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by Ann Nightingale
Most naturalists know that flowers often sport ultraviolet (UV) trails to lure birds and insects to visit and pollinate them. However, even experienced birders are usually surprised to find out that birds also display UV patterns in their feathers. Even the often-criticized European Starling can rival its more spectacular tropical counterparts under UV light. But who would have ever guessed that a nocturnal bird would also display an impressive UV wardrobe?
The strictly nocturnal Northern Saw-whet Owl is one of the least commonly seen common North American birds. It's a rare treat when birders happen onto a day roosting Saw-whet hidden in dense foliage. A few lucky landowners may have a nesting pair on their properties. Otherwise, the best chance to see this owl is after one has hit a window or been struck by a car. For a fairly abundant bird, this species can be maddeningly difficult to lay eyes on. But owl banders, who face the dark and sometimes cold nights to study this charismatic owl, know one of this bird’s best kept secrets.
At Rocky Point Bird Observatory (RPBO) in Victoria, BC, we have been monitoring the fall passage of Northern Saw-whet Owls over the southern tip of Vancouver Island since 2002. In fewer than a total of 60 weeks of banding, we have netted and collected data on more than 4100 passing owls, including 850 from mid-September to the end of October in 2012 alone. Saw-whets are believed by many to be more nomadic than migratory, but there is a somewhat organized southbound movement in the early fall. With a dearth of banding stations in the west, we’re still in the process of determining the primary migration corridors.
One of the most important pieces of data we collect when banding birds is the age of the individual. For most species of birds, there are two choices in the fall: a bird hatched this year (hatch-year) or an adult of indeterminate age (after-hatch-year). Adult birds of most species typically molt all of their feathers after breeding, making it difficult to impossible to determine a more precise age. Owls and other raptors, however, do not follow this pattern. They molt only some of their flight feathers each year, often in a predictable pattern, making it possible to determine their age more accurately–up to several years in some cases. Since Saw-whets are fairly prolific breeders–laying between four and seven eggs in each nest–we expect to see a much higher percentage of hatch-year birds than adults in the fall. Typically, 70 to 90% of the RPBO captures are hatch-year birds, but we’ve had the proportion of young birds drop into the 50% range during some monitoring seasons, suggesting a poor breeding season or low fledgling survival. This kind of information can sometimes be correlated to food supply, habitat changes, or weather, and may provide insight into potential conservation measures.
You can imagine that trying to assess the quality of feathers under dim light from headlamps, incandescent or fluorescent bulbs is something of a challenge, but until the mid 1990’s, this was simply how it was done. In 1982, researcher, Bruce A. Colvin discovered that porphyrin in the newly molted feathers of Barn Owls fluoresced under UV light. This organic compound fades over time and with exposure to light, making different generations of feathers easily identified using a UV light. Colvin’s discovery was shared among researchers and wildlife managers and ultimately came to the attention of David Brinker in the mid 1990s. He and other Saw-whet Owl banders along the Eastern Seaboard tried it on their smaller owls, and found that the light showed clear differences in the age of feathers, especially in detecting the differences between one- and two-year old feathers. Peter Pyle had published the expected patterns of molt in his Identification Guide to North American Birds—the “bible” to most bird banders, but using UV light made a difficult task much easier and more accurate, even for less experienced banders. Many of the stations operating under the loose affiliation of Project Owlnet are now using this tool to speed up and verify ageing during the banding process.
As helpful as this is for us, we’re pretty sure that the development of UV patterns was not for the benefit of bird banders. So how could this flashy pink signal be useful for the owls? The jury is still out on this one, but there is speculation. Since a young bird shows fluorescence across the whole wing, its age is revealed not only to banders, but to potential mates that can also see the pink porphyrin glow. During courtship, the male literally flies circles around the female, often repeating its flight display more that fifteen times. This would give the female ample opportunity to view the bright pattern of a young bird or the more muted and varied pattern of an older, more experienced bird. Even young birds are able to secure mates and raise young, though. We’ve also noticed that the intensity of the glow varies, even among birds of the same age class. It may be that the intensity is some kind of an indicator of overall fitness. We can see how this could be useful in mate selection.
However, larger owls, such as Barred Owls, also show the same kind of UV patterns, so presumably can see the glow of the new feathers as well. We also know that larger owls prey upon smaller owls like Northern Saw-whets. Is it possible that since the older birds show less color, they could be less likely targets of predation? Does the UV pattern offer the established breeders a bit of protection while sacrificing the young of the year? There are no guarantees, though, and recently the band of a third-year Saw-whet banded at Tatlayoko Lake Bird Observatory in British Columbia was found in a Great Horned Owl pellet 100 km south of Spokane, WA.
If you know where to look, Northern Saw-whet Owls can be found throughout the year in the Pacific Northwest. Despite being one of the most studied birds in North America, there is still much to learn about this secretive little owl!