Where did they come from? is the most-often asked question when people notice robins flocking to junipers and pooping all over their cars and porches. I never saw this many last summer, is the usual comment after the question.
The answer is that these are not our robins, that is, last summer's robins. The robins we're seeing and hearing in the junipers and backyards during winter are down from the Far North , probably from Canada and Washington, perhaps a few from Alaska and the Northwest Territories. And they're here for only two reasons: companionship and food. When one robin finds food, everyone finds food, and food means juniper berries (and other fruit).
There is one vital element in the robin's winter survival that must be available or they are in serious trouble: water. Juniper berries without ample water will ferment (and rot) in the robin's innards (probably gizzard and crop) and this causes a chemical reaction in the bird's blood, leading to confusion, head-bumping, window-crashing and even death.
Here, in Central Oregon, we have the perfect combination for robins to survive. Juniper trees for shelter and food, irrigation water, the Deschutes, and as a last resort people who leave (unfrozen) water out for other birds.
All the robins flitting about the countryside are also a blessing to the Accipiters (ah-sip-eh-ters), a tribe of hawks that prey on birds for a living. There are three species of them, the largest is the Northern Goshawk, one of, if not the best, bird-eaters Nature has yet to come up with. They put the fear of God in large birds, such as grouse, and if there is any such bird as a chicken hawk, I'd say that's it.
The middle-sized accipiter is the Cooper's Hawk, another expert at catching and eating robins, pigeons and such. People who keep a birdfeeder well stocked for backyard birds sometimes complain when they witness a Cooper's Hawk, or their smaller cousin, the Sharp-shinned Hawk, go blasting by the feeder carrying a junco or finch. (Evidence of Darwin's "Survivor of the Fittest" in action.)
The main reason Accipiters are good at catching birds is their prey cannot escape without a lot of luck. Accipiters have large grasping talons, a long tail and short, blunt wings. When a robin dashes into a thicket to escape pursuit, it often stops in the midst of the twigs and limbs, maybe thinking it is safe. Not so! The hawk pulls in its wings and goes right into the thicket after said robin, junco or finch, steering with its long tail, keeping up the remarkable speed with those blunt wings and pouncing on the unsuspecting victim with those needle-sharp talons.
So, life isn't all roses and cream for the thousand of robins you are seeing around us right now (even with the phenomenon of "safety in numbers"). But, about mid-February the survivors will begin their trek back to the Far North, and "our" robins, who have been lying about in the warmth of the Sacramento Valley like true snowbirds, will begin their trek back to Oregon. And the thing that triggers all this moving about is the Sun, the great and all-powerful Giver of Life on this Grand Old Earth.
As the earth swings around our Sun the angle shortens, and duration of light striking the surface lengthens. It is that change that triggers hormones in just about all animal life in the Northern Hemisphere, the biological urge to begin the process of begetting their kind.
Your robins will be back by March, you'll hear the males first singing their heads off, telling other male robins that, I have come home to defend my territory! That's just one reason why it's important to leave old robin nests where you find them, they're a sign to this year's couple that your back yard is a safe place to raise babies.