of where the bird can sit or perch; some feeders will have adjustable roofs or domes over the seed so you can lower it to the point where a large grackle can’t fit.

Always keep in mind that bird feeding often takes a bit of trial and error to find out what works for you, or more specifically, for your local birds. Some combination of all the recommendations above, after years of testing, and I bet you’ll find just the right setup to get what you want. Or you’ll give up and join me in enjoying all the winged creatures that descend on your feeders!

Do you have a nature question for Doug? Email questions to [email protected] and visit maineaudubon.org to learn more about backyard birding, native plants, and programs and events focusing on Maine wildlife and habitat. Doug leads free bird walks on Thursday mornings, 7 to 9 a.m., at the Gilsland Farm Audubon Sanctuary in Falmouth.


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