Feeding Ecology of Eastern Red Crossbills

The funky bills of crossbills give them unparalleled access to seeds in conifer cones, making them among the most dietarily specialized birds on the planet.

But within crossbills, there is variation in bill size and shape that represents adaptations for feeding on different conifers. For example, type 2 has a slightly deeper bill and wider seed husking groove than type 5. As a result, type 2 consumes large seeds from large ponderosa pine cones 66% faster than type 5. 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 eat a variety of conifers, including white spruce, red pine, and white pine.

There is also 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. 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 problem is that spruce cone crops are a fleeting resource – most cones lose their seeds by mid-winter and crossbills need another resource to “fall back” on. This is when conifers that retain seeds through winter and spring (like ponderosa and lodgepole pine) become critical for survival.

Spruce cone crops are also 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 likely occurs for the other western call types.

But what does the situation look like for the eastern call types, especially 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 red pine in another year.

A reasonable hypothesis is that eastern forests favor generalized call types – birds that can readily switch between different conifers within or between years. Another possibility is that eastern crossbills have adaptations specific to conifers that are most reliable year after year and retain their seeds in cones during seasons when food is scarce.

This is where we need you! Although type 12 has only just been described, it’s known that these birds are feeding on several different conifers, including white pine, red pine, Jack pine, pitch pine, red spruce, white spruce, tamarack, and more. However, it’s not 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 diet 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. 

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 we 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 (empidonaxdvg "at" gmail.com). We will be checking eBird records of type 12 each month, but we encourage folks who may need some help identifying crossbills to call type to contact us. One of our main goals with this project is to track what conifers type 12 is feeding on at different times of the year, so observations from all time periods are valuable.

For folks who aren’t eBird users, we’ve also created an iNaturalist project where observations can be uploaded.