Monday, September 29, 2014

Four and Dispersal

Four back in Decorah. Photo credit Bob Anderson.
Last week, Four took a little trip 22 miles northwest of her natal nest, although she has since returned and reconnected with Mom and Dad. What was Four doing? Will she ever permanently leave Decorah? How do we define and classify the behavior of a highly nomadic species like the bald eagle? I can't tell you what Four is thinking, but I can tell you a little bit about eagles and their movements.

Dispersal is the movement of individuals from their point of origin (natal area) to where they reproduce or would have reproduced had they survived and mated. Navigation is an animal's movement around its territory or home range. Migration is the regular, seasonal movement of animals between breeding and non-breeding areas. While D1 hadn't yet bred, she was clearly migrating back and forth between a non-breeding territory (Polar Bear Park) and a territory in which she would be likely to breed (NE Iowa).

It is theorized that animals disperse for one of three reasons:
  • To avoid inbreeding
  • To avoid kin competition
  • To colonize new habitat
A detailed discussion of these theories can be found here, but all three dispersal strategies are linked to reproductive success. Inbreeding takes a toll on reproductive fitness in the long run, and kin competition can have immediate deleterious consequences, especially if kin are competing over limited resources. Colonizing new habitat disperses a population across a wider area, so local extinctions have a lower chance to kill every individual in a population.

Having said that, D1, D14, and Four didn't sit down and check off a list of reasons prior to dispersing. Dispersal seems to be triggered by a number of factors, including hormones, body condition, reduction of parental provisioning, increased locomoter activity, and external factors that include favorable winds and full moons for night flyers. In short, our eagles learned to fly and hunt, Mom and Dad decreased feeding, an adrenally-produced hormone called corticosterone soared, and the eagles dispersed. Reports of Four and the other eagles playing 'tag' with Mom and Dad shortly before dispersal may really have been a hungry, hormonally-charged young eagle chasing down its parents in an attempt to secure food from them. Suddenly tag seems a little less playful.

Unlike dispersal, migrating animals usually move from one geographic region to another without using intervening habitat.  But are dispersal and migratory behaviors mutually exclusive in an highly nomadic animal that usually doesn't breed until it is four years of age or older? What limited data we have indicates they aren't. D1's first year had characteristics of both dispersal and migration. She left her natal territory on August 13, 2011 and flew 262 miles northwest, arriving at her northernmost destination on September 6th. She spent the next four months slowly traveling south, with long layovers at Yellow Lake and Black River Falls. She arrived back in NE Iowa on December 24th, 2011.
D1 in 2011
D1 spent her winters navigating throughout NE Iowa and SE Minnesota, but she refined her north/south movements considerably. By 2014, her 808-mile trip north to Polar Bear Park took just 16 days. Although she wasn't yet breeding, her trips fulfilled the criteria for migration: they were seasonal, endogenously controlled, and didn't use much intervening habitat.

RRP board member Brett Mandernack is researching the migratory behavior of immature bald eagles. If there is a difference in immature vs. adult eagle migration, when does this change occur? Is it gradual over the first several years of the eagle’s life or is the change abrupt? D1, D14, and Four are part of his work. The paper from his first study, which examines data collected between 1999 and 2006, was published in the Journal of Raptor Research in 2012. An abstract can be found at

So will our eagles choose to nest in Decorah as adults? It's not impossible, but it isn't likely either. Dispersed young may return to the vicinity of the nest, but they don't appear to spend time there as sub-adults or adults. A study of 878 bald eagles found a median natal dispersal distance of 42 miles (69.2 km) overall, with females dispersing farther than males. Unlike human families, the young do not return for a visit with grandchildren in tow.

Eagle cluster map. Decorah is marked by the red 375, but note other favorite spots
Does an eagle's initial dispersal or overwintering perambuation tell us where it might nest as an adult? D1's favorite wintering area was near Elkader, Iowa, about 35 miles south of her natal nest - in line with average disperal distances. Unfortunately, her transmitter stopped talking to us. We'll try to get a visual on her this winter but we can't guarantee it.

As I mentioned at the beginning of this post, Four is back in Decorah and reunited with at least one parent, although we haven't seen them feeding her. We'll let everyone know if she leaves again.

Things that helped me learn about this topic: