Animal Migration in the Sonoran Desert Region

Many of us are anticipating the arrival of sun-seeking visitors from afar due to the holiday season. But what if your winter visitors had to swim 6,000 miles to get here? That is exactly what our region’s largest winter visitors do. Each winter, the Sonoran Desert bears witness to the greatest mammal migration on Earth. Gray Whales travel some 12,000 miles round trip between summer feeding grounds in Arctic Alaskan and Siberian waters to winter calving and breeding lagoons off Baja California’s Pacific coast. These warm, shallow, salty bays are ideal calving grounds. The warm temperatures allow babies to quickly build up fat from their mothers’ rich milk. Shallow depths prevent intrusion by the whales’ predators, Orca Whales. Extra-salty water provides buoyancy for baby whales to nurse, rest, and hone their swimming skills.

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A Gray Whale. Photo: ASDM/Julie Xelowski-Brooker

While the babies quickly gain weight, adults rarely feed in their nursery or on migration, and instead live off of stored fat. By late April or early May, they are ready to embark for the north. Along their route they face threats from Orca attack, collision with ships, pollution, and subsistence whaling in their summer feeding grounds. But the Arctic waters they seek are rich in small, shrimplike amphipods and tube worms, which Gray Whales, like hydrodynamic bulldozers, scoop up in enormous mouthfuls of sea floor sediments and filter with their baleen. On a daily diet of nearly a ton of tiny organisms, they can regain up to 30 percent of their body weight and build up enough fat to sustain them for the return trip. By October, shorter day length, colder temperatures, and encroaching sea ice signal that it is time to swim south. In late December and early January they reach the warm cradle of desert waters off Baja. Migration is about balancing risk and reward, and the benefits outweigh the costs of such a long journey.

How Gray Whales navigate on this epic trip is still a matter of scientific debate. Like many migratory species, they probably rely on a combination of senses.  Baleen whales have excellent hearing. They use low-frequency sounds to communicate with each other and can orient to the sounds of waves along the coastline. They also have a built-in magnetic compass – minute crystals of magnetite in their brain that help them detect the Earth’s magnetic fields. In addition, whales “spy hop,” thrusting their heads above water to survey landmarks. No matter the means, their ability to travel across vast distances and return to the same place every year is remarkable.

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White-winged dove. Photo: Rhonda Spencer

While no other migrations in the Sonoran Desert region reach the superlative extremes of the Gray Whale’s, many creatures’ peregrinations mark subtle seasonal changes here. It is a sure sign of spring when Turkey Vultures and Lesser Nighthawks return from their wintering grounds further south. White-winged Doves and Lesser long-nosed bats are not far behind. Their migration follows the flowering and fruiting of saguaros and other columnar cacti in late spring. Nectar, pollen, and resulting fruits and seeds provide most of the moisture and nourishment these bats and birds require to raise their young.  Mexican Free-tailed Bats also spend summers here, feeding on abundant insects in urban areas and agricultural fields.

Some animals are altitudinal migrants, following seasonal resources up and down the slopes of our region’s sky island mountains. Anna’s Hummingbirds are year-round residents, but they move up into the mountains when migrating Black-chinned Hummingbirds arrive for summer, descending again when they depart. Phainopeplas and White-tailed Deer move up the mountains in summer to where it’s cooler, then descend in winter to seek food and warmth in the lower desert. In mild winters, deer may stay high.  Similarly, Red-tailed Hawks from northern Arizona will migrate to southern Arizona in winter to find warmer temperatures and more active prey, but may remain north in mild winters.

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Anna’s Hummingbird. Photo: Jay Pierstorff

We are just beginning to understand the migration patterns of some of our region’s species.  Monarch butterflies, an iconic symbol of the mysteries of animal migration, appear here in late summer and early fall en route to overwintering grounds. Monarchs in the eastern U.S. and Canada generally migrate to a handful of fir-topped mountains in Michoacán, Mexico, where they overwinter by the millions. Smaller western populations overwinter along the California coast. To understand how Monarchs in our region move between and among these two populations and better protect important migratory corridors, biologists tag and track their movements – a true challenge with an animal so small.

Animals face many hazards as they migrate, including storms, drought, food scarcity, predators, and the physical demands of the journey. Human activities – hunting, habitat destruction, construction of obstacles such as skyscrapers and wind turbines, and climate change – exacerbate these threats. We can help animal migrants by protecting habitat along their routes, slowing climate change, and creating sound or visual barriers that help animals avoid obstacles. Pollination gardens and landscapes of native plants are both café and motel for weary travelers in need of fuel and resting places. These efforts will not only help migrating species on their amazing journeys, but will keep alive that spark of wonder they bring us from afar.

Written by Robin Kropp, ASDM Education Specialist


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