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DEVELOPMENTAL COMPARISONS & PREDICTING SEX

A PHOTOGRAPHIC GUIDE TO AGING AND SEXING SNOWY OWLS FROM HATCHING TO FLEDGING:

Monitoring plumage and growth rates in Snowy Owl chicks

Solai S. Le Fay, Chloe Y. Hernandez, Denver W. Holt

In June of 2022, the Owl Research Institute (ORI) began monitoring a Snowy Owl (Bubo scandiacus) nest in Utqiaġvik, AK (formerly Barrow) as part of a 31 year ongoing research study. This was the only nest located within the 100 square mile study area during the 2022 breeding season. ORI’s 2022 field crew Solai Le Fay, Chloe Hernandez, and Denver Holt conducted nest checks every three days (weather permitting), to monitor growth rates, survival, and cached prey.

 

To date, ORI has recorded 285 Snowy Owl nests and all observations have been compiled into comprehensive guides to aging and sexing snowy owl chicks (see publications). Here we use the techniques and data outlined in those publications, as well as refined methods and new observations from the 2022 nest, to create a photographic and descriptive guide for field researchers studying Snowy Owls.

 

Eight distinguishable age/development stages were recognized by Holt (Holt et al. 2016) and are further outlined in this guide. We delineate 7 developmental stages below (stage 8 missing due to researcher's departure from the field site), from the first chick pipping on July 17, 2022 to the last nest check on August 27, 2022. All photos are from the only Snowy Owl nest in Utqiaġvik in 2022.

STAGE ONE: 

days 1 - 4

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Photo 1: July 17, 2022 - Chick 01 (pictured) was breaking out of the egg and likely finished hatching on July 18th. Chick 02 likely began to pip (break the egg open) the next day on July 19th, hatching the following day on July 20th.

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Photo 2: July 20, 2022 - Chick 01 (top) and Chick 02 (bottom) had hatched, estimated at 3 and 1 days old, respectively. The bottom egg in the center can be seen with a small fracture as the chick inside begins to break out with its egg tooth.

DEVELOPMENT & BEHAVIOR

Protoptile (first) down is all white, and pink skin is fully visible beneath.
The chicks have a calcified white egg tooth on the tip of their black bill (as shown in Chick 01, photo 2). This ‘egg tooth’ is not a real tooth, but a specialized structure used to break out of their shell.

Cere (fleshy covering at the base of the bill) is a light pink-gray color (see photo 5).
Eyes are fully sealed shut. Not shown in the photos are whitish talons and pink, featherless metatarsal pads and feet.

Nestling movement is very limited at this stage, but a soft twittering sound can sometimes be heard, likely the chicks begging for food or communicating with the female that they are hatched.

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Photo 3: note the pink wing

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Photo 4: note the body covered in protoptyle down

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Photo 5: note the closed eyes, the light pink-gray cere, the egg tooth, and the pink skin

STAGE TWO: 

days 4 - 7

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Photo 6: July 23, 2022 - Chick 03 has finished hatching. Chick 04 (center egg) has begun to fracture its egg and begin the hatching process.

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Photo 7: July 23, 2022 - The estimated ages of the nestlings are: Chick 01 (center) 6 days old, Chick 02 (right of center) 4 days old, Chick 03 (underneath Chick 01 on the left) 2 days old.

DEVELOPMENT & BEHAVIOR

Gray mesoptile (second) down can be seen subcutaneously (underneath the skin), and eyes begin to slit open. Egg tooth is still present, and cere is still a light pink-gray. Metatarsal pads and feet remain featherless.

 

Nestlings are still helpless in the nest, but are able to move a bit. Vocalizations remain only a soft twittering.

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Photo 8: the wing shows subcutaneous gray mesoptile down and a lack of flight feathers

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Photo 9: from above, you can see mesoptile down subcutaneously and the lack of  rectrices (tail feathers)

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Photo 10: note the closed eyes and the light pink-gray cere

STAGE THREE: 

days 8 - 14

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Photo 11: July 28, 2022 - Four chicks can be seen with one underneath the rest of the chicks on the left. 

The two oldest Chicks, 01 and 02, are now in stage three, whilst Chicks 03 and 04 are in stage two, and Chick 05 in stage one. Estimated ages: Chick 01 (top right) 11 days, Chick 02 (top left) 9 days, Chick 03 (bottom left) 7 days, Chick 04 (bottom right) 5 days, Chick 05 (unseen) 3 days, Chick 06 (egg) pipping front and center.

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Photo 12: July 31, 2022 - All 6 chicks have finished hatching. Chicks 01, 02, 03, and 04 are now in stage three. Chick 05 is in stage 2 and Chick 06 is in stage 1.

Estimated ages: Chick 01 (left) 14 days, Chick 02 (right) 12 days, Chick 03 (center back) 10 days, Chick 04 (center top) 8 days, Chick 05 (center middle) 6 days, Chick 06 (center bottom) 3 days. 

