Birds, Cones, and Climate

Some birds have very unpredictable winter ranges. One of these, the pine siskin, primarily eats seeds, and in many winters can be found throughout the boreal forests of Canada and the northern United States, where it eats the seeds of firs, spruces, and pines. But in some years, the pine siskin is almost absent from the boreal forest, and instead is found in patches up in the mountains of Appalachia and the Adirondacks. Why is there so much variation?

The thought is that the major changes in the pine siskin’s ranges are due to the cycles of seed production by boreal forest trees. Trees like firs, spruces, and pines produce seeds in cones and these cones take a lot of energy for the trees to make. So instead of making cones every year, these trees make them every two to three years. Sometimes, the climate conditions are perfect for cone-making and so all the trees across the continent all make cones at the same time. This is called “mast” year, and it is great for pine siskins and other seed-eating birds. But the following year, there are very few cones because the trees have used up so much of their reproductive energy. Birds like the pine siskin respond by expanding their range to find food wherever they can.

That’s the thought anyway. But until recently, it’s been hard to really prove what’s going on. I just read a cool paper that came out in the Proceedings of the National Academy of Sciences earlier this year. In this paper, the scientists explain how they tied the pine siskin’s range changes to climate variation.

In the study, data were taken from Project FeederWatch, which is a great citizen science project that asks volunteers to study birds at backyard feeders throughout the winter. In particular, the scientists gathered all the data on where pine siskins showed up at feeders in North America from 1989 to 2012 and analyzed it to see what patterns they could find over these 23 years.

The data definitely showed the variation in pine siskin winter locations. Here are six of the years. Light yellow shows the extent of Project FeederWatch data. (There aren’t so many people living up in northern Canada to make feeder observations.) The darker oranges and reds show where pine siskins appeared at feeders that year. And the dark black line shows the extent of the pine siskin’s range in the boreal forest.

pine_siskinAnd what the scientists found out was pretty neat. It turns out that there are two ‘types’ of pine siskin range movement: one that goes north-south and one that goes east-west. The north-south movements were due to differences in spring precipitation in the year leading up to the winter and winter temperatures in the boreal forest. The east-west movements were mostly due to summer rainfall and temperatures in the previous year. So these major range changes do seem to be due to climate patterns.

But what about the cones and their seeds? In this study, the scientists had to calculate seed production based on what they know about how trees produce seeds and the climate data that they had. It would have been even better if they had data on cone production throughout the boreal forests, but unfortunately that data doesn’t exist yet. But one day that data may exist.

When you help identify cones in Season Spotter images you’re helping to fill in this data gap. With enough data on when cones are present on boreal trees, we could more directly link climate to cone production and cone production to the movements of birds and other animals who depend on the cone seeds for winter food.

Courtenay Strong, Benjamin Zuckerberg, Julio L. Betancourt, and Walter D. Koenig (2015) “Climatic dipoles drive two principal modes of North American boreal bird irruption.” PNAS 112: E2795-E2802.


About Margaret Kosmala

I am an ecologist exploring the complex dynamics of plant and animal systems. I am especially interested in understanding how species communities change over time and how humans impact them.
This entry was posted in Science and tagged , , , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s