A Red Crossbill Feeding Note From Klamath County, Oregon:
By Konshau Williams Duman:
On the morning of May 30, 2023, I watched a group of Type 2 Red Crossbills feeding on the staminate cones (see very bottom for more photos) of a Ponderosa Pine along a Forest Service Road just east of the Klamath Marsh National Wildlife Refuge in Klamath County, Oregon. Or so I thought.
From our camp nearby, I had wandered around the Ponderosa forest that morning, following the calls of a few Crossbills up in the canopy in search of the small flock that I had stumbled upon there the previous evening. Fortunately, I was able to get views of a couple birds flying between the trees around an open area and I eventually saw them land in a Ponderosa and begin feeding around the branch tips. Better yet, the tree was already full of crossbills, and at this point I counted nine individuals feeding throughout the tree.
I got closer, set my spotting scope on them, and watched them hop to a cluster of staminate cones at the tip of a branch, and quickly split each cone open and eat something from within. Of course, the pollen sacs, I thought. I had read that staminate cones are a food source for Red Crossbills this time of year (Benkman 1987). I switched to watching different individuals throughout the flock each fed in exactly the same way: hop to a branch tip, split the staminate cones down the middle lengthwise (or sometimes chopping them off crosswise), and eat something from inside. This seemed a bit strange since the pollen sacs are around the outside of the cone, and they were only visiting each cone for a fraction of a second to cut it open and eat the something inside.
After a few minutes watching them, I was able to spot what they were eating when a wriggling whitish grub appeared in thetip of one of their bills for a tiny fraction of a second (see 6th photo below)before being ingested. I spotted grub extractions again and again throughout the duration that I watched them feeding. It seemed as though a grub was captured only in every second or thirdcone, with the birds splitting open many cones and immediately moving on without clearly extracting anything.
This flock of 9 fed in a single Ponderosa that was laden with fresh, purple, unripe staminate cones for another 15 minutes before flying over to another ponderosa stand a couple hundred meters away where they did not feed much. In this second spot, I was able to observe one male pry at a seed cone from last year that was still mostly re-closed. He left the cone in less than a minute, but I couldn’t see if he got any seeds or not. I saw a couple birds poke at the staminate cones on these trees as well, but they never fed on them clearly. After about 10 minutes in this spot, groups started breaking off and leaving. I followed a few of them back to the staminate cone tree where a group of 6 resumed splitting staminate cones for another 20 minutes. Two of them left and the remaining 4 continued feeding for 10 minutes. Two more left, and the last two fed for another 5 minutes before they too left. At that point, I inspected the ground beneath the tree before leaving the spot to search for other feeding locations, but I could not observe any feeding by any of the smaller dispersed groups that I found in the Ponderosas there.
The ground under the focal Ponderosa was lightly littered with halved staminate cones. In many of them, I could see the cross section of the larval mine with frass left behind in the cone axis. Other staminate cones were halved with no larval mine. I found three staminate cones that were broken off without being fully split open. In one that had been chopped crosswise at the base, I was able to actually find the live grub in the intact part of its mine. Another grub was inside an intact cone.
When I got home to Davis, CA, I identified the grubs as the larvae of a Xyela sawfly using a few forestry insect references (Furniss and Carolin 1977, Keen 1958) and eliminated similar sawfly genera using the key provided by Smith (1967). Under magnification using a dissecting scope from the UC Davis Herbarium, the 8.5 mm long larva was fairly distinctive, with the thoracic legs reduced almost to a short bristle (though tarsal claws are still present), and the prolegs appearing as low bumps on the ventral side of each abdominal segment. The head was very small and light brown with darkened mandibles, and the eyes were minute dots barely visible even under magnification. The body was cream colored. The small larval mines restricted to the inside of the staminate cone axis with no clear damage visible from the outside was also a good trait that eliminated many other bud-tip miners.
To my surprise, a quick search revealed that Red Crossbills have been documented feeding on Xyela larvae once before in Marin County, CA (Chapelle et al. 1956). It seems that Xyela do not draw much attention since they cause no economic damage to the pine forests they dwell in, so they are mostly just mentioned as a side note next to other more destructive forest insects. However, given their wide range across coniferous forests in North America and the fact that their larvae actively feed during part of the period of lowest conifer seed availability (Keen 1958, Fowells 1965), it would be interesting to see how often Red Crossbills use them as a food source during this critical period of their life cycle. Hopefully I will find out by continuing to study their diet this Spring and Summer and in years to come.
And to that point, we have started a FiRN North America Crossbill Foraging Project in iNaturalist: https://www.inaturalist.org/projects/red-crossbill-foraging-in-north-america
Photos By Konshau Williams Duman
Benkman, C. W. (1987). Food profitability and the foraging ecology of crossbills. Ecological monographs, 57(3), 251-267.
Chapelle, F. O., Thornburg, F., Hartman, F. A., Chatwin, S. L., Fox, W., Eisenmann, E., … & Cade, T. J. (1956). From Field and Study. Condor, 71-78.
Smith, D. R. (1967). A review of the larvae of Xyelidae, with notes on the family classification (Hymenoptera). Annals of the Entomological Society of America, 60(2), 376-384.
Fowells, H.A. (1965), Silvics of forest trees of the United States. No. 271. US Department of Agriculture, Forest Service.
Furniss, R. L., & Carolin, V. M. (1977). Western forest insects (No. 1339). US Department of Agriculture, Forest Service.
Keen, F. P. (1958). Cone and seed insects of western forest trees (No. 1169). US Department of Agriculture.
Starting from the top left: Xyela larva ventral view, Xyela larva in its mine viewed in situ in a dissected cone that was collected intact and preserved in alcohol, Xyela larva head, larval mine visible in a split piece of staminate cone found under the tree, Xyela larva dorsal view, Xyela larva being extracted by a foraging Crossbill, Red Crossbill prying open a split staminate cone.
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Become a Finch Forecaster, Finch Food Assessment: https://www.inaturalist.org/projects/become-a-finch-forecaster-finch-forecast-food-assessment
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