Fish crows (Corvus ossifragus) are a species of crow native to the southeastern United States. Their name comes from their habit of feeding on fish and other aquatic animals. Fish crows are smaller and more slender than the familiar American crow (Corvus brachyrhynchos). One thing that puzzles many people about fish crows is whether they migrate like many other bird species do.
Quick answer
Fish crows are partially migratory. Some populations, like those in the northernmost parts of their range, are migratory and travel south for the winter. However, most fish crow populations are year-round residents that do not migrate significant distances.
Do fish crows migrate?
The answer is that some fish crow populations migrate while others do not:
- Northern populations: Fish crows in the northernmost parts of their range, such as southern Ontario, migrate south for the winter. They travel to more southern U.S. states like Tennessee, Kentucky, and Arkansas.
- Coastal populations: Fish crows along the Atlantic and Gulf Coasts of the U.S. are generally year-round residents. They may wander short distances in winter but do not make true migratory journeys.
- Inland populations: Inland fish crow populations away from the coasts, such as those in the Ohio River Valley, also tend to be year-round residents.
So in summary, the northernmost fish crow populations exhibit migratory behavior while most other populations are non-migratory or only partially migratory.
Why do only some fish crow populations migrate?
Several factors likely influence which fish crow populations migrate versus staying put year-round:
- Climate: Colder winters in the northern part of their range make migration advantageous there to escape freezing weather and find more abundant food.
- Food availability: Coastal and inland crows have more reliable winter food sources like aquatic prey, grain fields, and human settlement garbage.
- Predation: Migratory crows avoid predators like hawks that are more abundant in winter in the north.
- Habitat: Non-migratory crows occupy suitable year-round habitats like coastlines and rivers.
In essence, crows in the warmer and more food-rich southern parts of the fish crow range have less need to migrate than their northern counterparts.
What is their migration pattern?
The migratory behavior of fish crows generally follows this pattern:
- Breeding grounds: They breed during spring and summer as far north as southern Ontario.
- Fall migration: They start migrating south in September and October.
- Winter range: They winter as far south as the Gulf Coast states.
- Spring migration: They fly back north in March and April.
Their migrations cover a distance of several hundred miles between their breeding and wintering grounds. Fish crows primarily migrate during the day and may gather in large flocks at night during migration.
Do they always migrate to the same areas?
In many bird species, individuals show high site fidelity to their breeding and wintering grounds. In other words, they return to the same locations year after year. More research is needed, but limited evidence suggests fish crows exhibit site fidelity to some degree.
Banding studies have found fish crows returning to prior wintering sites. However, they may not be as tied to specific wintering sites as they are to breeding sites. Some wintering latitude may vary depending on weather conditions each year.
How do fish crows migrate?
Fish crows migrate during the day in flocks of 10 to 100 or more birds. Their flight style involves alternating flapping and gliding. Some details on how they migrate include:
- Departure: They leave breeding grounds on clear days with tailwinds.
- Speed: They may migrate at speeds of 20-45 mph when aided by tailwinds.
- Distance: They can cover 200+ miles on a single day with favorable winds.
- Stopovers: They stop to rest and feed in wetlands, fields, and woodlands.
- Arrival: They reach wintering grounds after 1-2 weeks of migration.
Younger fish crows often migrate slightly later in fall and earlier in spring than older adults. The timing and pace of migration can vary significantly depending on weather conditions in a given year.
What routes do they take?
Research into fish crow migration pathways and routes is still quite limited. However, some of the main flyways they likely follow include:
- Ontario to the Southeast U.S.
- Atlantic Coast to the Gulf Coast
- Mississippi River Valley south through Louisiana
- Ohio River Valley south to Tennessee and Arkansas
Radio-tracking studies of migratory movements between specific breeding and wintering sites would help shed more light on their exact migration routes. Routes can be quite flexible from year to year.
How do they navigate during migration?
Fish crows use some fascinating methods to navigate during their long migrations. These include:
- Geomagnetic senses – They may detect subtle shifts in Earth’s magnetic fields to help guide direction.
- Landscape cues – They follow coastlines, rivers, mountains, and other landmarks.
- Celestial cues – They use the position of the sun and patterns of stars.
- Smell – Odors help lead them to suitable stopover sites.
- Memory – Young crows likely learn routes and sites from older birds.
Fish crows, like many migratory birds, have a remarkable “GPS system” involving all their senses and abilities to navigate thousands of miles annually.
Do they migrate alone or in flocks?
Fish crows primarily migrate in flocks rather than alone. Flock sizes range from about 10 to over 100 crows during migration. Travelling in flocks provides several advantages such as:
- Safety from predators
- Ability to find food and shelter sites
- Navigation help from experienced crows
- Aerodynamic drafting to reduce energy expenditure
Younger fish crows may join flocks with older birds to learn migration pathways and sites. Larger flocks typically break into smaller groups temporarily when stopping to rest and feed during migration.
How far do they migrate in a season?
The total migration journey fish crows make each spring and fall can span several hundred miles between their breeding and wintering grounds. Some examples of estimated total migration distances include:
- Southern Ontario to Tennessee: ~700 miles
- New York to Louisiana: ~1,200 miles
- Massachusetts to Florida: ~1,000 miles
These journeys require great energy reserves to complete without stopping. Inclement weather like storms can also lengthen the migration distances from year to year for some flocks.
What dangers do migrating fish crows face?
Fish crows face many threats and challenges during their annual migrations. Some of the main dangers include:
- Exhaustion – Flying hundreds of miles is tiring and they risk using up energy stores.
- Starvation – Finding enough food at stopovers is critical.
- Predators – Falcons, eagles, and hawks prey on crows during migration.
- Severe weather – Storms or high winds can blow flocks dangerously off course.
- Collisions – Power lines, wind turbines, trees, buildings, and other objects pose collision risks.
Natural selection has favored individuals and flocks that successfully navigate these migration dangers and conserve enough energy to complete journeys.
Do they stop to rest and feed during migration? If so, where?
Fish crows need to periodically stop to rest and feed during migration to replenish their energy reserves. They forage and roost in a variety of suitable stopover sites, including:
- Fields – They feed on seeds, grains, and insects in agricultural fields.
- Marshes – Shallow wetlands provide aquatic prey like frogs, fish, and mollusks.
- Lakeshores – They consume fish, crustaceans, and other food along lakes.
- Forests – They eat seeds, fruits, and arthropods in woodland habitats.
Crows may gather in the hundreds or thousands at favored migration stopover sites. They rest in trees and other sheltered spots before moving on. Most stopovers last anywhere from a few hours up to a couple days.
Do they fatten up before migrating?
Yes, fish crows go through a period of intense feeding and fat storage just prior to fall migration. They may increase their body mass by 30-50% by building up fat reserves.
This helps provide the energy needed to fly long distances without starving. Some ways crows prepare include:
- Eating more – Consuming extra food to store fat
- Foraging more – Devoting more time to finding food
- Storing food – Caching surplus food to eat later
- Lower activity – Resting more to conserve energy
After a period of hyperphagia or excessive eating, the crows are ready for the high energy demands of migration. Those with the largest fat reserves tend to have the highest migration success.
Conclusion
In summary, some northern fish crow populations are migratory, traveling hundreds of miles southward in fall from their breeding grounds to wintering habitats. Most southern fish crows are non-migratory and remain year-round on breeding territories. Migrating crows use sophisticated navigation strategies and stopover behaviors to complete their long seasonal journeys, which are driven by climate, food availability, and other factors.