Marbled murrelets are opportunistic feeders and utilize prey of diverse sizes and species. This diving seabird feeds primarily on small schooling fish and invertebrates in shallow, near-shore marine waters. Small schooling fish (such as Pacific anchovy, Pacific herring, candlefish, and Pacific sand lance) make up most of the diet, which may include small crustaceans when fish are not abundant.

During the breeding season, males and females have sooty-brown upperparts with dark bars. Underparts are light, mottled (or “marbled”) brown. During winter (the non-breeding season) adults have brownish-gray upperparts and white scapulars (top of wings). The plumage of fledged young is similar to that of adults in winter. Chicks are downy and tan-colored with dark speckling.

The marbled murrelet is a small Pacific seabird belonging to the family Alcidae. They are fast fliers with rapid wingbeats, short wings, are about 10 inches long and have a slender black beak.

The marbled murrelet spends the majority of its time on the ocean, resting and feeding in near-shore marine waters and come inland to nest. Marine foraging areas are usually within 1.2 to 3 miles of shore, typically in waters less than 100 feet deep. They spend the vast majority of the non-breeding season on the ocean. They have also been detected on rivers and inland lakes, but this is rare.

Marbled murrelets generally nest in old-growth forests characterized by large trees, multiple canopy layers and moderate to high canopy closure. Nest stands vary in size from several acres to thousands of acres; larger unfragmented stands appear to be the highest quality habitat for marbled murrelet nesting. Trees that have large branches or deformities are used as nest trees. Murrelets don’t build nests, but lay a single egg on a mat of moss, lichen or debris accumulations on these branches or deformities. Nest stands are dominated by Douglas fir in Oregon and Washington, and by old-growth redwoods and Douglas-fir in California. These forests are located close enough to the marine environment for the birds to fly to and from nest sites. Nests have been found inland from marine waters up to a distance of 53 miles in Washington state. In the non-forested portions of Alaska, murrelets can also nest on the ground or in rock cavities.

The primary cause of past marbled murrelet population decline is the loss and modification of nesting habitat due to commercial timber harvests, human-induced fires, land conversions and through natural causes such as wildfires and windstorms. In general, forest management practices that maximize timber production cut forest stands every 40 to 60 years, then replant. Since it takes 100 to 250 years to grow marbled murrelet nesting habitat, this time frame frequently does not allow older forest characteristics to develop, thus eliminating large forest management areas from providing future nesting habitat. Continued harvest of old growth and mature forests also perpetuates the loss and fragmentation of remaining habitat. Increased forest fragmentation can reduce nesting success by allowing increased predation of nests by raptors like great horned owls and sharp-shinned hawks, and corvids, like jays, ravens and crows. In the murrelet's marine habitat, harmful algal blooms, prey availability, oil spills and gill net fishing due to entanglement also threaten the population.

Climate change is likely to exacerbate the impacts of continued nesting habitat loss and fragmentation, as well as negatively alter marine habitat conditions. In particular, anthropogenic climate change climate change
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.

Learn more about climate change has the potential to substantially affect the coast redwood forests in California and Oregon, in which this forest type is projected to experience a reduction of nearly one-fourth its range. Climate change in the marine environment is projected to result in changes throughout the marine food web, further reducing prey quality and quantity.

Marbled murrelets are long-lived seabirds, living about 15 years, that spend most of their life in the marine environment, but use older forests for nesting. Courtship, foraging, loafing, molting and preening occur in near-shore marine waters.

Marbled murrelets nest asynchronously, meaning not all at the same time, across a prolonged nesting season from approximately early April to late September, depending on latitude. Murrelets achieve sexual maturity at approximately age 2 or 3, and lay a single egg on a nest platform. Not all adults nest every year, and renesting is rare. Both sexes incubate the egg in alternating 24 hour shifts for approximately 30 days. During this time they exchange incubation duties under cover of darkness, presumably to avoid leading predators to the nest. Upon hatching, murrelet chicks are virtually helpless and rely on the adults for food. The adults feed the chick at least once per day, flying in, primarily at dawn and dusk, from feeding on the ocean and carry one fish at a time. A large proportion of nests fail due to predation by corvids and raptors. The young fledge alone, unaccompanied by an adult, and fly directly to the sea. Marbled murrelets can’t achieve flight from the ground, only from water or an elevated tree limb. If they fall to the ground, they can’t get back in the air. Many chicks have died during their fledging flights.

Marbled murrelets are opportunistic feeders and utilize prey of diverse sizes and species. This diving seabird feeds primarily on small schooling fish and invertebrates in shallow, near-shore marine waters. Small schooling fish like Pacific anchovy, Pacific herring, candlefish and Pacific sand lance, make up most of the diet. When fish are not abundant, marbled murrelets may also feed on small crustaceans.