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.

8/29/16 – Migration in Bald eagles

Migrating waterfowl 8/23/16 Charlo, MT. Image courtesy of Explore and Owl Research Institute
Migrating waterfowl 8/23/16 Charlo, MT. Image courtesy of Explore and Owl Research Institute

8/29/16 – Migrating in birds is all about flying south for the winter, right? Those classic V-formations of honking geese heading south as a harbinger of winter is deeply ingrained in American culture. Well, as with many things in life, migrations can be more complicated than that.

Bald eagles are found across much of North America and they face many different types of environments and challenges. Bald eagles exhibit a wide variety of behaviors when it comes to migration.

Those hatched in the southern areas have enough time to migrate north during the latter part of the summer, before turning around and heading south for the winter, back to where they started. Eagles that live farther north only have enough time in their season to head south in the fall and return to the north in the spring.

But do all Bald eagles migrate? No, it is not that simple either!

In areas that ice covers during the winter eagles do move to ice-free areas, but these can sometimes be found nearby in areas with turbulent waters. In Nova Scotia and Maine eagles may remain for the winter, possibly instigated when wildlife officials offered pesticide-free carrion for eagles to feed on during the post-DDT recovery era.

Eagle warning sign courtesy Hal Heatwole
Eagle warning sign courtesy Hal Heatwole

In the Aleutian Islands and Chilkat River in Southeastern Alaska eagles may find enough food to remain for the winter, or move locally among the rivers of the Pacific Northwest to exploit various salmon runs. While eagles hatched much farther south, from California to Arizona, and north to the Greater Yellowstone Ecosystem also move into the area to feed on spawning salmon.

Eagle and bridge over Iliuk River courtesy of Hal Heatwole
Eagle and bridge over Iliuk River courtesy of Hal Heatwole

So Bald eagles migrate to feed on salmon? Yes, but it is not that simple either. Farther east, from heart of the Great Plains to the western front of the great eastern Boreal forests Bald eagles funnel into the waters of the UpperĀ  Mississippi River to feed on Gizzard Shad during the winter. In other locations the creation of reservoirs and dams and the introduction of fish has drawn eagles to feed during the winter. They also feed on other food types as well, exploiting waterfowl or jackrabbits in some locations.

Here in Pittsburgh the adult eagles are residents, meaning they are not migrating to new feeding grounds like eagles from areas with harsher winter conditions. Although, trail reporters and nestwatchers of the neighboring Hays pair do report an absence of eagles for a short period in the fall which may correspond with them exploiting a more abundant food source, we can’t say it is definitely a migration.

Eagle pair in Unalaska courtesy Hal Heatwole
Eagle pair in Unalaska courtesy Hal Heatwole

However, when eagles do migrate they often use traditional flyways, like other birds. This allows eagles to soar on thermals that rise along the coasts, the major rivers and mountain ranges. One of these traditional flyways runs along the Appalachian Mountains, at Kittatinny Ridge. Because these are traditional flyways used by a number of raptor species, you have fantastic viewing opportunities, and researchers have opportunities to track movements and numbers of migrating birds. If you are interested in watching the migrations or participating in their research, please check out Hawk Mountain Sanctuary, located about an hour from Harrisburg or 5-6 hours from Pittsburgh.

Where do eagles from the eastern US go? The Center for Conservation Biology is studying just that question.

For a migration study in the mid-west, see the American Eagle Foundation migration study.

To learn about the migration of eagles in the western US, try Sutton Avian Research Center.

Read more about how and why some Bald eagles (and Peregrine falcons) migrate, and for more in-depth information about avian migration in general, check out this page from Cornell.

 

8/5/16 – When will the juvies leave?

8/5/16 – We are still getting reports of the Harmar juveniles being seen on the hillside near the nest, but we don’t know how much longer this will happen. EagleStreamer, who watches the Hays eagles in person and visits the trail almost daily, reports the Hays juvies may have left the area. Remember, the Hays eaglets fledged about three weeks ahead of the Harmar eaglets, but the Harmar juvies are likely not going to stick around too much longer. If you live in the area and are thinking of seeing them, you had better go soon! Thank you to chatter jamary for sending this photo of one of the juvies carrying grass to the nest! Already practicing nest building skills, how about that?

Harmar juvie carrying grass on 8/3/16
Harmar juvie carrying grass on 8/3/16

If you remember back to when the Harmar eaglets fledged, we saw Hr 3 leave the nest on July 5th, but on the afternoon of July 6th Hr 2 disappeared, but we did not know if the eaglet had fledged, or just gone off cam, as we had a very limited view of the nest at that point. On July 7th, the Harmar female visited the nest and neither of the juvies appeared. At that point we strongly suspected that Hr 2 had fledged also. We did have a field report of one juvie being seen sitting high in the nest tree, then flying to an adult in the roost tree and making a nice landing (thanks Gerry!). It wasn’t until July 8th that Hr 2 returned to the nest, confirming that he indeed fledged! Later, the female makes a food drop and Hr 2 returns to claim the food. Soon Hr 3 returns also, confirming that both eaglets are fine after fledging. The next day they are still having meals on the nest, here Hr 3 eats while Hr 2 shows us a take-off and fly-by. Sometimes the juvies were even returning to the nest to simply hang out. When food is around, Hr 2 often pounced first, here he stays true to form when the male makes a food drop on the nest. When Hr 3 arrives and wants in on the meal, Hr 2 is not ready to give it up just yet.

Make sure to check out the pictures that chatters annette and jamary sent from the field. I will continue to pass on reports of the juvies as long as I continue to get them.