How to Age an Owl

Around 80% of the owls we've banded so far during our new Northern Saw-whet Owl migration survey have been hatch year (first-year) individuals.  Although there are still a lot of ‘unknowns’, this phenomenon can be partially explained by two important aspects of Saw-whet Owl biology: the first is that Saw-whets exhibit a pattern of differential migration, wherein young birds tend to migrate different distances or follow different migration pathways from adults; the second is that a large proportion (perhaps as many as 4 out of 5) of these tiny owls don’t survive long enough to celebrate their first birthday.

A first-year Saw-whet Owl © Tyler Christensen

We know this stuff because owl banders all over the Northeast have been recording the ages of every Saw-whet Owl they encounter during fall migration. 

But how do you tell the age of an owl?  No matter how politely you ask, the owls just don’t tell.

Instead, we have to infer an owl’s age by examining its wings.  Owls, like many birds, exhibit age-specific patterns of feather replacement.  Normally discerning these patterns is a subtle science, but thanks to an unusual pigment found in owl feathers called porphyrin (which causes freshly replaced feathers to fluoresce bright pink under ultraviolet light) we can see these patterns easily when the wing is viewed under a black light.

© Tyler Christensen

The wing in the photo above belongs to a first-year Saw-whet owl, just a few months after hatching.  The wing feathers all grew in at the same time, starting while the bird was still in the nest.  As a result, all the wing feathers fluoresce a similar shade of pink.

© Tyler Christensen

Second-year Saw-whets replace the outermost and innermost wing feathers, but retain a block of juvenile wing feathers in mid-wing; the freshly replaced outer and inner feathers now glow brighter pink than the mid-wing feathers left over from last year.  The wing in the photo above belongs to a second-year owl.

© Tyler Christensen

Owls in their third year or older begin replacing their feathers in less predictable patterns, resulting in scattered blocks of bright pink new feathers, dull pink intermediate feathers, and very old feathers without any pink whatsoever.  When a bird exhibits this pattern, it is classified as ‘after second year’.  The wing in the photo above belongs to an owl that is at least three years of age.

The use of ultraviolet light makes aging owls a cinch.  Most birds, however, don’t contain fluorescent pigments, which makes aging a much more meticulous process.  Either way, collecting demographic data throughout a species’ range and across the seasons helps us understand some of the big-picture aspects of bird ecology.  Saw-whet Owls are just one particularly colorful example.

WCAS's Saw-whet Migration Survey

Last night was the official start to Washington Crossing Audubon’s Northern Saw-whet Owl survey.  WCAS launched this survey to gain insight into the magnitude of Saw-whet Owl migration in our region.  Given their nocturnal habits, secretive nature, and the fact that they sing only on their northern breeding grounds, gathering data on Saw-whet Owls can be very difficult using passive observation alone. 

Northern Saw-whet Owl (Aegolius acadicus). © Tyler Christensen

The survey involves capturing owls using mist nets, taking a series of measurements, and fitting them with uniquely-numbered aluminum bands, which they will carry with them for the rest of their lives, allowing individual identification upon future encounters.

We were hoping for at least one owl during our first survey night – we caught eleven!  Our last net run yielded five different individuals.  Particularly exciting was that one of the owls captured had been previously banded by another Saw-whet Owl bander.  A major goal of owl banding projects is to gather data on migration behavior, and recapture data is essential to this end.  We look forward to finding out where this owl was first banded!

© Tyler Christensen

Presumably all the owls encountered last night were in active migration, and had stopped at this site to regain fat reserves before continuing south.  This survey will take place through peak Saw-whet migration, from now until early December, while their abundance is at its highest.  Hopefully future nights will be as successful as last night!

Saw-whet Owl after release © Tyler Christensen

The Swan Chasers

Last week, a few of the Washington Crossing Audubon board members got together to find a group of Trumpeter Swans that had been reported at the Assunpink Wildlife Management Area near Robbinsville, New Jersey.  This small posse consisted of myself, Hannah Suthers, Brad Meritt, Fairfax Hutter, and Sharyn Magee.

As you may remember, Wednesday the 12th was a day of unprecedented gorgeousness for mid-March, reaching a high temperature in the mid-sixties, complete with full sun and a gentle breeze.  It was a perfect day to go birding with friends and enjoy the weather.  These birders, however, chose the following day for our outing which, thanks to forces of cosmic humor, reached a high temperature in the mid-twenties with winds at 35 miles per hour causing a chill factor of -9°.  This group of fearless birders was not deterred.

Left to right: Fairfax Hutter, Sharyn Magee, Brad Meritt, and Hannah Suthers

Within moments of arriving at Assunpink Lake we had spotted the trio of large and conspicuous immature Trumpeter Swans, foraging on the far side of the lake with another trio of Mute Swans.  The much more slender necks and larger black bills of the Trumpeters made them readily separable from the Mutes.

