Irruption of Redpolls

Nature Watch
January 14, 2009

By Susan Benson,CNHM Director of Education

Do you have a larger number of pine siskins or redpolls at your feeder this winter? Have you seen siskins or crossbills in flocks along the roadsides? After checking with some local birders, it appears that siskins are present in area feeders, although less redpolls this winter. Crossbills are being seen along the roadsides, and sometimes are casualties as they don’t quite make it out of the way of cars. Irruptions, the seasonal movement of winter birds to our backyards happens with more than the owls I mentioned the past two weeks.

An irruption is comparable to bird migration, but it takes place more irregularly, usually every two to four years, depending on the species, rather than every year. Most irruptions result because of food shortages in areas where some bird species usually spend their winters. When they cannot find sufficient food, these birds move south in larger numbers. In many bird species, small numbers move south every winter, but irruptions are noticeable through their great abundance of the species south of its usual range. During winters when seed crops, especially of deciduous trees, are poor in the north, food is scarce for seed-eating species. This occurrence adds a dramatic level of excitement to winter birding. Some of the birds we are most likely to see in these winter irruptions are the winter finches such as common redpolls, pine siskins, pine grosbeaks, red crossbills, white-winged crossbills, purple finches, and evening grosbeaks. Other species shift their typical winter grounds on occasion, such as the bohemian waxwing, varied thrush, red-breasted nuthatch, or the Clark’s nutcracker. When many species irrupt during the same year, it is called a "superflight."

The common redpoll, named for the red cap on its head, is a regular irruptive species that usually comes south into our area every few years. Redpolls feed primarily on the catkins of birch and alder trees in their wintering grounds. In a year of poor catkin production, redpolls will move south, frequenting our bird feeders for seed foods. Redpolls love thistle, or nyger seed, and will swarm thistle feeders in large numbers. On irruptive years, feeder flocks can reach 40 or 50, and upwards towards 100 birds. Redpolls have throat pouches for temporarily storing seeds, so when at feeders they may fill their pouches with seeds quickly, and fly to a more protected area to eat the seeds. During mid-March, redpolls will begin their journey north again.

Red and white-winged crossbills are two irruptive species that depend so completely on conifer seeds for food that they regularly wander far and wide during winter. They are the only birds in North America that have bills crossing at the tips like confused scissors. One story is that the bill shape arose because crossbills tried to extract nails that held Jesus to the cross, but biologists explain that it enables birds to pry apart cone scales for seed extraction with their spoon-tipped tongues. They can also hold cones with their legs, and hang upside down, using their bills to cling to branches – making them seem a bit like a northern parrot-like bird. Since cone-bearing trees are unreliable in their cone crop, this makes crossbills more nomadic and unpredictable. Certainly, with the pine cone crop we saw fall this year, it is not a surprise to be seeing or hearing about crossbills in the area this winter. This morning, Museum staff observed flocks of probably over 250 crossbills in the conifers around the Museum.

Scientists at the Cornell Lab of Ornithology are studying why these irruptions occur, and are monitoring these events. Citizens can find an online bird irruption survey form to submit on their BirdSource website. Although we might not be experiencing a “superflight” year, keep an eye out at your feeder, or along roadsides for any of those fun-to-see irruptive bird species.

An added note of caution: Please watch for flocks of birds resting on our local highways during the extreme cold. They are ingesting gravel which helps with their digestive system. They are slow to move and we have been noting many casualties.

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 13470 County Highway M or on the web at http://www.cablemuseum.org/ to learn more about exhibits and programs.