How Caribou Decide When to Migrate South in the Fall

Caribou adjust their migration timing to the weather and conditions around them. NPS/M. Cameron.

The question

Caribou make some of the longest migrations on Earth, but how they decide when to head south in the fall has been something of a mystery. Many hoofed migrants time spring migration to the green-up of new plants, moving together; autumn migrations are less synchronized, and their cues less studied.

The study

National Park Service biologists and colleagues at the University of Alaska Fairbanks studied the Western Arctic Herd, Alaska's largest.

• GPS collars on females tracked them from 2010 to 2018 — 389 caribou-seasons across 9 years.
• Each location was paired with the weather and conditions there at that moment — temperature, snow depth, wind, barometric pressure — and a measure of vegetation.
• A movement model classified each move as migration (long, persistent, directional) or shorter moves such as feeding, and related it to the conditions each caribou experienced.

What they found

• Falling temperatures and the first snowfalls of winter were the main signals to migrate — strong and consistent from year to year. Wind, vegetation and barometric pressure mattered in some years but not others.
• Caribou keep deciding along the way. If they reached a place that was warmer or snow-free, they might slow down and linger, moving on when it cooled and snow fell again. They effectively pace migration along gradients of temperature and snow.
• Why fall timing varies so much: in autumn caribou are spread out, so individuals meet very different conditions.
• A mirror of spring: pacing to the approach of winter parallels the better-known way migrants follow the wave of spring growth.

Why it matters

The herd's fall migration has come later over the last three decades, and this work suggests the trend will continue as the Arctic warms.

Sources

Based on "How Do Caribou Decide When to Migrate in the Fall?," Gates of the Arctic National Park and Preserve, National Park Service, summarizing M. D. Cameron, J. M. Eisaguirre, G. A. Breed, K. Joly and K. Kielland, "Mechanistic movement models identify continuously updated autumn migration cues in Arctic caribou," Movement Ecology (2021); the NPS summary is a work of the United States government in the public domain, and the paper's abstract is paraphrased.