It’s early November and all of our eagle couples – Mr. North and DNF, Ma and Pa FSV, the T’s, HM and HD, and Mom and DM2 – are working on their nests. Bald eagles are famous for their whirling nuptial flights, but nest-building is where the rubber really hits the road: it puts mates in close contact with one another and shows off an eagle’s strength, inter-individual cooperation ability, and skill level. We know that imprinting, instinct, experience, and site and material availability influence nest site selection and structure. Does culture also play a role?
October 25, 2022: HM and HD do the stick tango! He’s left – see his smaller beak? – and she’s right.
A recent study found that neighboring groups of White-browed Sparrow Weavers, a sparrow that nests in central and north-central southern Africa, created very different-looking nests. The researchers concluded that nest-building choices weren’t solely controlled by instinct and the environment, since the weavers seemed to learn rules for nest-making that were passed down within a family group from generation to generation. This fits the definition of animal culture as defined by the Convention on Migratory Species: information or behaviors that are shared within a group or community and acquired through social learning. These traditions – songs, migratory pathways, foraging strategies, nest-building techniques, nest structures – build family and group cultures so distinct that outsiders can perceive them.
Bald Eagles: Pretty social for a non-social species!
Culture requires sociality and Bald Eagles aren’t a social species as we define it, but they are social at key points in their lives. From hatching through dispersal, eagles learn important skills from parents and siblings that build on innate behaviors: how to self-feed, how to steal and defend food, how to take food, how to fly, how to navigate, how to forage, what danger looks like, and so on. Newly dispersed juveniles form loose, highly mobile eagle associations where social learning continues as they navigate finding food and safety in unfamiliar landscapes, with unfamiliar eagles. As the cold deepens and water ices over, migratory eagles gather in large multigenerational winter roosts, where eagles perch, hunt, and fish together, squabble and steal from one another, and share resources, albeit in a very eagle way. In short, social learning and knowledge transmission happens in the nest, post-fledge, pre-dispersal, on winter roosts, on summering grounds and on migration routes, or almost everywhere that eagles find other eagles.
Foraging culture and social learning
Does social learning develop into or constitute culture? To form culture, knowledge must be shared within a group or community, acquired through social learning, and passed on down to subsequent generations. Although it isn’t part of any formal definition that I could find, the resulting culture must also be recognizable and definable by outsiders: namely, us.
Given that definition, there are some tantalizing hints of foraging-based culture in Bald Eagles. For example, some groups of eagles tolerate people. These eagles have learned – and transmitted – the knowledge that people are generally harmless and can (mostly) be ignored: a phenomenon we call generational habituation. An even smaller habituated group has learned to follow fisherfolk in boats to scoop up damaged and dead fish releases and cast-offs, turning a specific category of people from ignorable to interesting. This behavior isn’t common to all or even most eagles and appears to include regional variants: a localized, unique behavior that we might call culture. Eagles are fishers par excellence: perhaps in time, we’ll see habituated wild eagles partner with humans to lead them to the best fishing spots by perching or circling over them. The prospect sounds far-fetched, but these sorts of cross-species partnerships are not unknown: https://english.elpais.com/science-tech/2024-08-22/dolphins-are-no-longer-helping-out-fishers.html.
How about hunting strategies? In ‘The Bald Eagle’, Mark Stalmaster states: ‘A most unusual foraging ploy takes place in Alaska. Eagles excavate seabirds from their burrows by digging in the ground and yanking them out.’ He also mentions eagles walking through sagebrush deserts trying to flush black-tailed rabbits in Utah – although they apparently haven’t learned to dig them out of burrows yet – and eagles walking along the edge of rising flood waters to grab fleeing animals in Oregon. Grubb and Lopez from the US Forest Service documented eagles ice fishing at Mormon Lake in Utah – a unique foraging strategy used by adult, subadult, and juvenile eagles documented only at that lake: https://www.fs.usda.gov/rm/pubs_other/rmrs_1997_grubb_t001.pdf.
