Object Play During PFD: FSV57 Goes “Surfing”
August 21, 2026
On August 14, strong winds at the Fort St. Vrain nest gave us an unusual look at what may be another part of development during the post-fledging dependence period (PFD).
FSV57 repeatedly grasped and manipulated a flat piece of bark in the nest as gusts caught and lifted it. Rather than simply picking up or moving the bark, the juvenile repeatedly repositioned its feet and grip and adjusted its balance as the bark responded to the wind. The behavior continued for an extended period and certainly looked like play.
From an animal-cognition perspective, this type of exploratory object play may involve learning about the physical properties—or “affordances”—of objects: what can be grasped, lifted, resisted and repositioned.
But the timing of the behavior may be just as interesting. FSV57 was already fledged and in PFD when we recorded this behavior. Flight itself may be an important prerequisite for this particular kind of play. Once fledged, a juvenile has acquired a new combination of balance, coordination, responses to wind, and integrated use of its wings, feet and body. “Surfing” a piece of bark being lifted and shifted by gusts appears to draw on many of those newly acquired abilities.
That raises an interesting developmental question: does the transition to flight not only give juveniles new ways to move through their environment, but also open up new forms of exploration and play through which those abilities can be practiced and refined?
There is another particularly interesting part of this observation. Our video records show that this same piece of bark was originally brought into the nest by FSV53, one of two juveniles that fledged from the FSV nest during the 2025 season. Both FSV53 and its sibling, FSV54, subsequently manipulated and “surfed” the bark in much the same way we observed FSV57 doing during the 2026 season.
Thus, all three juveniles we have documented engaging in this behavior were fledged and in PFD. FSV57 also had no opportunity to observe the 2025 juveniles using the bark. Three juveniles across two breeding seasons therefore independently interacted with the same object in a remarkably similar way.
That raises another intriguing possibility: particular physical characteristics of the bark—its broad, flat shape, weight, and especially the way it responds to wind—may themselves invite this form of exploration and play. In the language of animal cognition, the object may provide a particular set of “affordances,” or opportunities for action, that the juveniles independently discovered.
A few observations cannot tell us whether flight is actually a prerequisite for this behavior or what developmental function the behavior serves. Nor does repeated use of the bark demonstrate learning about physical causality. But the observations provide another reason to think about PFD as much more than the weeks during which a fledged eagle remains dependent upon its parents for food. It is also a period following the transition to flight when juveniles continue to gain and refine physical and behavioral skills they will need for independence.
And yes, watching FSV57 repeatedly ride that piece of bark in the wind, we couldn’t resist calling it “surfing.”