Sorting Through Siskins

Ian Stewart

Bird watchers throughout Delaware have had their eyes fixed on their feeders this fall as we are in the midst of an ‘irruption year’. This is a year when many of the finches that normally spend the winter far to our north flood south in large numbers in search of food, presumably because the crop of cones they normally rely upon for seed has been poor. The most obvious invader has been the Pine Siskin (Carduelis pinus), a small streaky brown bird closely related to the familiar American Goldfinch. Flocks of over 50 of these twinkling, feisty finches have been reported at feeders throughout our state with some flocks containing over a hundred birds!

9 Pine Siskins and 1 Purple Finch jostling for food

As well as being a beautiful and lively bird to watch, Pine Siskins are a nice example of how you can sometimes distinguish young birds from old birds if you get a good enough look at them. Aging birds presents challenges that you don’t get with humans. As we get older our skin wrinkles and our hair may turn white or drop out altogether, which makes it easy to tell a 20-year old man from a 70-year old man. However, birds replace their feathers at least once a year during a process known as molt so you can’t tell a 2-year old Cardinal from a 10-year old Cardinal just by looking at it. Still, you can sometimes place songbirds into two broad categories. At this time of year, some birds can be classified as either ‘Hatching Year’ (i.e. hatched in the summer of 2020) or ‘After Hatch Year’ (i.e. hatched before 2020) based on the color or shape of certain feathers or the contrast between adjacent groups of new or old feathers. Sometimes older birds have more brightly colored feathers, as illustrated below in the bright yellow wing stripe usually only found in After Hatching Year Pine Siskins.

In Pine Siskins (and in many other songbirds) the tail feathers possessed by Hatching Year birds are the same ones they started to produce in the nest when they were just a week old and tend to be narrow and pointed, and dull in color. By contrast, the tail feathers possessed by After Hatching Year birds were freshly molted this fall when the bird was an adult and tend to be broad and rounded with a bright yellow base. This is quite easy to see when you are holding the bird which we do as part of DelNature’s Bird Banding project (see images below).

Surprisingly however, the difference in tail shape can often be seen when observing feeding Pine Siskins as this species is remarkably tame and allows you to approach it quite closely as long as you move slowly and don’t make any sudden movement. The bird shown below is probably a Hatching Year bird as it has pointed tail feathers (indicated by red lines).

By contrast, two of the birds shown below are probably After Hatching Year birds as they have rounded tail feathers (indicated by red lines), and one of them also has extensive yellow patches on the wing and tail.

If you want to attract these charismatic birds to your yard so you can try to age them yourself the best food to use is nyjer (thistle seed) or sunflower chips. They seem to prefer tray feeders like the one in the top picture but will readily come to hanging feeders or finch socks, all available at Wild Birds Unlimited in Hockessin. Otherwise the bird blind at Ashland Nature Center is a great place to observe or photograph Pine Siskins as well as several other species, especially on a cold day when the birds are hungry and feeding non-stop. Pine Siskins can be so tame that some people have been feeding them by hand, as seen in this incredible video taken at Ashland Nature Center by our hawk watcher David Brown!

To read the original study showing tail shape is a reliable method for aging Pine Siskins, see Yunick RP (1995) Rectrix Shape as a Criterion for Determining Age of the Pine Siskin. North American Bird Bander 20, 101-105.

If you want to learn more about winter birds such as Pine Siskins and how DelNature manages our land to increase biodiversity please click on this link for a new 3-part program featuring a lecture and guided field trips to our private Coverdale Farm Preserve!

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