During the past week several birds have returned to Big Valley from their winter grounds – turkey vultures, red-winged blackbirds and tree swallows, among others. Saturday my definitive harbingers of spring finally arrived. February 16th is this year’s “official” date for the return of the sandhill cranes (Grus canadensis). Never mind that snow remains on the ground and that temperatures at night still are well below freezing! For me, once I hear the sandhill cranes’ prehistoric calls echoing across the valley, spring is on its way.
Although approximately 80% of all North American migrating sandhill cranes stage and pass through the Platte River Basin in Nabraska, there is a sandhill crane population that winters in the Central Valley of California. These are the cranes that return to Big Valley and the Pacific Northwest as far north as Alaska.
While a sharp north winds blows I watch the cranes pick around in the snow and wonder why these magnificent birds did not simply stay in the warmth of the Central Valley. More snow is predicted for this week. These cranes will remain here throughout the breeding season and summer. Why not delay summer vacation until it is summer?
Birds have several migratory cues: photoperiod or length of day, temperature, circannual rhythms, and internal cues such as fat reserves. In addition other factors (food supply, changing habitat, presence of other species members, for example) can influence migration. I believe photoperiod (the absolute or changing amount of light) is a prime stimulus for migration. Yet I also feel there is not a single determinant initiating migration, but rather an unknown and subtle interaction between many influences.
Over the last seven years the sandhill cranes have returned to Big Valley between February 11th (2007) and February 25th (2009). This is the second year Leonard and I first noted them on the 16th of February, the other time being in 2008.
These sandhill cranes were photographed at Ash Creek Wildlife Area near Pilot Butte (Modoc County CA).