All of these examples – unique to a geographic area, complex, and multigenerational – involve social learning, knowledge transmission, and lifeways that are handed down from generation to generation and recognizable to outsiders; i.e. us. If your definition of culture requires human intellectual achievement, then Bald Eagles and animals more generally don’t have and can’t develop culture. But if you consider culture more broadly as discrete, unique knowledge and behaviors that are learned socially and transmitted to succeeding generations, Bald Eagles and many other animals have regional foraging cultures. Perhaps the next one hundred years or so will tell us more. It’s also interesting to speculate that, given a long enough time, culture could be among the factors that drive speciation. Perhaps the next 100,000 years will tell us more.
October 6, 2023: Side by side nestorations!
Back to nest-building!
Back to nest building! In the study ‘Architectural traditions in the structures built by cooperative weaver birds’, researchers found that social groups of white-browed sparrow weavers built nests that varied in shape and size among distinct groups living just meters apart. The nests were not driven by genetic relatedness nor similarity in environmental conditions, but instead reflected group-specific preferences passed down through generations. This is, as far as we know, unusual in birds: most social learning revolves around predator recognition, song, mate choice, and foraging.
Eagles are fairly social away from their breeding grounds, but white-browed sparrows are cooperative breeders and social all of the time. They form small territorial groups that consist of a dominant breeding pair and 5-20 helpers, which might be offspring or unrelated birds. Unrelated helpers participate in group defense of territory: related helpers defend territory and feed young. Their unique breeding system – just three percent of birds are cooperative breeders – lends itself to establishing nest building traditions, although we don’t know why. But then we often don’t understand the roots or reasons underlying our own local traditions. Maybe unique nests are a family signifier: a way to establish your family apart from the other 16 to 20 families living in the same tree.
October 9, 2023: I love how you can see the ‘rings’ of previous year’s nestorations – not all that different from tree rings or varves in sediment! A great example of the WTN? shape and style.
Bald Eagle nest shapes: Cylinders, bowls, disks, cones, and What The Nest?
Mark Stalmaster identified four Bald Eagle nest shapes. Cylindrical nests are usually built between two upright branches that are parallel to one another, bowl nests are cradled on several sides by supporting branches and the tree trunk, disk nests occur close to the trunk and sit atop several strong branches, and conical nests occur where tree branches cause the structure to be small on the bottom and larger towards the top. I’d like to add one more category: WTN or ‘What the Nest?’ a category for the truly bizarre nest shapes that don’t fit anywhere else. The original Decorah North nest – a giant heap of sticks – was an excellent example of that last one!
October 24, 2024: The Trempealeau Eagles are busy with nestorations. Based on Stalmaster’s illustrations, I’d categorize this as a bowl-shaped nest!
Not culture: Still cool!
Do these nest shapes reflect nest-building traditions or cultures? Almost certainly not: nestling eagles see their parents building and ‘play house’ in the nest, but they don’t actively help construct the family nest and don’t learn nest-building in a social way. Site and tree selection might be influenced by nest site imprinting, but imprinting isn’t culture. Perhaps technology will improve enough to let us learn whether nest site imprinting joins tree height, shape, and location as site selection factors. We’ll see what the years bring!
October 21, 2024: I think this is a disc nest. Will some nesting eagles eventually learn that humans will build nests for them? This is the second nest we’ve built at this site and Mr. North was present for both of them.
Interested in eagle nest building? We have the perfect opportunity to watch and compare styles, shapes, and sizes! The Decorah North Eagles are nesting in a nest we built for them in an oak tree in Iowa, the Trempealeau Eagles are nesting in a white pine tree in Wisconsin, and the Fort St. Vrain Eagles are nesting in a cottonwood 30 or so miles from Colorado’s Front Range. Each site has its own unique characteristics, each tree comes with different building challenges, and each eagle couple handles nest-building a little differently. We look forward to seeing you at our nests!
Things that helped me think and learn about this
The Raptor Resource Project