Irruptions

Some species of birds are residents that remain in the same place all year long.  Other species migrate from breeding grounds to wintering grounds and back again.  Of these migrants, some are latitudinal migrants that move from north to south and then south to north, and others are altitudinal migrants that move down-slope in winter to avoid harsh conditions higher up, and then move up-slope in summer to take advantage of the short, but intense, mountain growing season.  Many species have a mix of both resident populations that occur at lower latitudes or altitudes staying put, and other populations of the same species that are migratory and occur at higher latitudes or altitudes which move to warmer areas when they have to.  Some species are nomads that wander across a landscape in search of food with no reliable annual pattern.  And then there are the irruptive species that respond to food and/or weather conditions and can move far outside their usual geographic ranges in order to avoid poor conditions and enjoy favorable conditions.

These irruptive species generally belong to one of the other types of movement groups.  Snowy Owls, for example, are usually residents except for when food is scarce when they can move south quite far into the lower 48 states.  No one is sure why some species are irruptive and other are not; they are scattered across many different bird families, and have all kinds of different natural histories.  Irruptions certainly do not occur every year, and even when they do occur, they can be of different magnitudes, but if they are going to occur it is fall and winter because if food is going to run out, that is when it will happen.  This year we seem to be having a bit of a Red-breasted Nuthatch irruption in the Davis area.  Last year we had only a few of these birds hanging around the Davis area in winter, but now they are of lots of them in Davis and West Sacramento as well.  From what I saw last weekend while visiting my mom in Berkeley, they are having a bit of a Pine Siskin irruption there.  She had a flock of at least 25 birds constantly swarming over her feeders where as I have seen years when there were few to none to none at all in Berkeley.  Last winter we have a significant Evening Grosbeak irruption which was quite unusual.  Since I am studying this species, I got sent a number of recordings of the flight calls that these irruptive birds were making.  Interestingly, the birds that were showing up in the central valley or on the central coast of California were not making the Type 2 flight call that is most commonly found in Sierra Nevada (which is the closest population).  Instead, they all seemed to be making Type 1 flight calls which is generally found in the Pacific Northwest and the central Rockey Mountains!

Tracking these irruptions is not only exciting, but it may be able to tell us about the stability of the habitats that the birds come from.  In this time of rapid global climate change, understanding which habitats are remaining relatively stable and which are not could be very important in bird conservation.