2007
Western Bluebirds (WEBL) most commonly breed in CA, NM, and AZ, where 35%, 21%, and 17.8% of the population can be found. CO comes in a far-distant 4th, with a measly 3%. In our area, Mountain Bluebirds (MOBLs) have used our nest boxes since 2000, when we put up our first 4 boxes. (We now have 120+.) In mid-April, 2007, though, we saw our first WEBL—actually, a pair—just outside our living room windows. I was very surprised and asked our ornithologist friend what we should think of their appearance there. He said, kindly, that we should be just very grateful to have seen this pair passing through on their way to somewhere more appropriate for breeding. They weren’t going to nest here. Alas.
In late May, Zell was walking around the property and spotted a pair of birds obviously feeding nestlings in one of our boxes. He didn’t have his binoculars with him—now really, who goes outside during breeding season without binos?—so he headed back to the house to describe the birds to me. He said some kind of odd bird seemed to be helping a female bluebird
feed nestlings. I had no idea what he could be talking about; we grabbed our binos, scooped up the camera, and headed out to the spot. Lo and behold—a pair of WEBLs, taking turns bringing various insects and caterpillars to the noisy kids. (Up to that point, I had only seen the female around the box and had just assumed that she was a MOBL. My bad for not looking more closely!) The male had a solid blue back, with a wash of rufous across the chest and a blue belly. (According to Sibley, some WEBLs have the chestnut coloring on their backs; this dude just had it up to his scapulars.) Zell took some photos of the male to send to our ornithologist friend, to let him know we had this unusual nesting. (Not an “I told you so.” Really. I promise.) Woo-hoo!
Ironically, our friend just happened to call later that day to say he was heading from his home in the plains to visit his son on the other side of the mountains. Since he was traveling right past our house, he stopped by for a chat. We hustled him out to the nest box and he watched in humbled amazement as the WEBL pair went about their business. Score! It was even more of a triumph because we were able to confirm this unusual nesting for the CO Breeding Bird Atlas that year. Double score!!
2009
We saw neither hide nor hair (skin nor feather?) of WEBLs in the 2008 nesting season. But on April 2, 2009, Zell spotted a pair again at the edge of our property, not too far from a group of boxes. Could it be? Might we be that lucky again? The only other time we’ve seen them, they stuck around.
Lo and behold, during a weekly nest box check on May 17, I spotted a pair of WEBLs at one of our more isolated boxes, with a pretty complete nest. If you compare this male to the male we hosted in 2007, you’ll notice that the 2007 male had a blue belly, as is for WEBLs. The 2009 male has a much lighter belly, though. (It looks white here. But in a different photo, I could see a light wash of blue on his belly when I zoomed in the photo.) The white belly made me think of an Eastern Bluebird (EABL); however, this fellow clearly has a blue throat (rather than orange-red, as an EABL would) and has chestnut on much of his back. As far as I know, WEBLs and EABLs don’t hybridize. So I guess this one was just an intriguing aberration. Below, you see the female in the box, the male, the female, and the pair.
The nest looked pretty much like our typical MOBL nest, made primarily of sculptural swirls of strips of juniper bark. In many areas, bluebirds use grasses to make their nests. However, since our area has very little grassy vegetation, birds here use juniper strips throughout—starting with broader strips as the foundation and finishing with finer strips in the nest cup.
During the May 22 check, 4 eggs were present. (You can get a bit of a sense of the finer juniper strips in the nest cup 
in the photo on the left.) On the May 30 check, 5 eggs were present. (See photo, below right.) With these two dates, I could make a pretty 
good guess about when the first egg was laid. Songbirds typically lay 1 egg a day, most often in the morning. Since I found an extra egg on the may 30 check, I can assume that the female was still laying eggs during the May 22 check. I always check in the afternoon, to avoid disturbing the female if she is laying. So I can assume that the fourth egg was laid on May 22. Counting backward, I calculate that the first egg was laid 3 days earlier–May 18. I’m not always that lucky—if your first check with eggs is the full clutch, all you can know is that the first egg was laid sometime during the week (unless the full clutch has exactly the same number of eggs as the number of days between your counts—not something that happens in our boxes.) In those instances, you just have to guess. But a guess accurate to within 7 days is still useful information.
Assuming my “first egg” calculations were correct, this clutch hatched on the far 
end of the range of incubation (12 – 17 days). On the June 7 check, the babies looked to be only 2 or 3 days old. If they were 3 days old, they would have hatched June 4 or thereabouts, with a resulting incubation period of 18 days; 
if they were only 2 days old, then 19 days. (An alternative possibility is that the female may not have started incubating as soon as she laid the last egg.) In the photo on the right, you can really see the dark bluish natal down on the crown and back (as well as one nestling gaping). I count at least 4 nestlings, but I’m often not very sure at this age. Since one is obviously hungry (that nestling wouldn’t gape unless it were ready for food), we quickly close up the box and move away, hoping an adult is waiting in the trees with some food.
The next check 5 days later found development advancing nicely. Since their eyes are still not open, they are probably 7 or 8 days old, which fits with my estimate of the ages at the previous check. (It’s a lot easier to figure these things out after the fact, looking closely at photos and checking Web sites such as Cornell’s Birds of North America Online.) And, as you can see in the photo on the right, there are indeed 5 nestlings—so all 5 eggs had hatched, even though I could only see 4 at the earlier check. No problem—I can just go back and correct my notes from last week. (Photos like these are my favorite kinds. All are sleeping peacefully and nobody is gaping at our movements, begging for food.)
The next check found the kids at about age 16 days. WEBLs fledge between 20 and 22 days after hatching, so this was my last view of the kids. They are really looking like bluebirds now. You can see their tails are starting to grow—always a sign that fledging is near—but they are still stubby. Those stubby tails are an easy way to identify almost any recently fledged songbird. I couldn’t clearly count the heads and, since they were getting so big, I didn’t want to risk keeping the door open too long and causing a premature departure. So I figured I’d see if there were any dead nestlings in the box when I checked after they had fledged. If not, I’d count all 5 as having made it out into the world. (And indeed, there were no dead nestlings—so a very happy ending.)
© 2010 Tina Mitchell
