Thursday, November 03, 2016


Filed under #musings. A recent study on animal personalities got me thinking. Are the differences we see from nest to nest a product of our imaginations, the surrounding area, or their personalities?

Between 2003 and 2015, we watched bald eagle families on four territories: Decorah and Decorah North in Iowa, Fort St. Vrain in Colorado, and Eagle Valley in Wisconsin. They were very similar in their broadest outlines, since all four territories:
  • Were inhabited year-round by a territorial pair of eagles.
  • Were close to bodies of water.
  • Had distinctive seasons that included cold snowy winters, short unpredictable springs, and hot summers.
  • Were near what appeared to be ample food supplies: the trout hatchery in Decorah; a river, watering hole, and prairie dog colony at Fort St. Vrain, a trout stream and farms at Decorah North, and the Mississippi river at Eagle Valley.
However, watchers of the Decorah and Decorah North nests observed some big differences between the two last year.

At Decorah
We have been collecting data on this territory since 2006, when Neil and Bob began filming American Bald Eagle. We don't know Dad's age, although he is older than Mom. Based on her barely-adult plumage when she began nesting here in 2007, she turned 13 in 2016. Both eagles are experienced parents.
  • Both parents spent a lot of time on nestorations: bringing in sticks, arranging sticks, and moving sticks. 
  • The 'pan-tree' was almost always full. It wasn't uncommon for the nest to contain four or five prey items - not surprising, since Dad was observed bringing in as many as five fish in one hour! 
  • Both parents fed young on a regular basis. While Mom tended to take over feeding, Dad fed when he had a chance. The eaglets were fed together and separately. Human watchers believed that Mom and Dad tried to make sure both eaglets got sufficient food. 
  • A parent - usually Mom but sometimes Dad - was always present in the nest until long after the eaglets could thermoregulate.
  • The eaglets showed less aggression towards one another.
  • Fish made up a larger part of the eagles' diet.
  • Egg-laying started on February 18.
At Decorah North
We have been collecting data on these two eagles since February of 2016. We do not know how old either eagle is (although both appear to be full adults), how long they have been together prior to last fall, or how long they have been parenting.
  • Less time was spent on all aspects of nestoration during the period we were able to watch both nests. Fewer sticks were brought in and the eagles spent less time moving and rearranging them.
  • The parents did not cache nearly as much food in the nest. 'Field-dressed' prey was more commonly brought in than entire chunks or whole bodies that needed defurring or descaling.
  • Young were not fed nearly as often and separate feedings were not as common. Human watchers believed that Mom and Dad did not try to allocate food evenly between the eaglets.
  • Parents were not always present in the nest. Human watchers believed that they were more attuned to the brooding needs of the two older eaglets versus the youngest one.
  • The eaglets showed a great deal of aggression towards one another. Sibling aggression was a factor in DN3's death.
  • The prey base was wider and did not include as much fish as the Decorah nest - although based on a journal by Sherri Elliott, fish still comprised around 66% of their diet. It included opossum, muskrat, raccoon, fawn sections, rabbit, mink, cow placenta, birds, waterfowl, a cat, and unknown pieces of meat.
  • Egg-laying started on March 11.
  • The North Nest had more nest intruders and slightly harsher weather.
At the time, we believed that the difference in parenting styles was driven primarily by food availability at each nest. Since the Norths brought less food into the nest, it made sense that less food might be available overall, and their relative absence could be explained by the need to spend more time hunting. Food was also clearly a big factor in eaglet aggression, which tended to determine who was fed first or at all.  Our conclusions were consistent with earlier studies of bald eagles conducted at Besnard Lake, in Alaska, and on Chesapeake Bay. You don't have to watch eagles very long to realize how important food is. But could eagle coping styles/personalities also be playing a role? 

