Tuesday, May 03, 2016

Decorah North: DN1, DN3, and today on the nest

Watchers of the Decorah North Nest got quite a shock today after DN1 went after DN3 with a ferocity we haven't seen at any of the bald eagle nests we watch. It looked liked this:
  • Mom North lands on the nest, chasing away a sparrow that was stealing straw from the nest. DN1 and DN2 are huddled together at about 7:00 on the left side of the nest, while DN3 is laying at about 4:00 on the right side of the nest.
  • Both eaglets rise to a sitting position.
  • DN1 dominates DN2, pecking and pulling on DN2's left wing. DN2 submits and DN1 stops.
  • DN1 shoots poop, staying at roughly 7:00..
  • DN3 sits up and walks over to the turkey wing at about 3:00, where it appears to be look up at Mom. Keep in mind that the eaglets are walking on their tarsi, not their feet.
  • DN1 walks from 7:00 to 3:00 and begins a dominance interaction with DN3. In addition to pecking, DN1 bites and pulls on DN3, who submits. 
  • As DN3 submits, it rolls against DN1, scraping DN1 with its talons in what appears to be an accident.
  • DN1 resumes the dominance interaction, pulling and biting at DN3's wing and body and flipping DN3 over. DN3 remains in a submissive posture.
  • DN1 pulls DN3 up by the 'scruff' of its neck four times
  • Mom North, who has been watching the entire interaction, walks over and looks at DN1.
  • DN1 ceases the interaction, although we don't know to what degree Mom North interfered with it. Was DN1 ready to quit given the lack of a response from DN3, or did Mom's arrival distract DN1?

Following this event and subsequent battles, the ugly specter of siblicide - one sibling killing another - leapt to everyone's mind. So how common is siblicide in bald eagles? Sources disagree, with some referring to it as relatively common (University of Nebraska, American Eagle Foundation) and others calling it fairly rare (Hornsby). It hasn't yet occurred at any of our cams - not in Decorah (since 2008), not at Decorah North (since 2016), not in Fort St. Vrain (since 2003), and not during the one year we were able to watch Eagle Valley (2013). Our nests have different parenting 'styles', levels of food availability, surrounding environments, and nest invaders, but to date all eaglet fatalities have been caused by hypothermia, predators, and suspected disease. Why did DN1 take after DN3 and (to a lesser degree) DN2 today? We don't know, but as tough as it was to watch, it wasn't especially prolonged and didn't lead to death.

People also worried about a lack of food deliveries to the nest. While we don't know why parents don't pile up the pantree here as they do in Decorah, birds of prey (even young ones) can go a long time between meals. The eaglets remain healthy, alert, and pooping, which tells us that their digestive systems are working just fine. They are moving around the nest, interacting with one another and with parents, and showing interest in their surroundings. I'm sure they would like a meal, but they aren't starving yet and won't be for some time.

Several people referred to Dad and Mom as 'bad' parents. Again, different nests have different parenting styles, but eagles don't divide neatly into human narratives of good and bad behavior. It is a warm day, the young don't need brooding, and either Mom or Dad have been perched nearby much of the day. They are taking care of their young as they see fit, and we know from studies on human-raised birds that young birds have very different needs than young humans.

We've also been asked if we will rescue DN3. Absolutely not. We might consider interfering if the situation were human-caused, but what happened today was completely normal from an eagle perspective. If we rescued every eaglet we were concerned about, there would be no wild eaglets left to watch. It is very important to keep today in perspective. This is one of the first days we have seen a lot of concern about Decorah North. For the most part, the parents have acted according to our expectations: feeding, interacting, brooding, and in general caring for their family. It is also the first day - the first time - we have seen that level of aggressive interaction in one of the bald eagle nests we watch. But none of it was out of line or outside the parameters of normal eagle behavior. If eaglets died after a warm day with just one meal, we wouldn't have eagles to admire and worry about.

In 2011, a follower sent me a lovely watercolor painting of the fledglings at the Decorah nest. It said "You are braver than you believe, stronger than you seem, and smarter than you know." It remains a favorite artwork of mine and I often think of it when I am concerned about the eagles. As anthropomorphic as the painting may be, eagles are truly stronger than we think and often smarter than we know. As much as we love them, they are only their own and what we saw today was simply part of the way they live their lives.

Note: Hornsby follower gzebear compiled observations from 147 nests checked in an 8 year period and found only a 2% incidence of siblicde in hatches of 2, and 3.8% in hatches with 3. Follow the link to read gze's data, references, and additional comments from contributors to the thread. I think it might be time to revisit the whole subject of three-egg versus two-egg clutches. How common is one versus another? Are there regional differences? Is it an inheritable trait? Do we have more three egg clutches than we used to? It's off-topic here, but I subject that interests me quite a bit.