A number of you have contacted us about the SW Florida and Trio eagles. While these aren’t our nests, they raise interesting questions about eagle social, nesting, and bonding behavior. I hope this blog helps answer your questions!
Last year, Harriet and her mate M15 reclutched after their lone eaglet, E14, died of rodenticide poisoning on January 15. While Bald eagles at this latitude normally hatch in late December or early January, E15 and E16 hatched in late March and fledged in June. E16 dispersed in September, but E15 never really left. As of this writing, she is chasing other subadult eagles and owls away from the nest area, stealing food from her parents (who are also still feeding her), and roosting around the nest. Her parents are courting, copulating, and building a new nest. Will nest events precede on schedule this year despite last year’s disruption? When will they chase E15 away? Will E15 prevent egg laying?
We talk a lot about how daylength light influences eagle behavior, but we haven’t talked a whole lot about how mates influence one another. After E14 died last year, I asked Board member Jim Robison, who breeds Gyrfalcons, whether or not he thought the two would reclutch. “It all depends on the male,” Jim told me. “If he’s feeding her and bringing her into condition, they could!”. M15 brought Harriet food, copulated with her, and worked on the nest. His diligent attention and interest stimulated egg production and most likely raised her prolactin levels, leading to a second clutch. However, this one-time event is extremely unlikely to change Harriet’s overall breeding schedule. As I write, the two are already copulating and M15 is faithfully bringing food! Look for eggs to come in mid-to late November, following Harriet’s usual schedule.
Could E15’s presence delay or prevent nesting? Stress can raise corticosteroid levels, which negatively impacts egg production and brood care. But neither adult seems especially stressed out about E15’s presence. She isn’t limiting food availability – like Decorah, SW Florida is rich in food sources – preventing food deliveries, or challenging her mother for the territory. Eagle biologist Brett Mandernack told me that first-year eagles have been documented hanging out with the next year’s nestlings and being fully tolerated by the adults, even while adults are feeding young. Whether or not Harriet and M15 eventually chase E15 away, I’m guessing that life will go on as normal in the SW Florida nest. I hope whoever is poisoning mice or rats stops this year: rodenticide is a far bigger threat to nest success than E15.
The Trio story began with two eagles, female Hope and male Valor 1. In 2012, the pair’s first year together, V1 didn’t spend much time incubating, feeding Hope, or feeding his eaglets, who eventually died. When Valor II showed up in the fall of 2013, Hope quickly accepted him as a mate. Hope and V2 raised young while V1 hung around in the background, neither helping nor interfering with activities at his former nest. By 2016, Hope had re-accepted him as a mate and the three formed an eagle thruple. When Hope was killed by another eagle in March of 2017, the two Valors raised their young together. Would they stay together after their eaglets fledged? They did! The two recruited female Starr to form a new thruple in September of 2017.
Back when we started this project in 2008, everyone knew that eagles were monogamous and mated for life, although an eagle would recruit a new mate if an existing mate died. Cooperative breeding was almost unknown: we thought that Bald Eagles would not share mates or tolerate other eagles near or around their nests. It turns out we were wrong! While most bald eagles are monogamous, some bald eagles divorce and others form thruples like the Trio nest. The males have never been documented bonding with one another, but they are clearly bonded to one another or they would have parted ways after Hope’s death in 2017. The thruple’s relationship has even withstood the loss of their nest, which was destroyed this summer in the derecho. As I write, they are building a new nest and preparing for 2021’s crop of eaglets.
Friends are like eagles: you don’t find them in flocks. Our image of the eagle as noble loner has been challenged at the Flyway, where we watch large convocations of eagles perching and hunting. While they often squabble over food, we’ve documented cooperative hunting of coots by juvenile and subadult eagles, the willingness of adult eagles to give food up to insistent youngsters (perhaps to avoid multi-eagle food fights), and local territorial pairs perching with eagles that appear to be drifting through. Again, this area is very rich in food, which probably reduces antagonistic behavior. Why risk a fight if you don’t have to?
It’s important to recognize individuals, especially in large social groups. Do I get along with that person or not? Are they likely to attack me or steal my food? We know that eagles display and respond to attitude – just think of Mom and DM2! – and that they establish a dominance hierarchy based on size, but do individuals recognize one another as they interact at relatively stable winter roosts and feeding grounds? At one point, I would have said No. Now, I’m not so sure.
Bald eagles don’t use tools, but they share with other intelligent animals a tool kit of reasoning, memory, flexibility, imagination, and insight that informs their lives. Before adopting mates and settling down, eagles are relatively social, moving through a world filled with other eagles to follow, perch with, hunt with, and steal food from. According to Safina, males of very few species directly enhance the survival of mates and young year round. Non-migratory mated male eagles are part of a very select group: they defend their mates and young from individuals who threaten their safety, protect their territories, help provide food to their mates year round, and feed their young until they disperse. Most of its members (think wolves and humans) are quite social. Eagles don’t socialize or form packs like wolves and humans, but they might be more social than we think.
Have eagles changed or did our earlier observations record unusual behavior as the population dwindled under the twin stresses of DDT and persecution, something that has been recorded in species like wolves? We’re looking forward to learning even more. Thanks for watching, sharing, learning, and especially for caring!
According to Agonistic Asymmetries and the Foraging Ecology of Bald Eagles, feeding eagles were more successful in defeating pirating eagles according to (1) whether their heads were up to prior to a pirating attempt, (2) how long their heads had been up, and (3) whether they displayed. I thought the ‘heads up’ factor was fascinating! We often see our eagles react to the neighborhood crow watch by putting their heads up and identifying whatever it was that upset the crows. Paying attention to their neighbors gives territorial eagles a talon up, so to speak, on potential intruders and competition.
Things that helped me learn about this topic