Project update (7/8/23) Thanks to observations from many folks, this project is off to a great start, with nearly 200 recordings of eastern crossbills and information on the conifers the birds were eating! Most observations are of the recently described type 12, which is the crossbill I'm most interested in for this project. Pictured below is a stacked bar chart illustrating the contribution of different conifers to the diet of type 12 based on the available data.
Although much more data are needed to fully describe type 12's feeding ecology, there are some interesting patterns. Notably, these birds eat lots of white pine seed in late summer-fall just after cone crops mature. However, white pine is known to rapidly lose seeds from its cones, and crossbills switch to other conifers in late winter-early spring. Red pine, red spruce, and Norway spruce all become more important for these birds at this critical time. This is somewhat expected, as these conifers are known to retain seeds in cones for longer, meaning they are available to crossbills when other resources aren't.
Still, major questions remain, and much more data are needed. For example, these data are lumped from across several years and several regions in the eastern US. Therefore, it is difficult to evaluate how this picture might vary geographically and temporally. Another issue is the likely over-representation of Norway spruce and pitch pine observations. Norway spruce is planted in areas that birders frequently visit (e.g., parks, cemeteries, college campuses), meaning there are lots of these observations on eBird and iNaturalist, even though this conifer probably represents a small fraction of what most type 12 crossbills feed on. The high number of pitch pine observations reflects a recent irruption of type 12 into the pine barrens of places like Massachusetts. Lots of birders went to find these irrupting crossbills, meaning there are lots of eBird and iNaturalist observations out there.
The only way to resolve these issues is through more data collection. So, if you come across crossbills in the east, please considering recording their calls and documenting which conifer the birds are feeding on. Observations can be uploaded to eBird (I check eBird records several times each month), my iNaturalist project, or emailed to me.
Project Background
The funky bills of crossbills give them unparalleled access to the seeds in conifer cones, making them among the most dietarily specialized birds on the planet.
But within crossbills, there is important variation in bill size and shape that are adaptations for feeding on different conifers. For example, type 2 has a slightly deeper bill and wider seed husking groove than type 5. Because of these differences, type 2 consumes large seeds from large ponderosa pine cones about 66% faster than type 5 can. On the flip side, type 5 extracts smaller seeds from smaller lodgepole pine cones 20% faster than type 2.
These differences matter to the birds. Over several years of fieldwork throughout the central and southern Rockies during large pine cone crops, 95% of the type 2 I encountered were feeding on ponderosa pine and 80% of the type 5 were feeding on lodgepole pine. Seems straightforward enough, right?
Not quite. It’s long been known that the feeding behavior of crossbills is more complicated. First, diets within a call type can vary geographically. In the east where type 2 occurs in small numbers, these birds eats a variety of conifers, including white spruce, red pine, and white pine.
There is also pronounced seasonal variation in diets, and types 2 and 5 nicely illustrate this in the Rockies: when Engelmann and blue spruce produce large cone crops in late summer, both call types switch to feeding on spruce and ignore ponderosa and lodgepole pine, respectively. And why wouldn’t they? Spruce cones are easier for both call types to feed on and their cone crops are often massive, meaning there is an abundance of easily accessible conifer seed.
The only problem is that spruce cone crops are a fleeting resource – most cones lose their seeds by mid-winter and crossbills need another resource to “fallback” on. This is when conifers that retain seeds through winter and into spring (like ponderosa and lodgepole pine) become critical for their survival.
Spruce cone crops are also extremely unpredictable and occur less often than cone crops of ponderosa and lodgepole pine. This combined with the roughly 27 million acres of ponderosa pine forest across the west means that a type 2 experiencing a cone crop failure in Arizona can probably find a ponderosa pine cone crop somewhere – whether it’s in Utah, Colorado, New Mexico, Wyoming, Idaho, Montana, Oregon, Washington, South Dakota, etc. It’s easy to see why type 2s with a bill size best suited for feeding on ponderosa pine would have an advantage in most years and thus be favored by natural selection. A similar dynamic probably occurs for many of the other western call types.
But what does the situation look like for the eastern call types, especially type 1 and the recently described type 12? Unlike the Rockies and the Pacific Northwest, there aren’t many millions of acres dominated by one conifer in the east. But what the east lacks in abundance of any one conifer species, it makes up for with higher local diversity of conifers. Because different conifers often produce cone crops in different years, a crossbill in New York could go from feeding mostly on white pine in one year to red spruce the next, and maybe even red or Jack pine in another year.
A reasonable hypothesis is that the ecology of eastern forests should favor generalized call types – birds that can readily switch between several different conifers within or between years. Another possibility is that eastern crossbills have evolved adaptations specific to the conifers that are most reliably available year after year and retain their seeds in cones during parts of the year when food is scarce.
This is where I need you! Although type 12 has only just been described, it’s known that these birds are feeding on several different conifer species, including white pine, red pine, Jack pine, pitch pine, red spruce, white spruce, tamarack, and more. However, it’s less clear whether certain conifers are especially important at certain times of the year, and whether certain conifers form a consistent part of type 12’s diet across years. We also don’t know how much their foraging ecology differs across their range – are birds in the western Great Lakes doing something different than birds in the Northeast? Getting a detailed picture of type 12’s diet within and across years is key to understanding the ecology and evolution of this call type, as well as its conservation and management needs. We also need more data on the feeding ecology of type 1 throughout its range, from the Northeast to the Southern Appalachians.
Generating the amount of data needed to tackle these questions is too massive a project for any one person. But individual efforts by many people can generate some incredible insights into these birds.
Here’s what I need: If you encounter crossbills in the Great Lakes, Northeast, or Appalachia, record their calls (here is a great resource for tips on recording birds with your phone) and add this recording to your eBird checklist. Next, watch the crossbills, determine what conifer species they’re feeding on, and add this information to the notes in your checklist.
It’s important to make sure the birds are feeding on cones of a particular conifer species, as sometimes birds will perch for long periods of time in a tree and not feed on it at all. Not quite comfortable identifying all the conifers in the east? No problem! A picture of the tree, especially a good shot of its cones, is all that is needed (also, check out this guide to the conifers of the east)!
Finally, email the checklist to Cody Porter ([email protected]). I will be checking eBird records of type 12 each month, but I encourage folks who may need some help identifying crossbills to call type to contact me. One of my main goals with this project is to track what conifers types 1 and 12 are feeding on at different times of the year, so observations from all time periods are valuable.
For folks who aren’t eBird users, I’ve also created an iNaturalist project where observations can be uploaded.