Fielding calls about “winterkilled” Bluebirds in nest boxes
by MOBS President Steve Garr
The artic blast we received here in Missouri and other areas in mid February was certainly hard on our Bluebirds. Scattered reports have started coming in of finding dead bluebirds in nest boxes. Periodically, areas have winters with extended severe cold that result in this situation with Bluebirds in particular. Often, in these circumstances the Bluebird “hosts” feel they have done something wrong or didn’t do something they should have. Please know that is truly not the case in the vast majority of these cases. I think rarely is lack of food or water the cause. For the most part it involves the habit of our Eastern Bluebirds of piling into a cavity (whether a nest box or corner or piece of equipment or whatever) to stay warm. Even when you have SEVERAL nest boxes available within feet of each other they will all pile into just one or two. In these super cold temps for such a long time, sadly, the birds on the bottom can just get smothered.
Please do not think this means you need to close up nest boxes! They provide such valuable winter cover for many birds. However, while Carolina Wrens or Downy Woodpeckers or Chickadees may roost in the boxes for warmth they rarely roost in them with more than one or a few individuals. ( I have never seen a box with more than one Downy at a time roosting in the box.) Bluebirds on the other hand can sometimes have 20 or more birds inside a standard size nest box. (sometimes the last one in still has their tail sticking out of the box). This is OK unless they are in there for an extended length of time under extreme temps. If you have a very successful breeding season (which is great!) and lots of Bluebirds, you are more likely to see more dead bluebirds in the boxes in these unusually cold years.
There has been some research and much speculation as to whether or not these “winterkilled” birds in the nest boxes are local resident birds or if in mid to late February some of them could be birds that were on their way back north to their breeding grounds. I don’t know the answer to that question positively, however I do know that here in Missouri we generally have the highest number of Bluebirds coming to our feeders, bird baths, and yards in the month of February.
My good friend (and MOBS member) Bob Peak has perhaps done more research and documentation on “winterkill” in Bluebirds than anyone I know. Bob and his wife Judy monitor an extensive nest box trail at Land Between the Lakes National Recreation Area. They also maintain a trail in Henderson, KY . The Peaks have a vast number of boxes (over 200) and decades of winters and nesting season data from which to pull when researching the loss of bluebirds in the winter in nest boxes. Naturally when I began getting calls a few weeks ago about dead Bluebirds in boxes I reached out to the Peaks. Given the size of their trail and the number of boxes they monitor, it was not surprising to hear they had also been finding dead bluebirds in some of their nest boxes. Bob wrote a very interesting paper concerning winterkilled bluebirds which can be found on the NABS Website in the Summer 2016 issue of the Bluebird Journal.
Roosting boxes ( boxes designed for roosting and not nesting) and roosting box plans are available fairly widely. Some Bluebirders feel they are very beneficial to use– others do not. Perhaps that is a good topic for a future newsletter.
This I do know: As important as it is for us all to turn in our Trail Summary Report each season to MOBS’ nesting data chair Greg Samuel—this year it will be even more important. Enclosed in the Winter 2021 newsletter is a recording form for you to make copies and use on your trail if you would like. (this is not the SUMMARY report form you will turn in to Greg. That form can be found by clicking this link and is distributed annually in the newsletter). How ever you choose to collect nest box data on your trail, be sure to send your summary report to Greg at the end of the 2021 nesting season.
I know this is not happy news or a fun part of birding but I thought it was important info to have and I hope Bluebirders will take heart in knowing their efforts for our native cavity nesters ARE worthwhile and beneficial. Thank you all who have been hosting bluebirds year -round in Missouri.