Blog #463 Intruders: Helpful or Harmful?

May 11, 2024

Male intruder testing the occupants of the nest, courtesy of Cindy Sedlacek.

April through July can be stressful months for nesting Ospreys in the Finger Lakes, as marauding unmated Osprey intruders (or ‘floaters’) go from nest to nest searching for nest vacancies and weaknesses in the colony (technically, a semi-colony). Intruders serve an important part in Osprey society by filling nest vacancies where partners have perished. Intruding Ospreys are indicative of healthy, growing populations such as those around Cayuga Lake. They also have a way of disturbing a peaceful morning.

Intruders are birds without nests ranging from young birds hatched two years before and older adults and make up about 30% of the colony. However, these highly aggressive birds are doing more than just checking the situation out. They are making full-on assaults to take over nests, severely testing the physical strength of the resident Ospreys and their tenacity and determination to remain on their nest. Salt Point’s

An intruder aggressively attacking the breeding pair, courtesy of Cindy Sedlacek.

Orpheus and Ursula show their mettle and unity by putting up brilliant defenses against such onslaughts.

Intrusions are of varying intensities. A low-key response might be triggered by neighboring Ospreys or intruders flying too close to the nest. This action may result in Ursula and Orpheus, if he is near, staring at the intruder and giving high-pitched chip guard calls. Ursula will mantle her wings, spreading them to cover the nest and protect its contents. If the menacing persists, Orpheus will take it up a notch by dashing after the intruders, chasing them away from the nest.

This is a common behavior at the Salt Point nest as inquisitive neighbors as well as intruders fly overhead. If tensions continue to rise, Orpheus and Ursula raise their voices, shrieking at the interloper(s). Intruders may escalate the aggression by diving at the nest defenders with legs jutted forward and talons out, ready to oust the breeding pair. These encounters at the nest can be especially dangerous, crushing eggs or injuring hatchlings or the adults.

Orpheus chasing an intruder who landed on the nest, courtesy of Cindy Sedlacek.

Orpheus chases the intruder half-way across the lake, courtesy of Cindy Sedlacek.

Some intruders are particularly persistent, bothering the same nest pair for weeks, trying to create a division between the breeding pair. Tenacious intruders may try to

block a pair mating or relentlessly dive at a female attempting to lay her eggs. These ongoing disturbances can stress a pair, inhibit egg laying and cause a pair to abandon their nest and eggs.

A last look at Ophelia in her nest before leaving, courtesy of Cindy Sedlacek.

Several years ago, Orpheus’s original mate of ten years, Ophelia, returned from migration five days after Orpheus only to find a young female, an intruder nicknamed Ursula, taking her place in her nest. Ophelia easily ejected Ursula from the nest and kept her out overnight. Two days later Ursula began her campaign of harassment against the nesting pair by purposely diving at the nest and flying off when chased by Orpheus. She repeatedly baited the pair seven times in one day and 12 times for the next two days.

Stressed, Ophelia stayed glued to her empty nest. The following day Ursula responded with a brutal attack on Ophelia while asleep. Ursula repeatedly dove at the nest, strafing

over Ophelia’s back with open talons. Ursula pushed her way into the nest, turned aside and beating her with outstretched wings. Ophelia finally had enough. Defeated, Ophelia left the nest with Ursula in pursuit.

The two females had battled physically for over a week for the rights to the nest, with little interference from Orpheus. Ophelia, who was probably in her prime at 13–14 years old at the time, abdicated to the younger Ursula. Ophelia has not been seen again after her decade-long reign at Salt Point. She may have joined the roaming class of intruders and found another nest with a willing mate. Despite successful mating, Ursula laid no eggs that year, but has done well ever since. Apparently, stress and anxiety are harmful to birds as well as people.

Orpheus (with eyes closed) and Ursula (R) on their nest, courtesy of Cindy Sedlacek.

Eyes to the sky!

Candace

Candace E. Cornell

Cayuga Lake Osprey Network

cec222@gmail.com

Guest photographer:

Ithaca’s Cindy Sedlacek brilliantly depicts the Ospreys of Cayuga Lake with the intimacy of a true bird lover. Enamored by the animal’s gorgeous wings and graceful flight, her work is a tribute to the power and grace of these stunning raptors.

A black and white drawing of an eagle

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