I don’t know how I’ve made it this far in my blog without posting about bluebirds. Maybe I’m just too easily distracte…a wasp on the window!
Ok, I can focus for a little bit longer than that.
Every day on my way to and from work, I see a pair of Eastern Bluebirds. All spring, in fact, they’ve been a common sight for me. It wasn’t until I was visiting with my parents that I really began to think about them, though. They have a bluebird house on a post in their backyard (only feet from their back deck) and there’s a bold, little pair of birds that has used it for several years. Last time I was there, I watched both the male and female go in and out (their young must have hatched, already). The male also spent some time just sitting on the roof of the house while the female was inside. They were, on the whole, fairly forgiving of our presence in the house or even on some parts of the deck.
The thing that actually struck me about their situation was that the bluebirds were doing well in their place. Eastern Bluebirds (well, all the bluebirds) are cavity nesters and in many urban landscapes, natural cavities become limited because people often prefer prettier trees and posts, rather than the rotting ones that contain those cavities. That limitation brings with it some other issues and the biggest one is competition. Carolina Wrens are also cavity nesters, though they usually prefer nests of a different size. The two biggest competitors shouldn’t even be around: the House Sparrow and the European Starling. Both of those birds are considered invasive, having been introduced from Europe and both of those birds are also cavity nesters. Starlings usually take (and compete for) larger cavities than the bluebird will normally use, though there is certainly some overlap. It’s the House Sparrows that really cause the biggest problems for the bluebirds.
So far, luckily, the pair of bluebirds at my parents and the one here on Selwyn seem to be doing just fine. All of this background leads me to the question I asked myself (and I may have done it out loud- it’s possible that I sit alone too much). ‘Will they be back next season?’ hmmm…and the follow-up question was, ‘Hmmm…do my parents clean out the nest box at the end of the season?’ I assume they do, but I actually didn’t remember to ask them. I saw something shiny and got distracted.
A quick check on the literature surrounding bluebirds and their nesting patterns brought up something interesting:
First of all, bluebirds do prefer their nest sites clean! They do like to re-use the same nesting sites again (if they were successful the first time), but will switch to a new site to avoid nest parasites.
The second thing that came up (that was truly fascinating) has to do with the type of nest boxes that people put out. I’ve seen ‘bluebird boxes’ sold in a number of places. There are, however, different kinds of bluebird boxes and some of them do have an effect on the nesting success of the birds. One study actually compared the success rates of birds using different kinds of boxes (over 30 years) and found that open boxes have a higher success rate than closed boxes. Now I actually didn’t know what an open box was, so I had to look. Here’s what they look like:
That opening on the top (even if it is covered with mesh) lets light into the cavity of the next box. That increase in light doesn’t seem to bother the blue birds at all. Well, it has a couple of interesting effects. Birds that nest in open boxes have smaller clutches than those in natural cavities (but larger than in standard boxes) AND, more importantly, they have higher survivorship of the young. The success rate in open boxes is higher than in any other kind of house. Part of the explanation for this phenomenon is that the bluebirds actually experience less damage from House Sparrows in the open boxes. House Sparrows have a strong preference for enclosed cavities and the problems that they cause decrease in open boxes.
So, if you’d like to keep your bluebirds of happiness (or if you’d just like to keep your bluebirds happy), get ’em an open box and clean it out when they finish. They’ll keep coming back!