Crossbill Foraging Project:
Monitor Crossbill Foraging in iNaturalist: Help track the feeding ecology of Red Crossbills across North America!
By Caleb Centanni and Matthew Young
Red Crossbills are among the most enigmatic denizens of the conifer forest. These colorful finches follow shifting seed crops of spruce, pine, hemlock, and other conifer cones. Like these cone crops, Crossbills often appear and vanish in different locations, and it is sometimes hard to predict where you will find a Red Crossbill in any given year.
As it turns out, underlying these erratic movements is a labyrinth of taxonomic complexity. Since 1990, scientists have described 12 call types (or ecotypes) of this species in North America. One of these, the Cassia Crossbill, has since been elevated to a full species. Each call type has unique geographic and ecological areas with certain conifer species they often associate with, but researchers at FiRN and elsewhere are still working on decoding these differences. This makes Red Crossbills one of the most fascinating species we can study to learn more about ecology, evolutionary biology, and taxonomy.
The key to understanding the Red Crossbill complex may lie in understanding which types most commonly feed on which trees, and how this changes between years as conifer cone crops change across the North American landscape. However, unlike Crossbills and conifers, Crossbill scientists are few and far between, leaving us without an easy way to track foraging patterns across the enormous scale of space and time in which these finches operate.
That’s where you come in! If you’re a birder, a feeder watcher, ornithologist, or a natural-history enthusiast with an interest in finches and trees, you can help scientists learn about Crossbills by contributing to our new iNaturalist project, Red Crossbill Foraging in North America. All you need is your smartphone and an iNaturalist account!
Any time you observe Red Crossbills foraging directly on a food item, make a sound recording of the Crossbills and upload an image of the foraging Crossbill or its food item into iNaturalist. Since this project is focused on ecotypes, we are only asking for observations where an audio recording was obtained, for now. We are also only looking for observations where definite foraging was observed (i.e. the birds were visibly opening their bills and consuming food items). Even if you miss photographing the Crossbills, we can accept an observation of their foraging item along with a recording, so getting a recording of foraging birds is the first priority in contributing to the project. Here are the basic steps to add your observations:
1. Make an audio recording of Red Crossbills (or Cassia Crossbills) that you can see foraging on any food item. Eating at feeders is included. Upload this recording into eBird, XenoCanto, or iNaturalist.
2. Make an iNaturalist account and join the project “Red Crossbill Foraging in North America”. You can do this by visiting the project page and clicking the small link that says “Join this project” in the upper right hand corner above the blue “Add observations” button.
Here is the link for this project:
3. Make an iNaturalist observation and add either an image of a foraging Red Crossbill or an image of an organism (plant, animal, fungi) that it was foraging on. Submit your observation into the database. For help on eastern conifer ID’s, see this article here: https://finchnetwork.org/a-crossbills-guide-to-conifers-of-the-northeastern-forest
We hope to write one soon for the western part of the continent as well. Stay tuned for that.
4. Go to your observation on the iNaturalist website. Click in the search bar in the lower right that says “Projects” and select “Red Crossbill Foraging in North America”. Then fill out the observation fields. In the “Finch Forage Item” field, name what the Crossbills were eating (e.g. spruce seeds). In the “recording” observation field, copy and paste a link to your recording (in Macaulay Library, Xeno Canto, etc.) of the exact flock of Crossbills that was foraging in this observation. Click “Add” next to each foraging field, then click “Add to Project”.
If you have a good photo of a cone crop or even a food crop that other finches like to eat, please add any of those sightings to the iNaturalist Winter Finch Food Assessment Project/Become a Finch Forecaster here: https://www.inaturalist.org/projects/finch-forecast-food-assessment
We greatly appreciate you taking the time to contribute to these projects! We know you’re busy, and we appreciate any help. Your observations will help scientists continue to unravel the enigma of Crossbill ecotypes as well as enhancing our understanding of ecology, evolution, and speciation in general. And where is better to observe finches and the natural world than the beautiful and remote forests that Crossbills call home! If you have any questions about this project, how to record Crossbills, or other ways you can help, please contact me at email@example.com or Matt Young at firstname.lastname@example.org or check for info on the FiRN home page. We can’t wait to see what you discover!
If interested, also check out the Evening Grosbeak foraging project: https://www.inaturalist.org/projects/evening-grosbeak-foraging
Cover Photo Christian Nunes
More Background on the Red Crossbill Foraging Project
Crossbill foraging behavior is complex, and the types’ diverse regional conifer associations are not fully understood. Diets within a call type can vary geographically….and bill sizes, even within call type, can vary depending on these diets as well. In the east where Type 2 occurs in small numbers annually, the type eats a variety of conifers, including white spruce, red pine, Eastern white pine and Table Mountain pine. In the west it is most commonly found feeding on the three inland subspecies of ponderosa pine, P. p. scopularum, ponderosa, and bractyptera, but avoids the ponderosa pine subspecies benthamiana in Sierras, where it most closely associates with lodgepole pine, a conifer species most used by Type 5 and Type 9 (Cassia). In the Sierras it utilizes the lodgepole pine subsp. murrayana.
There is also pronounced seasonal variation in diets, and Types 2 and 5 nicely illustrate this in the Rockies as well: 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. They likely first search the West for these abundances, targeting and exhausting whatever major spruce crops exist in a given cone cycle (July-June).