DEVELOPMENT & BEHAVIOR

Gray mesoptile down begins to emerge from the skin and replaces the white protoptile down. Eyes can open now between 9-11 days, as seen in Chick 01, photo 11. At the end of this stage (around day 14), primary quill feathers on the wings begin to emerge (photo 13). Uropygial gland is large, void of feathers, and dark in color (photo 14). Legs and toes now have white feathers (photo 15).

Chicks are now more mobile in the nest and are able to hold their heads up. Chicks may begin to gular flutter to cool off, opening their mouths and vibrating their interior throat muscles and hyoid apparatus, comparable to panting. Chicks may start to bill snap when feeling threatened. Food begging is now a loud, long scream.

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Photo 13: note primary quill feathers beginning to emerge

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Photo 14: note lack of rectrices (tail feathers) around the uropygial gland

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Photo 15: note feathers beginning to grow on legs and toes

STAGE FOUR: 

days 15 - 21

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Photo 16: August 13, 2022 - Chick 06 at 16 days old after banding.

Photo 17: August 8, 2022 - Chick 04 at 18 days old.

DEVELOPMENT & BEHAVIOR

Chicks are now predominantly covered in gray mesoptile down.

The calcified egg tooth disappears (see photo 17), cere begins to darken in color, primary flight feathers erupt from their sheaths (photo 18), and tail feather quills are visible (photo 19). Legs and feet become more feathered (photo 20). The young experience a surge in body mass prior to nest departure.

 

The chicks are very mobile in and immediately around the nest. Pre-fledging departure from the nest begins.

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Photo 18: note primary feathers beginning to erupt from their sheaths

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Photo 19: note rectrices (tail feathers) beginning to emerge in sheaths

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Photo 20: note feathered feet and legs

STAGE FIVE: 

days 22 - 28

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Photo 21 : August 8, 2022 - Chick 01 at 22 days old.

Photo 22: August 13, 2022 - Chick 01 at 27 days old.

DEVELOPMENT & BEHAVIOR

Eyes are a deeper yellow; mesoptile down is a darker gray and juvenile plumage emerges, showing a white X around the eyes. Tail feather quills begin to erupt (photo 24) and primary feathers continue to emerge from sheaths. Feathers on the legs and feet grow longer.

Nestlings have departed their nest mounds on foot one at a time in the order they hatched. The chicks can now walk and run. They will generally stay within 1 km of their nest but are capable of moving farther, and will occasionally return to the nest area as they wander. The chicks will roost individually, near siblings, or occasionally in groups. The parents will usually stay in view of the young, however the females will roost closer to their young than males.

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Photo 23: primary feathers further erupt from sheaths

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Photo 24: rectrices begin to erupt from sheaths

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Photo 25: feathers on feet and legs continue to grow longer, with feathers beginning to grow past toes

Why do Snowy Owl chicks begin to leave the nest at 3 weeks?

Snowy Owls are an obligate ground nesting species. Research shows that Snowy Owl nestlings begin to leave the nest around 3 weeks of age as an anti-predator tactic. In the Utqiaġvik study area, Arctic foxes (Alopex lagopus), Pomarine Jaegers (Stercorarius pomarinus), and Glaucous Gulls (Larus hyperboreus) can prey upon young Snowy Owls. It is suggested that Snowy Owl nestlings depart early from their nests to minimize time predators have to locate a nest. Additionally, nestlings depart the nest asynchronously, likely a safeguard against nest failure if the nest were to be preyed upon.

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STAGE SIX: 

days 29 - 35

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Photo 26: August 18, 2022 - Chick 01 at 32 days old

Photo 27: August 24, 2022 - Chick 03 at 34 days old (defensive posture)

DEVELOPMENT & BEHAVIOR

All juvenile flight feathers are growing rapidly and synchronously, with primary wing feathers and coverts showing dark bars and spots. Cere is dark in color. Feathers around the eyes are beginning to look like a white mask.

 

Chicks now display a defensive posture with their wings outstretched when feeling threatened (photo 27).

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Photo 28: primary feathers erupt further from sheaths

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Photo 29: rectrices erupting from quills

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Photo 30: feathers on feet and legs have grown long

STAGE SEVEN: 

days 36 - 43

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Photo 31: August 27, 2022 - Chick 03 at 37 days old

Photo 32: August 27, 2022 - Chick 02 at 39 days old

DEVELOPMENT & BEHAVIOR

At this stage, wing and tail feathers are well developed and there is a notable difference in plumage between males and females (described in 'Predicting the Sex of Chicks').

Gray down is still retained on the head, back, and breast. Juvenile plumage shows a developed white mask around the eyes.

 

Chicks will begin hopping and their first attempts at flying.

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Photo 33: primary feathers are well developed

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Photo 34: rectrices continue to develop

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Photo 35: feet and legs are fully feathered

DEVELOPMENTAL COMPARISONS & PREDICTING SEX

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