Bluebirds in Maine’s Winter

Eastern Bluebirds are becoming an increasingly common sight during the winter in the southern-half of Maine. I wrote about this when we started this blog back in 2015 (“It’s Winter in Maine! Why do I see robins and bluebirds?“) and we’ve seen the trend continue since. Let’s take a look at a few citizen science projects that have been collecting data on Eastern Bluebirds in winter over the past few decades:

Christmas Bird Count

I referenced this data in 2015 but it is interesting to see the trend continue. There was a surprising dip in the number of Eastern Bluebirds reported in Maine during the 2016 Christmas Bird Counts but this could partially be a result of the weather that year. From my notes, the first Saturday of the 117th CBC saw a big snow storm and at least a quarter of Maine’s CBCs are run on the first weekend of the count period, so it is not surprising that observations would be down in such poor conditions.

Below you can compare the “abundance index” of Eastern Bluebirds from National Audubon’s CBC Trends Viewer. Their calculations help address seasonal issues by calculating an index where they “divide the raw number of birds counted by a function that describes the statistical relationship between effort spent and birds counted. Then [they] look at how those resulting, effort-corrected counts change over the time, to generate the population trend.”

A trend that is not visible on the charts above, is that Eastern Bluebirds are expanding their winter range northward. That is, as we are seeing more bluebirds each year, they are spreading from the southern-most counts and the number of counts that they are being reported in is also increasing. In the animation below, you can see the southern counts reporting higher numbers consistently each year, and the reach of smaller (approx. <25 individuals) wintering populations continuing north-northeast. There may be a few factors influencing this trend: Habitat is changing in southern Maine as suburban sprawl fragments our forests and creates more edge habitat (where bluebirds thrive). “Bird and wildlife watching” is increasingly popular with more people offering food like mealworms for bluebirds in winter (though this would more likely just make wintering bluebird more detectable which is why we compare raw numbers and abundance above). Bluebird trails continue to grow and are helping the breeding population grow in Maine. But let’s be clear: poleward range expansions are being documented in many species as a result of climate change.

eBird Science

These increases in Maine aren’t necessarily a good thing. The Christmas Bird Count is one of the longest running citizen science projects that produces fairly consistent effort annually, which makes their snapshots valuable but there is a “new kid on the block” in the form of eBird Science. eBird is a citizen science project where birders submit complete checklists of the birds they observer, where, when, and with how much effort, to allow for observations to be plotted over space and time. This database now has nearly 14 million checklists with over half of a billion birds reported. One of the new outputs from eBird are their “eBird Status and Trends” pages, like this one showing the Non-breeding trends for Eastern Bluebird:

You can see the increasing trends in Maine, very similar to what we identified above from CBC data. However on the national scale, you can see the sharp declines in abundance to bluebirds in the mid-Atlantic states.

How to Help

Collect Data – As you can see above, there is a lot of value in citizen science projects for understanding trends in bird (and other wildlife) populations. Consider participating in a Christmas Bird Count, submit checklists to eBird anywhere and anytime, or get involved in the Maine Bird Atlas!

Put up a Bluebird Box – The lack of nesting sites is a barrier to the success of Eastern Bluebirds in Maine. They are secondary-cavity nesters, meaning they nest in cavities but cannot excavate their own. They rely on primary-cavity nesters, like woodpeckers, to make a hole that the bluebirds can use in successive years. These cavities are often in dead trees along edges of fields or other open areas, but unfortunately those are often removed by humans because of their lack of visual appeal or risk of damage were they to topple. Can you think of a better gift for someone than a bird house (available at our Gilsland Farm and Fields Pond stores) that will bring something as beautiful as a bluebird to their yard?

Keep Cats Indoors – Speaking of attracting bluebirds to your yard… Outdoor cats (including owned cats, not just feral) kill over one billion birds per year in the United States. Since this is a completely anthropogenic cause – humans put those cats on the landscape, there is nothing natural about it – we need to take the responsibility of pet ownership to keep our cats indoors or at least not be attracting birds to yards where cats occur.

Plant Natives – Bluebirds’ diets are made up of primarily insects and fruit. Insects are especially important in the summer or breeding months when that is the food they need to raise their young. We need more native plants on the landscape (think of this as the bottom of the food chain) to support native insects, which in turn provides more food for our birds. The fruit bluebirds eat is also important: Attract them with natives in the winter like winterberry, and remove invasive species like asiatic bittersweet. You’ll see bluebirds consumer these non-native fruits which unfortunately leads to the dispersal of their seeds. So while it may look like they are benefiting from that food, there is a harmful impact in the long run. Learn more about the work Maine Audubon is doing to support native plants here: Bringing Nature Home.