Blog #415 Being Neighborly
Moana teased the geese and gulls, courtesy of Cindy Sedlacek.
The sky is filled with Osprey parents and fledglings, now that they are mobile. Lansing fledglings from the nearby Churchill, Myers Hill, and even Cargill nests fish around Salt Point and next door Myers Point, interacting with Moana, Makani, and Lani. The young Salt Point Osprey are now in the post-phase of the breeding cycle, a period about which little is known because of the mobility of the family. The post-fledging phase is a time for exploration, learning, and imprinting on the landscape. It is also the time to hone survival skills such as flying and fishing by playing and teasing other species. Although it looks quiet, the Osprey nest is still the site of regular family meals and lots of commotion. This same nest-switching is happening in all areas around Cayuga Lake.
Their explorations may take the fledglings 20–30 miles away from the nest as they learn the lay of the land. They gradually lose their affinity for the nest, although the nest continues to be the feeding station and family meeting place. Some fledglings sleep in the nest and others chose
the trees. One thing is for sure, their landings need work. Makani, in particular, tends to overshoot the nest, making audible thuds.
The Salt Point young are captivated by the water, frequently playing, bathing, and dragging their feet in it. The young are just beginning to catch small fish now, but they need much work before they can catch a whole meal. When not flying, the young usually perch where they can see the water and the creatures below. This is when they learn to distinguish prey items from inedible
Lani studies the stream below looking for images of fish, courtesy of Cynthia Sedlacek.
objects and develop search images for fish. All three fledglings are still highly dependent on Orpheus’s meal deliveries but must become competent fishers by the end of the summer.
Ursula has not been seen all week and is presumed to have migrated. The mother’s breeding duties are over for the year as soon as the last chick fledges. She will take a week or more to regain the weight she lost during incubation before heading south to her winter home in Central or South America. She will return next March barring a serious mishap.
Young Osprey stay within eyeshot of their protective father, but the older they get, the farther they wander. Yesterday a great blue heron crossed Lani’s flight path, and immediately Orpheus gave the heron the bum’s rush. He flew after it, diving occasionally, until the heron, squawking for dear life, was out of sight. Orpheus did the same “chase and dive” at a turkey vulture today, moving it out of the nest’s air space.
An extraordinary and fascinating change comes upon the Salt Point Ospreys and all other parental Ospreys after the young fledge: the parents are no longer territorial to neighboring fledgling Ospreys, and no longer chase them away. If an unknown intruder flies by, Orpheus will drive him off in seconds, but neighboring fledglings are welcome at his nest. The fledglings are invited to perch on any nest to rest, sleep, or socialize; if a fish is delivered, they are typically welcome to eat it.
Why do the Ospreys do this? Some bird species employ nest helpers (usually kin) to raise offspring, not theirs. However, examples of breeding birds that accept parental duties for non-offspring are much rarer. Bitterbaum and Brown (1981) reported that Purple Martins with young
of their own will feed other fledglings in their colony that intrude into their nests. Other
Three Salt Point fledglings and two neighboring fledglings on the nest, from the telephone of Ava Ciaccia.
researchers have documented the same in Ospreys, where fledglings switch nests to beg for food. This suggests that Osprey families are considerably more fluid after than previously thought. This adoption can especially benefit an older subordinate fledgling begging for food at a nest with younger occupants, reducing the competition for food and improving its survival changes.
A vagrant fledgling being fed by a neighboring adult in her nest, courtesy of Lorraine Minns.
Perhaps nest-switching is a consequence of colonial living since this occurs in both the colonial Purple Martin and semi-colonial Osprey species. In these situations, the fledglings visit other nests, often for food, but also just to perch and perhaps socialize with the resident fledglings. Fledglings from different nests play together, highlighting the recognition of colony members.
Why would nest-switching occur? Subordinate fledglings from large broods could easily improve their food intake, and, thus, their survival if they could become the dominant fledgling at a nest with younger birds. Interestingly, all intruding fledglings moved to nests within sight of their natal nests, suggesting proximity was important in this behavior, perhaps because seeing fish deliveries at other nests stimulated the behavior.
Why would parents tolerate this behavior of feeding unrelated young? Ospreys have excellent visual acuity and tolerate only their mates and not intruders at their nests. Many researchers have suggested that adult Osprey can distinguish their young and the neighboring young although convincing data is still needed.