The broad variation in foraging choices within and among call types raises questions for Crossbill scientists: Why do Crossbills choose different tree species at different times of year? And if one type forages on many conifer species, what key feature makes that type ecologically unique from other types? Two leading ideas of what distinguishes the types are the key conifer hypothesis and the geographic isolation hypothesis. The key conifer hypothesis suggests that each type prefers to forage on one key conifer species when possible, using other conifers opportunistically (Benkman, 1993, 2003). This hypothesis predicts that key conifers should be used most predictably when food is scarce in the late winter and spring, taking advantage of its ability to efficiently extract seeds from its key conifer. In contrast, the geographic isolation hypothesis contends that each type has its own “core range” where it prefers to forage, irrupting into other areas when resources are scarce in its core range (Knox, 1992; Martin et al., 2019, 2020). Each of these theories has been studied and supported in different regions, but no universal understanding of the types’ foraging ecology has emerged.
Conifer forests vary significantly from the Western United States to the Eastern United States. Like the West, the Northeast has a high conifer diversity, but the densities of conifers there are much lower when compared to the western mountains, where many of the call types are most common. Call types are thought to be specially adapted for feeding on key conifers in the west, but in the east, Types 1 and 12 are thought to very likely be generalists. Recently, Type 12 was described as the “Northeastern Crossbill”, and its range supports the idea that it is likely to be a generalist. Type 12 appears to be most closely associated with red and white spruce, and red, jack, pitch and white pine forests of the Algonquin PP, Maritime Provinces, Maine, New Hampshire, Vermont, and the Adirondack region of New York. It also commonly occurs in certain times of the year in the western Great Lakes.
Over the last 15 years, Matt has witnessed a predictable seasonal shift in the diet of type 12 that seems consistent across years. It can be most commonly found in July in the Northeast where Red Spruce crops have developed, and then sometime between August and December it shifts to areas where there are good Eastern White Pine crops, before eventually moving down the east coast in winter to utilize Pitch Pine or Japanese Black Pine or to areas where Jack and Red Pine are producing good crops in the Northeast and Great Lakes States (it exhibits these movements with regularity). It also will regularly use white spruce (and sometimes tamarack) in the east, and Norway spruce wherever it has been planted as well.
Focusing in on the late winter and early spring, Type 12 has bred regularly in the pitch pine forests of Massachusetts and Maine and in recent years northern NJ, Long Island and the pine barrens of Albany, NY. It also has regularly bred in jack pine areas of Maine and Wisconsin, and red pine areas of the Adirondacks of New York and elsewhere. Hard-coned pines (i.e. Jack, pitch, red, Japanese Black Pine etc.) often hold seeds longer than soft-coned conifers, which may explain Type 12’s close association for hard-pines late in the cone cycle.
As stated above, Call types are thought to be closely adapted to key conifers in the West but seem to have a more generalist strategy in the East. Except for Eastern white pine, none of the conifers in the East are particularly common and widespread, even red pine and red spruce, and so a crossbill being adapted for a single conifer seems unlikely.
The key conifer concept hasn’t been thoroughly tested across the North American landscape. As explained, Crossbills like to utilize hard pines in late winter and spring because they are often the only conifers with predictably reliable seed before new crops start developing again in May-June. Type 12 does this in late winter and spring when it shifts to jack, red and pitch pines. Recently call types 3 and 4, which are thought to be adapted for soft-coned conifers western hemlock and Douglas-fir, have been observed feeding on lodgepole and ponderosa pines in the west in the winter and spring. This raises questions about the key conifer hypothesis, which says that these types should stick to their key conifers in the late winter and spring. Unexpected patterns like this are why we’d like your help in assessing crossbill foraging across the continent. The more we can learn about foraging events like this, the better we can assess whether crossbills prefer core areas, key conifers, or a combination of the two.
As we learn more, perhaps the two hypotheses can be unified or clarified by a third hypothesis: the rich patch exploiter hypothesis. This idea holds that Crossbills are opportunists which move to pursue the most fruitful resources they can find across space and time (Cornelius et al., 2013). This hypothesis has different implications for the call types depending on which of the two previously described theories is more accurate. If call types are rich patch exploiters adapted to key conifers, then they likely make brief and unpredictable irruptions into plentiful cone crops on secondary conifers before predictably returning to their key conifer in the winter and spring months. If they are rich patch exploiters with some degree of geographic isolation tied to the multi-conifer ecosystem they most closely associate with, they should leave their core areas in opportunistic irruptions when food is scarce at home, but reliably return to a certain geographic area with these conifers.
The only way to resolve these issues is through more data collection….and of course getting outside to witness what these birds are actually doing! So, if you come across crossbills anywhere, please consider recording their calls and documenting which conifers the birds are feeding on. Observations can be uploaded to eBird, our iNaturalists projects, or emailed to us.
FiRN is a nonprofit, and was been granted 501c3 status in 2020. We are a co-lead on the Evening Grosbeak Road to Recovery Project, and have funded upwards of $10,000 to go towards research, conservation and education for finch projects in the last year plus. FiRN is committed to researching and protecting these birds and other threatened finch species like the Evening Grosbeak, Rosy-finches, and Hawaii’s finches the honeycreepers, and if you have been enjoying all the blogs and identifying of Evening Grosbeak and Red Crossbill call types (upwards of 15,000 recordings listened to), redpoll subspecies and green morph Pine Siskins FiRN has helped with over the years, please think about supporting our efforts and making a small donation at the donate link below.