Fall migration has wrapped up for all but a few bird species. This semi-annual rite of passage typically follows predictable timetables and geographic routes. Exceptions to the rule, "irruptive" species, are northerners that head this way certain winters, driven out of their home territories by food scarcity.
Each fall, New Hampshire birders await a forecast of irruptive bird movements made by Canadian field ornithologist, Ron Pittaway. For birds to leave familiar territory, the reasons have to be compelling. So logically, Pittway's predictions are based on the availability of food for birds in the northern forest. This year is unusual in that wild birds three main food sources are producing bumper crops: conifer cones, birch seeds and mountain-ash berries.
Irruptive species that head south when their preferred food is scarce will likely stay north this winter, widely dispersed and well fed, which is great news for the birds. In our excitement as as we watch our backyard feeders, crabapples and conifers flourish with exotic northerners, it's easy to forget that hardship drives them our way.
Mostly it's fledglings that are forced out in search of food in unfamiliar landscapes while the dominant adults stay put on their home territory. Survival odds that already are low young birds, drop even lower.
A bumper seed or fruit crop stresses a tree and usually is followed by at least one year of seed scarcity that again will drive certain northern birds our way.