Mountain Bluebird (MOBL)

Mountain Bluebirds are the sky-blue members of the bluebird family (especially if your sky is the sun-drenched firmament of Colorado).  CO is the winner in the MOBL competition, with 12% of the US population, followed closely by New Mexico and Wyoming (both at 10%) then Montana, Oregon, and Utah (all at ~9%).  In CO, they breed in pinyon/juniper woodlands, such as we have here, more frequently than any other habitat.  They typically first appear here in mid- to late March and begin nest-building in mid- April; eggs appear starting early May; and hatchlings emerge a couple of weeks thereafter.  This year, though, activity seemed to start 2-3 weeks later than usual.  Speculation by a state entomologist and several fly-fishing aficionados suggested that–due to a long drought, heavy snows the previous winter, and a cold spring (we had a fire in the wood stove on Memorial Day weekend!)–all sorts of spring-related bird and insect activities lagged a bit throughout the state.  Here’s an interesting MOBL factoid from Birds of North American Online:  Of the 3 bluebird species in the US, MOBLs are the most reliant on insects all year long.  Somehow, that makes me feel a bit better that I don’t offer any of the specialized suets that many in other areas do.  (Of course, that wouldn’t rule out offering mealworms—but I don’t do that either.) You can find lots of wonderful information about bluebirds at the Sialis Web page.

MOBL male

MOBL male

The males can be a brilliant blue (how dare Sibley refer to it as “a pale blue?”) all over, although the MOBLs in our area are more likely to be a bit less brilliant, with grayer tones.  But lovely, nonetheless.  Here’s a photo of a typical male for our area, posing against the CO blue sky.  (Click on any photo to get a larger version, then use your browser’s “Back” button to return to this page.)

The females are much grayer overall, with blue primarily on their wings and tails.  Here’s a female, also posing against the CO blue sky.

MOBL female

MOBL female

This year, we had 6 nesting pairs–a high number for us.  And, amazing but true, we had 5 pairs attempt 2nd broods.  (I don’t think 2nd broods are necessarily unusual for all MOBLs—just here.)  Perhaps the late arrival of early spring insect hatches resulted in a different food supply than would normally have been available at the time of 2nd broods here.  Who knows?  Only one of the 2nd broods fledged any young, though.  Interestingly, that brood was raised by a lone—and exhausted, I would imagine—male.  After the 2nd weekly check on the eggs, I never saw the female  at the nest box again, although the male always zoomed in to make his presence known.  Let’s hear it for dads—and especially single dads—everywhere.  I don’t feed supplemental mealworms to our insectivores, since I prefer that they eat the myriad native insects available (as they have evolved to do).  As well, I rarely interfere with natural happenings at our nest boxes; offering artificial cavities is all the manipulation we usually do.  However, had I had mealworms on hand, I would have helped this dude out in a heartbeat.  As it was, though, he fledged 2 youngsters that carry his hardy, tenacious genes.

MOBL nest

MOBL nest

Okay, enough sentimentality.  Here’s a photo of a typical MOBL nest.  They construct the most sculptural nests of all of the cavity nesters in our area.  I don’t think any of our photos do them justice.  The birds have a marvelous way of swirling long strands of juniper bark to form the outer base of the nest.   The nest cup is then lined with thinner strips of juniper and whatever meager vegetation they can find in the area.  (We have very little understory to speak of, so grasses and such are hard to come by.)

The eggs are classic bluebird—a lovely blue and about 3/4″ long.  Here’s a photo of an egg from an abandoned late 2nd brood of 4 eggs.  And below that, a clutch of eggs in situ.  Clutches here are typically 4 or 5 eggs.  Unmistakable as anything but MOBLs:  Unless an unusual Western Bluebird pair is nesting (which happened for the first time last year), no other cavity nesters have blue eggs in our area.  Interestingly, many of the MOBL nests incorporated a few feathers this year–not something that is typical here.  (Books I consulted mentioned that WEBLs on occasion add feathers to their nests—which made me wonder, early on, whether we might have had some WEBL pairs nesting.  But I saw no mention of MOBLs doing so.)  In the 2nd photo, you can see a Band-tailed Pigeon feather in the lower right, which were very plentiful because we had huge flocks of these behemoths at our feeders this spring and summer.  You can also see the sculptural swirls of the nest cup, in this one made up mostly of thin strips of juniper bark.

unhatched MOBL egg

unhatched MOBL egg

MOBL nest with eggs

MOBL nest with eggs

Next, a photo of nestlings about 2 days old. (Call me crazy, but I think all bird hatchlings look rather like embryonic dinosaurs at this stage. Can you really doubt their ancestry once you’ve gotten good looks at what emerges from the eggs?)

MOBL nestlings, ~2 days old

MOBL nestlings, ~2 days old

These nestlings are about 4 days old. In addition to several Band-tailed Pigeon feathers at the top, a Northern Flicker (obviously, red-shafted) feather adds a jaunty splash of color.

MOBL nestlings, ~4 days old

MOBL nestlings, ~4 days old

At last—a photo of nestlings that are actually starting to look like MOBLs. These little fellows are about 9 days old, with pin feathers emerging on their wings while still sporting natal down on their heads. There are at least 3 in that nest, although only 2 are visible from this view.

