Monday, February 20, 2017

Did we have a new eagle at the Decorah North nest last year?

Mom North rolling her egg
Hashtag this one #speculation! As Decorah North fans know, we had our first egg in that nest on March 11 in 2016. In 2017, first-egg timing retreated to February 19 - earlier than Mom and Dad Decorah! Why did it change so much?

Mom and Dad Decorah's first egg history looks like this:
  • 2/18/16: First egg
  • 2/18/15: First egg
  • 2/23/14: First egg
  • 2/17/12: First egg
  • 2/23/11: First egg
  • 2/25/10: First egg
  • 3/02/09: First egg
  • 3/08/08: First egg (note: this date is an estimation based on a photo of first hatch)
We know that Mom was laying for the first time in 2008. While we don't see a dramatic shift backwards, the first time she laid an egg also marked the latest time she laid an egg. Her nesting chronology slowly shifted earlier, yielding an average first-egg date on February 19 to date. 

Fort St. Vrain's first egg history looks like this:
  • 02/14/17: First egg
  • 02/16/17: First egg
  • 02/14/15: First egg
  • 02/21/14: First egg
  • 02/17/13: First egg
  • 02/16/12: First egg
  • 02/16/11: First egg
  • 02/14/10: First egg
  • 02/17/09: First egg
  • 02/27/08: First egg
  • 03/03/07: First egg
  • 02/17/06: First egg
Note that I didn't say Mom and Dad Fort St. Vrain's egg history. Looking at first egg dates, it appears we had a mate changeover in 2007, although we don't know whether it was Mom or Dad. As we saw in Decorah, nesting chronology slowly shifted earlier, yielding an average first lay date of February 15 to date.

How do peregrine falcons compare? Falcon nests experience a lot more turnover than any of the bald eagle nests we watch, making it difficult to develop data on partner nesting chronologies. For example, we've seen 11 mate changeovers in the 19 years we've banded at Xcel Energy's Blackdog plant in Eagan, MN. However, we can draw a couple of broad conclusions: 
  • A change-over in the resident male or female falcon is often (but not always) accompanied by a change in nesting chronology. 
  • Nesting chronology is somewhat more likely to move later in the first year of partnership and than move earlier as partners are paired over multiple years. 
  • Return timing and territorial fighting both appear to influence nesting chronology, and territorial fighting is one factor in shifting egg-timing later. If a gravid female falcon is killed by an invading female falcon, the resident male will need to court her and fertilize her eggs, moving the nest's chronology later for at least the first year. 
A lot of people speculated that Dad North was new on site last year. While we don't know for sure, the remarkable shift in first egg timing indicates that one of the parents - possibly Dad North - was new. We don't know whether territorial interaction played a role, and we also don't know whether a new male could impact timing differently than a new female. But we will move egg-watch for Decorah North earlier next year. Will we also see a difference in parenting styles and outcomes in the North nest this year? We look forward to finding out!

Note: The Eagle Valley Eagles laid later in the one year we were able to watch them. I wish we could have had at least a few more years to take a look at their nesting chronologies as well, since some of their behavior and provisioning seemed more like what we experienced in the North Nest last year than in the Decorah nest since 2010.