A Bluebird’s Dilemma by Catherine Dale
There’s something just a little bit strange about the Western bluebirds of the Okanagan Valley. In the late fall, when birds from all over North America are headed south to spend the winter months in warmer climes, a good number of our local bluebirds are settling themselves in for a long, cold Canadian winter.
This is a phenomenon known as partial migration, and although it’s actually quite common in the animal kingdom (especially in birds), scientists don’t know that much about how it works. Specifically, we don’t really understand what decides who stays and who goes in most partially migratory species.
Researchers studying some bird species (such as European blackcaps) have found that migratory strategy is genetically determined. However, other researchers, focusing on different species, have found that whether an individual migrates or stays may depend on its sex, age, physical condition or even personality.
For the past three years, I’ve been studying how partial migration works in the Western bluebirds of the Okanagan for my PhD thesis. Specifically, I’m interested in finding out what factors are important in determining whether an individual bluebird stays or goes.
To answer this question, I’ve spent the last few summers trekking around this beautiful valley, catching bluebirds at 15 sites stretching between Summerland and Osoyoos. (As you can imagine, I spent a fair amount of time every day just driving between sites!)
When I catch a bluebird, I mark it by putting four bands on its legs – a numbered metal band and a unique combination of three colour bands. This allows me to identify individuals without having to capture them again.
Of course, the main thing I need to know is whether the bluebirds I catch are migrants or residents. To do that, I take a small clipping (approximately 1mm) from each bird’s claw Believe it or not, a bird’s claw can give us information about where it has been over the past few months, using a technique called stable isotope analysis.
Isotope analysis depends on the fact that different locations have different isotopic ‘signatures’, which get incorporated into any plant or animal tissue produced there. So, because bird claws are constantly growing, by analyzing their isotopic signature, we can find out where birds have been.
Once a bird is banded and I’ve taken the claw clipping, I release it to get on with the business of breeding. Bluebirds are a great species to study because, unlike many other birds, they will willingly nest in human constructed nest boxes – meaning that it’s easy to follow their nesting attempts and document their success over the course of a summer. Between my 15 sites, I have over 400 boxes to check, which is enough to keep me (and a couple of assistants) very busy!
So what have I found so far? Well, I’m still in the process of analyzing most of my data. But it seems that migratory strategy in bluebirds is not determined by sex (male and female bluebirds are equally likely to migrate) – or by age (young birds are just as likely to migrate as older birds). It also doesn’t appear that migratory strategy is genetically determined – because a number of my banded birds switched from being migratory in 2011 to resident in 2012.
Currently, I’m investigating whether personality might be playing a role in whether a bird decides to migrate or stay. During the breeding season, I did an experiment to test how aggressive individual birds were. I tested this by simulating a territorial invasion by a competing box nester – the house wren. As anyone who has a bluebird trail knows, despite their small size, house wrens are more than capable of fighting bluebirds for their boxes. They also have been known to poke holes in bluebird eggs to convince bluebirds to give up a box.
My experiment involved putting a model of a house wren on a bluebird pair’s box, and playing house wren song over a speaker to fool the pair into thinking they had to compete for their box. Then I recorded how the bluebirds responded – for example, whether they hovered in front of the model, dive-bombed it, or even hit it hard enough to knock it off the box.
I’m still sorting through the data, but preliminary evidence suggests that there may be difference in aggression between migrant and resident bluebirds. If so, this might indicate that personality plays a role in determining who stays and who goes.
I still have lots more to do to figure out exactly how personality fits into the picture. However, before I continue my data analysis, I’ll be spending some more time in the Okanagan this spring, trying to track down my banded birds and find out what they did last winter. So if you happen to see a bluebird with coloured leg bands, please contact me – I’d really like to hear about it!
“Catherine Dale is a PhD student from Queen’s University in Kingston, Ontario, who is studying Western bluebird migration in the Okanagan Valley.”