MOBL nestlings, ~9 days old

MOBL nestlings, ~9 days old

This group is about 13 days old—the same group shown at 4 days old above. (You can still see the flicker feather at the top of the nest, although it’s looking a tad more bedraggled and less jaunty than before.) These are the oldest nestlings we have photos of. I was reluctant to open the boxes wide enough for photos once the nestlings were older than this, to avoid the possibility of premature fledging.
MOBL nestlings, ~13 days old

MOBL nestlings, ~13 days old

And in closing, one of my favorite photos of this season.  This female had just about had it with our messing with her nest.  When we approached the box for photos, she zoomed in and plunked herself down on a branch about 3 feet away from Zell, at eye level.  She stayed there while he took a photo of the nestlings.  After we had closed the box, she stayed put and Zell snapped this photo.  If looks could kill…

MOBL female outside nest box

©  2008 Tina Mitchell


8 Responses to Mountain Bluebird (MOBL)

  1. Susana Field says:

    Thank you so much Tina for your thoughtful and thorough reply! I REALLY appreciate it!

  2. Tina says:

    Hi, Susan–

    I can only speculate what might have been going on in that nest. (Even with monitoring every week, you’d be surprised how many unsolvable mysteries come up!). Possibly 1 of the adults was sterile. Eight eggs is probably too many for a single clutch. (I don’t think I ever saw more then 5 eggs, even early in the season.) I wonder if she laid a clutch, incubated it for a while, figured out they were infertile, and laid another clutch. It would be a bit odd to lay another clutch in the same nest, without at least covering the 1st set of eggs, but again–who knows? One good thing comes out of this–at least as of now, you don’t have nest box predators such as snakes, mice, or chipmunks checking out the box. If they had found this treasure, they’d have eaten them with gusto! I hope you put the eggs out where some critter could eat them. They’re full of nutrition and I always hate to waste a good meal for wildlife.

    The female checking out the nest box recently isn’t likely to be the same female, especially since you stopped seeing the pair considerably earlier. Cavity nesters are just hard-wired to check out cavities, pretty much whenever they find them. She could just have been passing through on migration and looking around. So no worries about cleaning out the nest. I would usually do that in October, when I was pretty sure the small mammals that also used our boxes (chipmunks and mice) would be finished with their breeding season. So Nov. is a fine time to do it, especially given the warm weather lately.

    And you’re right–you could have checked the box every now and then with little harm as long as your box opens from the side or the top. (I usually tried not to do it more than once a week.) Once they start building a nest, the adults get pretty dedicated to a box. And once an egg has been laid, it takes a LOT to run them off. I usually just make a small amount of noise as I approach a box to let them know I’m around and tap on the box while standing away from the hole before opening it. Both of those actions gives the female plenty of opportunity to leave if she wants to before you open the box. With a telescoping mirror (such as the kind you can find in an auto parts store), you can seen into even a rather deep nest. Just be cautious as you open the box and check with the mirror, in case the eggs or nestlings might be close to whatever side opens. I generally don’t check the box if the nestlings are more than 14 days old or are making a lot of noise. They’re probably pretty close to fledging and I don’t want to do anything that might make them leave prematurely. If you get a pair again next year, give it a try!

    Let me know if you have other questions.


  3. Susana Field says:

    Hi Tina, my husband and I live on the edge of a sagebrush-covered Mesa in Rangely CO and built a nest box for Mountain Bluebirds outside our house a few years ago. Although every year MOBL have come to check it out, this Spring for the first time nest building finally took place. We were excited but also afraid that our presence would scare them so we avoided being near them while they were active (although after reading your blogpost it sounds like we needn’t worry about that too much). But then they just stopped coming around and we never saw any fledglings. Now, it’s November 1st and my husband decided to clean out the nest box and found a nest with 8 eggs! He removed the nest, figuring it had been abandoned last Spring. Then just now a female MOBL came up to the nest box and looked in! Can you help shed light on this mystery? Thanks!!!

  4. Nice post. Haven’t thought of alot of these points before. Will come back and bookmark your site for future reference.

  5. Adhehootarfed says:

    Thanks for post. It is really imformative read.
    I really like to browse!

  6. Kathy Clark says:


    Thank you for helping Terry. He is a long-time friend of mine and Terry Neumyer is a BIRD 😀 I’d like to suggest putting your site in our next NL as ways for others to learn about BB’s around the USA. Thanks Tina!

    He’s been a great help to NABS!!


  7. Terry Neumyer says:

    Thanks for sharing your photos and dialog with the net world.

    At the North American Bluebird Society (NABS), we’re preparing a DVD for elementary school children to teach them about birds–bluebirds especially. May NABS use sections of the text in this DVD?

    Terry Neumyer
    Educational Committee Chairman, NABS


    Zell and I would be happy to help out however we can. Thanks for asking!


  8. Lawrence Herbert says:

    Nice work Tina.
    From what I have read, MOBL are regularly double-brooded.
    The male sometimes sits on the eggs too therefore helping to incubate. (I don’t know if the male has a brood patch per se,
    In the fall and winter they often wander to the Great Plains too. We have some western Missouri winter records!
    Good birding, Larry H. Joplin MO.

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