Junco Winter Adaptations

Juncos spend the winter spread across the United States.  They spend summers at breeding areas in southern Canada, and in fall males migrate to the northern states while females migrate farther south.  Males have several adaptations that allow them to more easily survive in colder climates, and allow them to return early to breeding areas to claim the best territory for attracting a mate among the females that come back a few weeks later.

Junco males are small, round-bodied birds with a slate gray back and are larger and weigh more than females.  According to Bergmann’s Rule, large bodies have a smaller surface area relative to their volume, and are therefore able to retain more heat than longer or skinnier bodies.  Being able to retain more heat for longer periods, an average-sized male junco can go two hours longer without food than an average female, and a large male may go more than ten hours longer between meals.  This can be an essential factor when living through a colder period.

Tree cavities provide excellent places to shelter in winter.  At night and in severe weather, juncos seek a small, tight space big enough for a single bird that provides protection from wind and moisture.  During daytime hours, while foraging, juncos can often be found in conifer trees whose year-round needles provide a windbreak and a place to remain out of sight of predators.

Juncos have a normal body temperature of 107˚F, well above the average outside temperature on a winter day which could be between -10˚F and +20˚F.  Maintaining body heat requires a high-caloric diet to create the heat and resources to not allow the heat to escape.  Birds have muscular control over their feathers and are able to fluff them, trapping air next to their skin.  Preening is a daily activity where oil secreted from a gland above the tail is rubbed on all of their feathers providing a moisture barrier that keeps water, snow, and cold winds away from their skin.  Both feathers and trapped air make up two layers of insulation that work efficiently to retain body heat.

During extreme weather, it may be advantageous to remain sheltered and inactive when the amount of calories burned to get a meal is more than the calories in the meal.  When sheltering for longer periods, juncos may intentionally lower their body temperatures a few degrees, entering a state of torpor.  This state lasts for only a few hours, but saves enough energy so that the bird does not need to forage for an extended period during a day.  Using muscular control, birds can generate body heat by contracting and releasing their muscles, similar to shivering in mammals, and bring their body temperatures back to normal and resume daily activities.

Northern U.S. flocks of juncos in winter are 70% or more males, while most females spend winters in the southern U.S.  This is also true for several other species that migrate with juncos including tree sparrows, song sparrows, and mourning doves.  Juncos will be leaving in late winter to return to breeding grounds across Canada, but for the coming few months we will certainly enjoy their presence in our area.