8/29/16 – Dispersal and Migration

8/29/16 -The Harmar eagle family is nearing the end of its nesting season. The Hays eagle fans have celebrated the ‘end’ of their eagle season knowing the two eaglets that fledged earlier in the year have now left the area. Thank you to chatter philipp for sharing some of the cake he made for the occasion! A warning: viewing these pictures may cause some serious cravings!

Philipp's eagle cake
Philipp’s eagle cake
Side view showing the delicate layering
Side view showing the delicate layering
White chocolate layer cake with currant jam filling
White chocolate layer cake with currant jam filling

There was a sighting of the Harmar juvenile eagles on 8/22/16. If they haven’t already, they will be leaving the area to disperse soon also. But the adult eagles in Pittsburgh seem to remain in the area, not migrating.

What is the difference between dispersal and migration?

This question is more complicated than it may seem as some biologists and ecologists use different terms for the same thing, and there is a great variety to the behaviors exhibited.

Dispersal can be considered the movement of an organism from where it is born (or hatched, in the case of eagles) to where it breeds. This is a movement that occurs only once during a lifetime and is often the first truly independent movement.

Migration is a seasonal movement between breeding and non-breeding home ranges or territories. These movements occur repeatedly in long-lived animals, but may occur only once in in a lifetime in some animals (Monarch butterflies take four generations to complete one migration loop).

Animals are taking a risk when they head off into the world, having to avoid being eaten in unfamiliar territory, while trying to find enough food and water to keep moving. Why do they do this? In the case of dispersal it is to avoid direct competition with their parents or breeding with siblings. In the case of migration it is an adaptation to changing resources, often food.

In order to avoid breeding with siblings or cousins, one sex usually disperses farther than the other, and which one moves farther depends on the mating system. With most mammals the males usually disperse farther. With most birds the females usually disperse farther. There are exceptions to both cases, but with Bald eagles, the females usually settle farther from their hatch place than the males. This holds true for Peregrine falcons and other raptors also.

Eagle on rock courtesy of Hal Heatwole
Eagle on rock courtesy of Hal Heatwole

Because it takes several years for Bald eagles to mature and settle into a breeding territory, their dispersal actually encompasses several migration cycles. Find out more about Bald eagle migration here.

Here in Pittsburgh the adult eagles are residents, the juvenile eagles, however, will move away, this is called dispersal. Here is more on the difference between migration and dispersal.

Juvenile Bald eagles have longer feathers and different wing shapes than adults, making them fly more slowly. This also means they are more influenced by wind direction than adults are too. Young of the year eagles do not migrate with the adults and often wander widely before actually migrating away from their natal grounds. Juvenile Bald eagles can migrate an average of 150-200 miles a day but this varies widely and they stop and feed along the way for varying lengths of time. Eagles will follow the same routes and visit the same locations each migration, but if needed they are capable of altering their route to exploit a better food resource or avoid some problem.

Eagle on cliff image courtesy of Hal Heatwole
Eagle on cliff image courtesy of Hal Heatwole

Where the juvenile Bald eagles from the Pittsburgh area go is unknown. They may head north towards the Great Lakes, or follow the rivers down the Ohio towards the Land Between the Lakes on the Tennessee and Kentucky border. Or maybe they head east towards the Chesapeake Bay, another location where eagles are known to congregate during the winter. Most juvenile Bald eagles will return to the area near where they were hatched the following breeding seasons, migrating between these locations each year until they settle into a place where they can breed. Eagles of adult age that are not breeding tend to wander more freely about the area, not being as tied to one location like those who are raising young.

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