Let's start with defining the term 'copying style'. Jaap Koolhaus (Koolhaas et al. 1999, 2010) describes a coping style as a correlated set of individual behavioral and physiological characteristics consistent over time and across situations. Researchers divide coping styles into proactive and reactive groups:
  • Proactive individuals respond to stressful situations with action. They build rigid routines, explore territories quickly, develop superficial maps, tend to be offensive towards conspecific rivals, are impulsive in decision-making, score high in frustration tests, take risks in the face of potential dangers, and are novelty seekers. Perhaps as a result of their routines, they are less responsive to short-term changes in their environment. 
  • Reactive individuals respond to stressful situations with immobility. They are less likely to build rigid routines, explore territories slowly, develop deeper maps, and tend to be less offensive towards conspecific rivals. Perhaps as a result of their flexibility, they are more responsive to short-term changes in their environment. 
So how might behavioral type affect local space use in bald eagles? Unsurprisingly, it is a complex question. As defined above, the North adults seem proactive (quick, active, aggressive, less responsive to environmental changes and external stimuli) and the Decorah adults seem reactive (slower, less active, possessing a deep understanding of the territory, very responsive to environmental changes and external stimuli). One of the things that watchers commented on was the absence of Mom and Dad North when compared to Mom and Dad Decorah. Several studies have found that proactive individuals show a wider ranging space use than their reactive counterparts. The North eagles spent less time in the nest and on the nesting tree last year. Was that driven only by food availability or did a deep-seated need to range also play a part? We also discussed the differences in parenting styles quite a bit. The Decorah eagles seemed to human watchers to be much more reactive to the needs and behaviors of their eaglets than did the Norths. Did personality have anything to do with it? 

A lot of the narrative around both eagle families seems to fit nicely into a proactive/reactive box. But what about prey piling and nestoration? Watchers identified prey piling as a big difference between the two families. The Decorah eagles piled prey high (and have ever since we began watching them), while the North eagles seldom cached prey in the nest (although left-over turkey feathers piled up, they didn't make a meal). Similarly, the Decorah eagles spent a lot of time working on their nest (Dad's sticky OCD, anyone?) when compared to the North eagles. These seem like rigid behaviors, not flexible responses to the environment, which puts them firmly in the proactive category. It could be argued that, unlike the Decorah eagles, the Norths didn't flexibly respond to changes in their environment (eaglets) with changes in their behavior (caching prey). However we categorize their behavior, both pairs of eagles appear to be using environmental information in different ways.

It hasn't been that long since it was taboo to think of animals as anything other than clinical study subjects. Changes in our way of thinking and advancements in research and study techniques have altered that. I am firmly convinced that personality plays a part in the unique behaviors seen at each nest (although food remains important) and I am looking forward to more research on the topic.

Note 1: Dana Bove of the Boulder County Audubon contacted us about bald eagle monitoring. He developed an Excel worksheet to monitor nests outside of the active reproductive period, i.e., when eagles may be in and around the nest, but aren't productively copulating, laying eggs, incubating, or caring for young. We were very interested in them given the differences in the Decorah and Decorah North nests. How big were those differences in behavior? We received permission to use his protocol and put it online here. Please feel welcome to start sharing your observations via the tool. This is a bit of an experiment for us, so feel welcome to share feedback as well!

Note 2: Some watchers believe that parental experience also played a role. Like many animals, eagles get better at basic parenting skills with experience. Could some of the differences in preparation and provisioning also be a matter of an experienced parent at the North Nest?

Note 3: Could nest invaders be another piece of the puzzle? Eagles enthusiastically steal prey from one another and hungry hawks will sometimes invade as well. Could the difference in prey piling and preparation be in part a response to potential competition and nest invasion? A lot to think about!

We see proactive and reactive behaviors in nesting peregrine falcons when we handle them for banding. Some falcons foot and bite the entire time, while others are very docile and don't put up a struggle at all. A few observations...note that these are anecdotal:
  • Highly aggressive females seem to replace highly aggressive females. I can think of at least three sites where female falcons were dangerously aggressive. We've never had anything but dangerously aggressive falcons at those sites. 
  • There is a general belief that highly aggressive falcons produce highly aggressive offspring, although to my knowledge it has never been formally studied.
  • Footy, bitey males tend to be older than female siblings based on plumage growth. 
A study on crows found that dominance-related parameters of age and sex did not coincide with proactivity. I wondered about this since older, bigger eaglets tend to be more dominant, and older nestling falcons are more likely to be aggressive when handled. In effect, a study on this would be looking at whether birth order influenced personality/coping strategies. We plan to start assessing docility in nestling falcons next year for comparison with their behavior should they survive to adults. We will also keep notes on dispersal - so keep those band numbers coming!