Hanover Bald Eagle Blog #24
May 17, 2019
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HANOVER BALD EAGLE BLOG
Harmony in the Nest: rule or exception?
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The Hanover eaglets, while both endearingly disheveled during this awkward phase in their development, are notably different in size and have been since birth. Each egg was laid roughly three days apart and followed a similar interval for hatching. Therefore, the nestlings are three days apart in age, and given the rapid growth rates of eagles, we can tell from appearance alone which nestling is older.
Even though the larger firstborn chick could exhibit dominance over the second-born, the two siblings cohabitate quite peacefully.
But do all raptors siblings get along this well?
Many raptors engage in “asynchronous hatching,” which is when one egg is laid sooner than the second and incubated immediately by the parent, as we saw with the Hanover eggs. “Synchronous” hatching is a different reproductive strategy used by many precocial birds (well-developed at birth), in which eggs are laid and incubated as close together as possible, often within a few hours. These two methods can have far-reaching implications for the development of the individual chicks. For asynchronous hatchers, the firstborn will have a head-start in growth over their sibling. In years where there are plenty of resources to go around, issues rarely arise. However, during periods of food scarcity, sibling aggression may surface, leading to physical harassment, food hoarding, or eviction initiated by the eldest chick. With enough persistence, these activities can result in death of the youngest.
This may sound brutal, but in reality, it’s quite normal. While the concept of siblicide, as this act is called, seems morbid to us, throughout the rest of the animal kingdom there are many examples in which the “circle of life” is rough around the edges.
The death of one chick may result in the survival of their sibling since all of the food brought back to the nest will then feed one mouth instead of two. Seen through this lens, by laying eggs asynchronously and giving one chick an advantage from the get-go, parents increase their chances of raising at least one healthy, reproductively capable youngster. For those birds that raise same-age chicks, all of the young are equally capable of fighting for food, which means that when there is not enough to go around, all chicks suffer equally, potentially resulting in zero young surviving. Asynchronous hatching is likely a built-in reproductive strategy that results in a higher number of healthy young leaving the nest and therefore providing an overall benefit to the population.
In order to understand how this unusual approach to parenting is beneficial, we must remember that reproductive success is not defined by the number of eggs that are laid, or the number of fledglings that leave the nest. Instead, success is measured by how many offspring are able to produce their own young. Therefore, if eagle parents raise chicks that are able to leave the nest, but are too weak to reproduce, then those parents have not ensured that their genes will make it out into the world that season.
The prevalence of siblicide in raptors is difficult to confirm without 24-hour surveillance of nests, and therefore we do not yet have a strong understanding of how often this occurs. On the bright side, while Bald Eagles have been known to engage in siblicide, it is suspected to occur in less than 5% of nests.
The fact that the Hanover eaglets get along so well and have not exhibited signs of aggression implies that there is enough food available for both. Their healthy appearance and agreeable interactions with each other also reflect well on Liberty and Freedom’s ability to provide enough food to support the whole family.
So, the next time we see the Hanover eaglets sleeping side by side, preening each other’s feathers, or politely taking turns grabbing morsels from mom, let’s remember the significance of their comradery. The nicer eaglets are to each other, the more grand-eaglets may be born in the future!
Cornell Lab of Ornithology. (2004). Handbook of Bird Biology. Princeton: NJ: Princeton University Press.
Estes, Wendy A., Dewey, Sarah R., and Kennedy, Patricia L. (1999). Siblicide at Northern Goshawk Nests: Does Food Play a Role? The Wilson Bulletin, Vol. 111(No.3), 432-436.
Hedgpeth, Dana. (2016, March 22). Two baby bald eagles show sibling rivalry at National Arboretum. Retrieved from: https://www.washingtonpost.com/news/local/wp/2016/03/22/two-baby-bald-eagles-show-sibling-rivalry-at-national-arboretum/?noredirect=on&utm_term=.21c46011cc2a
Thank you Hawk Mountain for this week's blog entry!
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