An eastern bluebird brings a pine needle to use in its nesting box in Freeport. Many factors will affect nesting success for eastern bluebirds. Robert F. Bukaty/Associated Press

Our first question this week is from Brian Jones of Gorham, who noticed some unhatched eggs while cleaning out bluebird nest boxes. This is a common question we get in the fall as people are tending to their nest boxes, and this provides an interesting glimpse into how the nesting season went in general. As a quick reminder, cleaning boxes (or birdhouses) is helpful for sanitation and makes a good starting point for birds next year, but it’s not required. Rodents will happily use old nests in the winter, for sleeping or storing food, and wouldn’t you rather have them do that, rather than nest in your walls? You may already find a new resident if you check the boxes now, but empty or not, it is interesting to look at the success – or lack of success – of these nests.

To help answer Brian’s question, I found some research led by Caren Cooper at Cornell’s Lab of Ornithology which looked into geographic and seasonal influences on nesting success in eastern bluebirds. An important number to start with from Cooper’s research was that 7.8% of eggs, from 7,231 nests checked, went unhatched. We’ll revisit this number, but it is a good one to keep in mind as you think about nest boxes you check and the rate you find unhatched eggs – although your sample size is going to be significantly lower than Cooper’s 7,000 nests.

One of the key takeaways from Cooper’s research is that “the ‘per-egg’ probability of hatching failure was highest late in the season” and “highest at lower latitudes.” You may think this late-season low-latitude combo would just mean higher temperatures cause nest failure, but bluebirds are remarkably good at maintaining proper temperature for the eggs while incubating, and despite it being a dry and hot summer last year, lethal temperatures for eggs are around 105 degrees. We do tend to see less experienced birds nesting later in the year, and they are less likely to be successful, so we may be biased by when we are checking the boxes.

Good ventilation is important, but as stated, temperature, especially at our northern latitude, is not a big problem for bluebirds. The more common thing we see as being a cause for nest failure in bird boxes is poor drainage. Any boxes you have out should have holes cut or drilled from the bottom corners to allow any water coming in to get out. Excess water can drown young, cool eggs, or cause mold and bacteria to grow. Many of the ‘novelty birdhouses’ we see for sale lack this very essential modification for letting water out. Fortunately, you can add a hole to each corner pretty easily with a drill.

One remarkable thing we’ve observed lately is the increase in bluebirds in Maine. I’ve written about the increases during winter before, and you can find it extensively covered on Maine Audubon’s blog but the increase in summer is interesting, too. In Ralph Palmer’s 1949 Maine Birds, he lists the bluebird nesting season as having two broods “from April 28 to July 11” with the first eggs laid on those dates. I’ll be excited to report findings from the Maine Bird Atlas soon, but we saw reports (from the past five years) stretch this nesting season by several weeks on either end. Eastern bluebirds, especially in southern Maine, are now successfully raising three broods per season.

Coming back to Cooper’s research, these later broods are expected to be less successful, so don’t be too alarmed if you’re finding unhatched eggs in the fall, and do consider factors like drainage in the box if you are seeing an abnormally high percentage of unhatched eggs.



It was fun to hear from Forrest Dillon again; it was last fall that we discussed the dearth of acorns as a possible reason for so many squirrels feeding on people’s porch pumpkins this month. This year, Forrest wrote in because he’s been finding an unusual (in their yard) amount of pitch pine tips on the ground. This is most likely the handy work of red squirrels, and it’s also fun to think about in the larger context of cyclical patterns we’ve discussed in this column over the years.

I often like to point to the memorable headlines of 2018 when we experienced the “squirrel-magedon,” an apparent population boom after an abundance of acorns led to an unbelievable amount of roadkill, but also was the primary (and novel) food source for the famous great black hawk at Deering Oaks. While population numbers of foxes are harder to track, we saw an increase in the number of fox (red and gray) sightings in Maine following that. Following the increase in predators, we then saw a crash in mast (especially acorns), which caused a change in squirrels’ diet, relying on anthropogenic food sources (like your Halloween pumpkins). Now, we are seeing an especially heavy crop of red spruce and eastern white pine cones, which will definitely benefit species like red squirrels, as well as avian specialists like red crossbills.

I suspect the increased activity Forrest has noticed recently is from these red squirrels taking advantage of an abundance of resources. Freshly-cut tips from pines and spruces will make great bedding and insulation for the incoming winter. Keep wildlife in mind as you prepare for winter too: remember to “leave the leaves” and maybe now we should “pass over the pines” too.

Do you have a nature question for Doug? Email questions to [email protected] and visit to learn more about birding, native plants, and programs and events focusing on Maine wildlife and habitat. Doug and other naturalists lead free bird walks on Thursday mornings, 7 to 9 a.m., at the Gilsland Farm Audubon Sanctuary in Falmouth. Starting Dec. 1, the walks will be from 8 to 10 a.m.

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