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How Do Animals Know When to Hibernate or Migrate?

More springlike weather is headed to the Northern Hemisphere. Temperatures are beginning to creep out of the chill, and parts of the region are beginning to see an increasing number of warmer days. Trees are slowly being populated with blooms, and the grass is greening as the season of regrowth and rebirth is in swing.

The spring shift in weather also affects annual behavior of animals as many emerge from hibernation and others prepare themselves for migration. Changes in temperatures can alter the cues used by species to regulate their behavior.

Dr. Hannah Carey, professor of comparative biosciences at the University of Wisconsin, said that an animal's instincts will outsmart the ever-changing spring weather cycle.

"When you're classified as a hibernator, you don't worry about the temperature outside, you know when it's time to sleep and time to emerge. These animals are on a circannual rhythm, or year-long pattern that allows them to not only hibernate but to do it well enough no matter the weather," Carey said.

Despite the below-zero temperatures of this past winter, Carey said species native to her research area in Wisconsin were prepared to hibernate even with temperatures dropping and snowfall. She said even the toughest weather will not disrupt the ground squirrel's wintry slumber.

"Ground squirrels will start to store up body fat [and energy] in the fall to get ready for an average winter of hibernation. But even this year, [when they emerge from hibernation] we don't expect it to be a terrible year for them even with the more-than-average days in the negatives.

Usually ground squirrels don't concern themselves with freezing temperatures or the potential for snow.

"These mammals sleep underground and use their body fat plus a combination of snow-covered areas as insulation. When they emerge, even a late snowstorm wouldn't put a dent in their annual rhythms," Carey said.

Carey attributed the skill set that allows ground squirrels to be successful hibernators to their ability to live in temperate environments where a variation of weather conditions are expected.

"Natural selection and evolution have prepared ground squirrels for this, but it has a lot to do with location. The temperate environment with varying temperatures and the extent of ice and snow cover are all things that prepare them to be good hibernators in a lot of different climates," Carey said.

Animals have adapted their life cycles to the seasons and resource availability. Some animals have developed behaviors to cope with winter conditions, conserve energy and deal with food scarcity by hibernating, or in some cases, migrating to a warmer climate.

Jeff Kelly, associate professor of biology of the University of Oklahoma Animal Migration Research group, said it may be a bit tougher on a species who migrate rather than hibernate, depending on climate and weather conditions.

"My research on song birds, for example, shows that they are long-distance migrators and based on the weather we've observed this year, they seem to be later in returning to their breeding grounds.

Fickle weather will keep the long-distance migrators at bay.

"Here in Oklahoma, we've had a spring of unpredictable weather where on a Saturday afternoon it could be 80 F, but then Monday it could be cold enough to snow. They aren't going to migrate where their food resources aren't in abundance because the weather hasn't let them grow," Kelly said.

Long-distance migrants, in particular, are at more risk with changeable weather. They are too small to carry the fat and energy they need and therefore residing in a warm climate to collect the food is exactly what they need, Kelly said.

"In contrast, short distance migrants will move on local temperatures, either north or south. Robins, for example, tend to migrate to the mid-Atlantic area and stay there based on the local conditions being conducive to their diet," Kelly said.

Hummingbirds are another easily recognizable migrator that call the northeastern United States their nesting grounds. Tom Auer, biologist of the Important Bird Areas Program of the National Audubon Society, said that this year this species may be delayed in their arrival.

"Migration is a very well-honed evolutionary process, but the risk lies in the shift in climate affecting the flowering schedule," Auer said.

Hummingbirds rely on nectar sources upon their time of arrival, so they have a primary source of food.

"They can survive a few days without food and can manage in frost-threatened areas but not if these conditions are long-standing. That's why most hummingbirds move on food and not temperatures in an area. They also work on a photoperiod, when the length of sunlight changes on a given day, and not warmth or cold," Auer said.

Now that we're entering the last quarter of April, migration should really start picking up pace in the most noticeable format, diversity. More species of birds should be entering the area as photoperiods and food sources become more favorable for migrants, Auer said.

The same can be said for hibernators as well. Be sure to keep an eye out for more wildlife to be gracing the backyard as fickle weather won't put a stop to the ever-continuous circle of life.