Irruptive invasions are sudden large movements of one or more species into an area where they are uncommon or not expected. We see this mostly with northern passerines, winter finches and owls. Most irruptions occur during the winter months and are normally due to food shortages on their normal wintering grounds. It does not happen every year, and for certain species it may be many years or even decades between irruptions.
In 2007 we witnessed a huge invasion of Red-breasted Nuthatches into the Delaware Valley beginning in the summer months. Something occurred on their breeding grounds, likely a complete shortage of invertebrates, that forced them to head south without even raising a brood. In some winters, like this one, we hardly get to see even one Red-breasted Nuthatch.
This winter we have begun to see good numbers of Pine Siskins and the highly nomadic White-winged Crossbills, spreading over our area. Finally we had beautiful day so I headed out this morning to Valley Forge National Park to look for them. Both these species occur in dense evergreen forest and in our area in winter are best looked for in areas with an abundance of conifer cones. One such area is the Washington Memorial Chapel Churchyard which has a few stands of pine, hemlock, and spruce amongst the deciduous trees.
I may have been the first one to arrive but others soon joined me. With the help of Rick, Eli, Ron, Liza, Philip and John we combed the area finding a flock of Pine Siskins. We eventually heard the noisy continuous rattling calls of White-winged Crossbills. Tracking them down we had a small flock of females followed by a wonderful group of mostly bright reddish-pink males amongst about a dozen birds. Since the bills of White-winged Crossbills are small, and do not have the variation in bill size that Red Crossbills do, they favor the smaller and softer hemlock and spruce cones. And this is exactly where we found them, using their unique cross-bills to pry between the scales of the cones as they foraged anywhere from midlevel to the top of the trees.