Hanover Bald Eagle Blog #8 - 2020

April 21, 2020
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"Don't Mess With Us."

Written by Zoey Greenberg • In Partnership with Comcast Business • Powered by HDOnTap 

It looks like Liberty and Freedom aren’t quite ready to move out. As many of you have noticed, both eagles have been contributing material, feeding, and generally hanging out. This level of activity is a welcome reprieve from the site of an empty nest. However, it may be a prudent practice for us all maintain expectations in line with Bald Eagle ecology, which suggests that a double clutch is possible, yet very unlikely.

Double clutching does occasionally occur within wild Bald Eagle populations. On the other hand, as mentioned in the latest blog, the window for fertility is a short one and depends upon the hormone levels of both the male and the female. The further the breeding season progresses, the less likely a second clutch becomes. That being said, wildlife enthusiasts should always remember to never say never.

If Liberty and Freedom are not planning on another clutch, then why all this fussing about the nest?

It’s a good question, and one that may be related to the high territoriality of Bald Eagles. We have seen floaters (non-breeding Bald Eagles) in the Hanover area for several years. If Liberty and Freedom were to leave right now, there is a high chance that a floater would snatch up their site and remain until they themselves were ready to breed. The Hanover nest is a hot commodity given that it’s already constructed, close to water, and well-positioned in a tall tree with good visibility.

By remaining active at the nest and adding new materials, Liberty and Freedom may be signaling to potential usurpers that they are still rulers of the Hanover domain. In a study of Black Kites - another long-lived raptor species - researchers determined that nest decorations serve as a strong indicator of “physical prowess.” Strong, healthy, breeding birds would add conspicuous materials to their nest, and in return experienced less antagonism from floaters. In essence, they seemed to be telling others “don’t mess with us.” Interestingly, there was a strong correlation between the age of individual kites, and the degree to which they used nest decorations: birds in their prime (around 10 years old) invested more energy in decorating their nests, while the youngest and oldest birds which were less capable of defending against intruders, often removed nest decorations that were placed there by the researchers in an experimental study. These weaker birds seemed to be deliberately avoiding sending any signals of strength or prowess so as to remain under the radar.

This research provides intriguing evidence that long-lived raptors, such as Bald Eagles, may send messages to floaters in their area by engaging in nest renovations even when they are done breeding for the season. This is the most likely explanation for Liberty and Freedom’s behavior this week, given that their chance of laying another clutch is low.

Other reasons for Liberty and Freedom’s recent behavior could be 1) sprucing of the nest to make it better for next season 2) further strengthening their pair bond or 3) practicing their domestic protocol. To this last point, it’s true that Liberty and Freedom are amazing parents with a strong bond and a well-oiled system. However, since something went wrong this year, they may be trying to polish some of their skills such as food provisioning to the nest.

Regardless of the exact reasons for their continued interest in the nest, their diligence suggests that they are intending on sticking with the Hanover site (pun intended). If they attributed their nest failure to inadequacies in the surrounding habitat, the availability of prey, or the nest itself, they would be spending energy scoping out alternative sites rather than investing more energy into this one. We may also hypothesize that Liberty and Freedom are relatively strong individuals since they are choosing to defend their territory against floaters, potentially inviting competition. Signaling is likely only worth it if you think you can win.

I think we all know these two would win.

Sources

Katzner, T., & Tingay, R. (2010). Eagle Diversity, Ecology, and Conservation. In Bildstein K. & Parry-Jones J. (Authors) & Katzner T. & Tingay R. (Eds.), The Eagle Watchers: Observing and Conserving Raptors around the World (pp. 1-25). ITHACA; LONDON: Cornell University Press. doi:10.7591/j.ctt7zjxp.5

Sergio, F., Blas, J., Blanco, G., Tanferna, A., López, L., Lemus, J., & Hiraldo, F. (2011). Raptor Nest Decorations Are a Reliable Threat Against Conspecifics. Science, 331(6015), 327-330. Retrieved April 18, 2020, from http://www.jstor.org/stable/40986679

And a special thanks to Dr. Todd Katzner for his knowledgeable insight. 

Raptor Ecology Specialist - Zoey Greenberg 

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