Trumpeter Swans (center) and a Mute Swan (right)

The significance of these swans’ appearance in our neck of the woods is that New Jersey is well outside their normal range, which consists of scattered populations in northwestern and central North America, with he highest numbers occurring in Alaska.  During migration they sometimes disperse widely, and are occasionally reported far from their usual range, as demonstrated by these three individuals. 

At one point a lone Tree Swallow flew over the ice along the south shore of the lake where we stood; this was the first Tree Swallow of the year for the entire group, a very exciting (but not very convincing) sign of spring on such a blustery day.  Among the numerous Northern Pintails, Ring-necked Ducks, Mallards, and Common Mergansers was a single duck that stood out as it flew across the lake.  This duck was predominantly gray, with a bright white speculum, reddish head, and a buff-colored stripe running from bill to crown; any guesses?

This was a drake (male) Eurasian Wigeon, a rare vagrant to the east coast of North America from the Old World.  Interestingly, our American Wigeon is a vagrant species that is occasionally sighted in Europe – something for the “twitchers” of the U.K. to get excited about.  This spectacular little duck was just more icing on the cake.

We only stayed for about 30 minutes before the cold and wind became unbearable, forcing us to retreat to our cars.  To celebrate the productive speed-birding session at Assunpink, we retired to Woody’s Towne Café in Allentown for a bowl of hot chili.  Who else is ready for warm weather?

Ducks at Roebling Park

This morning was the first of a series of fall, winter, and early-spring walks at Roebling Park in the Abbott Marshlands.  These walks are hosted by the Mercer County Park Commission and co-sponsored by the Washington Crossing Audubon Society and Friends for the Marsh.  The theme of these walks is winter waterfowl; when most of our summer songbirds have left for warmer climes, ducks and other waterbirds from the North begin to arrive in large numbers at bodies of freshwater throughout our region.  Hence, birders congregate at water to admire them.  Today's walk had over 30 participants, ranging from first-time birders to seasoned duck-watchers.

Without question, today's most abundant waterfowl species was Gadwall (we spotted over 100), followed by Green-winged Teal (23), Northern Shoveler (9), and Northern Pintail (6).  Altogether we spotted 13 species of waterfowl, which is unusually high for this time of year.  For many of us, today helped get our rusty waterfowl ID gears turning after a year of inactivity.

A resting Northern Shoveler and a male Mallard

A male Wood Duck tucked away in the reeds

The tidal North Marsh section of the park had both the highest diversity and abundance of ducks, including three new species for the day: A female Bufflehead, a male Ruddy Duck, and a female Lesser Scaup.

Female Bufflehead (center) and male and female Gadwall (front)

Not all today's finds were birds; young naturalist with a Spring Peeper

Other highlights from today's trip included several passing flocks of Red-winged Blackbirds totaling nearly 200 individuals, a flyover immature Bald Eagle, and nice views of a Merlin.

Click here to view the checklist of birds encountered during this field trip.  For more information on upcoming field trips and programs, please visit our website.

Happy birding!

Sparrow Spotting

This morning I went "digiscoping" with Washington Crossing Audubon's new spotting scope.  I wanted to experiment with the prospect of digiscoping during field trips and educational programs to share photos of highlights on this blog.  At 7:45 this morning I headed out to the Pole Farm in Lawrenceville for a digiscoping test-run.

WCAS's new Vortex Viper HD spotting scope

In typical late-October fashion, shrub rows everywhere were just dripping with sparrows and Yellow-rumped Warblers.  Every step down the path sent LBJ's retreating into the shrubbery.  This is one of most enjoyable phases of fall migration, characterized by an impressive abundance of kinglets, thrushes, Yellow-rumped and Palm Warblers, and sparrows over a colorful backdrop of fall foliage.  

Song Sparrow

Palm Warbler

At around 10:30 I encountered a male American Kestrel hunting grasshoppers from its perch atop a wooden post.  Each time he would spot one, he fly down from his perch, pounce on it, and bring it back to his perch to eat.  This went on for about fifteen minutes before he moved on.

Adult male American Kestrel hunting grasshoppers

On my way back to the car I encountered a small flock of Savannah Sparrows working a patch of Tickseed Sunflower at the edge of the trail.  These birds posed nicely for the best digiscoped photos I took all morning.  

Savannah Sparrow on Tickseed Sunflower

Savannah Sparrow

I rarely carry my bulky SLR on field trips, so the prospect of digiscoping as a way of recording field trip sightings for this blog is very exciting.  I'll try to put this into practice and blog about the highlights of the WCAS field trip to Shark River on November 16th.

Happy birding!