Blog #384 Everything you wanted to know about Osprey eggs, but were afraid to ask!

Ospreys typically lay eggs in the morning, so the bird is not constrained by the weight of the egg during the  daytime. Studies in New England show older females lay two weeks earlier than younger females, and pairs that previously nested together lay earlier than newly formed pairs. Early breeding is an advantage to this predominantly monogamous species, especially in areas with short breeding seasons. Early breeding pairs generally produce more young with higher survival rates. 

Some of the many colors and patterns on Osprey eggs, courtesy of YouTube, flicker, and nwrafting. 

Osprey eggs are considered one of the most beautiful eggs in the animal kingdom. They have a creamy  white to cinnamon-peach backgrounds and are heavily wreathed and spotted with rich red to dark brown with flecks of lavender and swirls of mahogany in an infinite number of patterns. The surface is satin  

More unique Osprey eggs, courtesy of acorn naturalists, flicker, and boneclones. 

rather than a glossy finish, and the eggs are about the size of a jumbo chicken egg ranging in weight from about 2.9–6.3 oz. The patterns on Osprey eggs are unique to the female and enable her to distinguish 

her eggs from others. The unique color and pattern are similar for the same bird year-to-year but differ  from those of other females. 

Ornithologist Paul Spitzer describes Osprey eggs in a nest writing, “Words fail me as to how beautiful they are...Clutch-mates tend to have some artistic consistency, but there is enough variation that one wants to admire each entire clutch as an avian work of art. Toward the end of the incubation period, gentle polishing and weathering have sometimes reduced the eggs’ appearance to old scuffed shoes. But— aha!—the life within is about to make its appearance.” 

The stunning beauty of osprey clutches was not lost on 19–20th-century egg-collectors or “oologists” who competed to find the eggs by climbing to nests in high trees and combing isolated marshes and beaches. The remains of this destructive hobby fill museum cases with dusty old blown-out eggs. Osprey eggs were great prizes, so they are well represented in these anachronistic curio collections. 

An Osprey’s “clutch size” ranges from 2 to 4, and roughly 80% of Cayuga Lake Basin’s females lay 3 eggs. It is known from banding studies that a 2–egg clutch reflects a young, first-time breeding female. 4- egg clutches occur in food-rich nesting areas. Typically, eggs are laid two days apart and hatch in the order they were laid. A three-egg clutch takes about 6–7 days to complete; a 4-egg clutch probably 8–10 days. 

Orpheus trying to nudge Ursula off the eggs. 

Both males and females incubate the eggs. When the male brings a fish to the incubating female, the pair  trades places and the female typically flies off with the fish. However, males like Orpheus are highly  motivated to incubate and may incubate up to 40% of the time. The females, however, incubate  throughout the night. The ideal incubation temperature for an Osprey egg to develop normally is 98.6  degrees—a few degrees below the normal avian body temperature of 104 degrees. Like most birds,  Ospreys warm their eggs by transferring their body heat through a temporarily bare area of abdominal  skin called the brood patch. Because they have relatively small brood patches, the Osprey incubation  period is substantially longer than expected for a raptor of this size. Extra time is needed to give each egg  adequate time at 98.6 degrees to ensure proper development.

The brood patch on female pied flycatcher, courtesy of Archive.com. 

How well the parent birds carry out their incubation duties probably impacts hatch dates. It takes about  35–37 days for the chicks to emerge, although exceptions of up to 43 days are recorded. It’s commonly  

Trading places to incubate in the early morning.

thought that parents only leave their eggs or young unattended to chase off predators like bald eagles,  the Salt Point nest cam has recorded otherwise. Incubating parents periodically leave the eggs unattended to stretch, fly around, and perhaps take a quick bath. However, these absences are never 

Orpheus is still excited about his recent encounter with an intruder. 

more than five minutes, and never during inclement weather. Their nests, exposed to the open, make it a  challenge to protect the vulnerable eggs while brooding. Eggs perish if they become cold or too damp.  Prolonged heavy rain can cause significant mortality of eggs and small young. Large hailstones can  easily crack the eggs if any part of them is left exposed.  

Ospreys can sense the temperature of their eggs with receptors in their brood patches. This information allows them to accurately regulate their attentiveness—the time spent incubating—more accurately.  Periods of attentiveness gradually decline as the embryos grow and generate their own heat. The  incubating adult turns the eggs every 20 to 40 minutes to warm the eggs evenly and to prevent embryonic  membranes from sticking to the shell. 

Interestingly, Osprey pairs throughout the U.S. produce about. twice as many young at artificial nest sites  than at natural sites such as trees and rock ledges. This is in part due to artificial sites being more stable  and less likely to blow down during and in between breeding seasons. Thus, Osprey populations with  access to artificial nesting sites, like those surrounding Cayuga Lake, tend to expand more quickly than  those confined to natural sites. 

Eyes to the sky! 

Candace

Candace E. Cornell 

Friends of SaltPoint, Lansing, NY 

Cayuga Lake Osprey Network 

  

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OnOspreyTime: Ospreys of Salt Point 

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Cayuga Lake Osprey Trail