Assuming that Ospreys can recognize their own recently born fledgling, there are several hypotheses that Poole (1980) puts forth to explain why Ospreys would feed strange nestlings at their nests. Some suggest that the use of artificial nesting platforms creates artificially dense colonies, promoting such feedings. However, the Salt Point nest experiences visiting fledglings, from other nests in the loose semi-colony.
While it is not difficult to understand why subordinate Osprey fledglings might benefit from switching nests, it is harder to explain why breeding adults that tolerate and feed strange young would not be selected against. “Vagrant” young might take the food available to a pair's fledglings, thus lowering that pair's fitness. These parasitic young could be detrimental to a pair’s parental fitness, but the adult Ospreys can distinguish young, just as many colonial birds are capable of doing.
A vagrant fledgling lands on the Salt Point nest while all three fledglings were also at home by CE Cornell.
One factor that might explain the altruistic feeding of non-offspring by adult Ospreys is kin selection. Ospreys show great fidelity to their natal site when they return to breed in the spring. 73% of males and 36% of females settle in or near (< 6 mi.) their natal colony. Poole (1980) found evidence indicating a high relatedness among colony members. If intruder fledglings are likely to be close kin to neighboring adults, then the inclusive fitness of adults that temporarily adopt fledglings could be potentially raised, assuming food is available. It is unlikely that polygamy, observed earlier in the season at Salt Point, would affect the relatedness of adult Osprey to vagrant young. Thus, it appears that the fidelity of breeders to their natal area is an adequate explanation of a high degree of relatedness among Ospreys nesting in colonies which in turn could help select for the altruistic feeding of vagrant fledglings.
Alternatively, since wandering fledglings are common in Osprey colonies, it might not be energetically advantageous for adults to chase every vagrant fledgling by the nest, especially in places or in years of food abundance where nest switchers would be of little threat to the survival of the adult’s offspring. If so, one would predict little nest switching. Ospreys are tolerant even in lean years.
Lani was chased in fun by a fledgling from a nearby nest, courtesy of Cindy and Karel Sedlacek.
Investigations need to be done on Osprey post-family units. Questions like how long a fledgling is adopted at any one nest, whether dominant or subordinate young are more likely to switch, how well intruder young are fed compared to residents, and the responses of resident young and adults to vagrant fledglings need study. Answers to these questions, coupled with more data on the relatedness of Ospreys in a colony would allow one to judge whether or not the altruistic adoption of fledglings is an evolved and integral part of Osprey reproductive tactics.
Eyes to the sky!
Candace
Candace E. Cornell
Friends of Salt Point, Lansing, NY
Cayuga Lake Osprey Network
Guest Photographer:
Cynthia Sedlacek, musician and Director of Data Administration and Reporting at Cornell University, is a surprising artistic talent. Cynthia’s affection and admiration for raptors are evident in her exciting photographs primarily of Ospreys nesting around Cayuga Lake and the red-tailed hawks of the Cornell Campus. Her photographs of local Ospreys are incomparable as she captures their essence, their jizz, and their inner bird. From the hard-to-see nestlings to the fast-flying adults, Cindy goes beyond the pale to catch all aspects of Osprey life in the Cayuga Basin. “My camera allows me to capture their beauty in many ways that the human eye can’t see,” she says, “like their spectacular wing positions and detailed feather patterns.” Cindy and her husband Karel are also avid nest monitors for the Cayuga Lake Osprey Network and video stream footage of the campus hawks.
References:
Bitterbaum, M.J. and C.R. Brown. 1981. A Martin house is not a home. Nat. History 90:64-67.
Poole, A. 1982. Breeding Ospreys Feed Fledglings That Are Not Their Own. The Auk, Oct. 1982, Vol. 99, No. 4, pp. 781-784.
EYES TO THE SKY!
WATCH!
READ!
On Osprey Time: Ospreys of the Finger Lakes
VISIT!
HELP PROTECT OSPREYS:
•Stay 300 feet away from nests during the breeding season. If the Osprey vocalizes, you are too close! Back off Immediately.
•Carry binoculars to view wildlife from afar.
•Help keep local waters clean, healthy, and safe.
•Recycle used fishing lines, twine, and nets, which can kill Osprey.
•Join the Cayuga Osprey Network and volunteer to help monitor Osprey nests: cec222@gmail.com.
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