Tuesday, March 22, 2016

Thoughts on the sub-adult nest-intruder

We had an intruder at the Decorah North Nest on March 13th. While Dad was in the nest incubating the pair's single egg, a roughly four-year old female eagle perched on a limb right next to the nest tree. He responded by covering the egg, softly vocalizing, and ruffling his feathers. The whole encounter lasted about fourteen minutes.

I was intrigued by Dad's soft vocalizations. Why didn't he call a little louder? I went to our panel of eagle experts for an answer. Jim Grier and Brett Mandernack both provided insights into Dad's behavior. Note that neither Jim nor Brett referred to the eagles by title (Dad) or sex. Those additions are my own.

Jim has extensively studied eagles in Ontario and along the Mississippi River Valley. He noted that the density of eagles is getting so high that it might be changing eagle "society" and interactions, or bringing out behaviors that don't occur in lower density [and perhaps limited food-base] situations.While we don't think of the Decorah North Nest as urban, bald eagles along the Mississippi river and its environs are urbanizing as the population density grows. Our initial ideas about the lonely majesty of the bald eagle may have come from observing lower-density populations in a landscape much less impacted by human beings.

Jim explained that the soft vocalizations sound more like typical young- or mate- calls made at close range between young and parents or between mates (as in food-begging by young toward adults or females toward males). He also pointed out the direction of Dad North's gaze. Dad North didn't spend much time looking at the eagle on the branch, almost as it he was familiar with her. To quote:

"Direct" gaze can include both truly direct, focused straight at -- with both eyes and holding the gaze -- or what I call indirect-direct, mostly a side-ways look with occasional looking away and often with raised hackles, all of which is the prelude to an attack if an intruder doesn't back off. When an eagle is presented with a new individual that it doesn't know but which it is not particularly concerned about, it will "study" the new individual, looking directly at it and looking at different parts of its body. Then it will look away briefly, then look back and study it some more, and alternate with increasing lengths of time looking away and shorter periods of gazes ... until it becomes "familiar" with the new individual and then, subsequently, usually only glance occasionally at the individual but mostly seeming to ignore the familiar bird nearby. 

As upset as Dad might look to us, his indirect gaze indicates he might not have been especially concerned with the intruder. There was just one brief threat-bout toward the bird on the branch when he stood up to bring in his wings and change position. Jim suggested that there could be some mutual extra-pair mate attraction going on between Dad and the bird on the branch, creating an interesting mixture of aggressive and compatibility behaviors (soft vocalizing=compatibility, while threat-bout=aggressiveness). While it had never occurred to me that the two eagles might be familiar to one another, Brett also pointed out the possible familiarity of the intruder with the incubating adult. A quick search of documented nest intrusions makes it obvious that this is far from a full-metal response. Here are three aggressive reactions for comparison. Bob was quite fascinated by the second one:

Jim went on to provide an alternative interpretation of the interaction.

"An alternative interpretation might be that there is a conflict between opposing behaviors, such as aggression toward the intruder in conflict with, or inhibited by, a higher drive that results in priority on protecting the egg (against either the outsider or accidental damage by the parent itself if it were to move around too much near the egg, as in the behavior of balling of the feet when moving around the egg). All of this is stereotyped action patterning that has been shaped by eons of evolution. The interactions have likely happened many times ancestrally with selection of survival and reproduction producing the optimal solution of trade-offs.

Contributing to all of this is the fact that the perched outsider, younger bird, isn't making moves toward the incubating adult and also not staring at it, but just mostly looking away ... as if they're familiar with each other, or that any serious interaction occurred earlier and have now run their course so we're just seeing the end of it.

All in all, it somewhat reminds of the reduced level (to my Ontario and previous MN experience) of aggressive interactions that I observed among close nests when we were doing the FWS aerial monitoring surveys along the Mississippi River in 2009. We might be seeing a new social order in eagles! (Or, at least new to us, behaviors that are more likely elicited in a new [to us] high-density eagle population [and high productivity of prey base, so food shortage is not a factor].)"

In short, what looked to our eyes like an aggressive interaction was not really very aggressive. Dad didn't challenge the intruder with a direct gaze, loud vocalizations, or an attack. He might be familiar with her (and possibly even attracted to her), or an aggressive response might have been inhibited by a higher priority to protect the egg. The intruder also wasn't aggressive. While she remained near the nest, she didn't try to enter it or engage Dad directly, and Mom displaced but didn't seriously attack her when Mom arrived at the nest. While we think of bald eagles as being non-social on their breeding territories, their interactions with and signaling towards visitors and 'neighbors' may be more complex than we initially thought, especially as the population becomes more dense and interactions become more common.

Many thanks to Jim and Brett for sharing their experience, wisdom, and insight!

I became curious about eagle-to-eagle interactions after reading Jim and Brett's comments. A few videos:
  • Sub-adult chased off by juvenile. This is an aggressive interaction, although no physical contact occurs. Note the loud vocalizing and direct attack: https://youtu.be/vnu0ZEJEAJw
  • Adult to sub-adult interaction. Another aggressive interaction. Note that eagle on the nest doesn't stand up to defend until physical contact is imminent, perhaps lending support for Jim's idea that aggressive responses might be inhibited by a higher priority to protect eggs.
  • Adult to sub-adult interaction. This is interesting and not especially aggressive, although it includes physical contact. The adult (the video identifies it as male Romeo) is watching something outside the nest. At 3:34, a sub-adult eagle lands in the nest. Romeo flaps at it, but it looks down and away without responding directly to him. For the most part, the two eagles avoid direct eye contact. At 5:31, the sub-adult appears to check out the adults talons. The sub-adult nips at the adult at 5:34 and the adult flies off. https://youtu.be/_7BssmkllQY
We are sure people are curious about whether or not the intruder could be offspring from a previous year. We can't dismiss it, since eagles are philopatric, but we also can't confirm it. Perhaps further research will shed light on the relationships between bald eagles and sub-adult offspring. 


Nora H said...

Great post Amy!! Thank you!

Bevy Neuhaus said...

Might the female eagle have been a daughter to this pair??

Bevy Neuhaus said...

How come they have not feed the new hatchling? I know baby chicks "live" for a few days on what is called the egg sack. This allows chicks to be shipped without any hunger "pangs" I have seen 2 fish but doesn't appear to have been eaten. Are they "aging" the fish?

Bevy Neuhaus said...

I am referring to the eagles at the Decorah IA hatch sight on the above comments

Helen Crowley said...

Perhaps the female intruder was a "sister" from the hatch of the father?

Flores Morris said...

Thank you very much! Beautiful video and beautiful words!

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