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Weather conditions lead to window strikes for migrating songbirds

Weather conditions lead to window strikes for migrating songbirds

A significant number of songbirds were silenced last week in Delaware when their southerly migration ran afoul of circumstantial weather conditions that led to their flying into glass windows causing mortal injuries to nearly a dozen species of thrushes and sparrows, common and rare birds alike.

As a cold front moved into the northeastern United States on the night of October 28, conditions were perfect for a mass migration of songbirds throughout the Mid-Atlantic region. After almost a week in which no strong bird migratory movements had been detected across Delmarva, many songbirds seized on the coming cold front to depart ahead of it, leading to a historic fallout of migrants in Delaware. Reports from birders indicate that migrant songbirds, particularly sparrows and thrushes, had descended on Delaware in record numbers as they worked their way south for the upcoming winter. Unfortunately, their epic migration also provided a glimpse into the dangers that many migratory birds face along their journey.

Just after daylight Friday morning, Oct. 29, shopkeepers, residents, and visitors of Rehoboth began noticing a number of dead birds along the boardwalk. Their concern led them to contact the Rehoboth Beach Police Department, which got through to the Delaware Division of Fish and Wildlife Enforcement Section. Later, as he identified photos of some of the dead birds as thrushes, Anthony Gonzon, a wildlife biologist with the Division’s Natural Heritage and Endangered Species Program, knew immediately that something disturbing had happened in the resort beach town.

For Gonzon, finding one or two dead songbirds almost anywhere would not be surprising, but an initial report of 12-15 birds at a localized site was highly unusual – especially occurring away from large cities where tall buildings and the potential for birds to strike windows would be much more prevalent.

Joined by Sr. Cpl. Tommy Penuel of the Division’s Enforcement Section and officers from the Rehoboth Police Department, Gonzon searched the Rehoboth boardwalk and side streets for dead migrants, eventually turning up 60 dead songbirds representing 11 different species. Counted among them were 20 White-throated Sparrows, 17 Hermit Thrushes, along with uncommon species such as the Lincoln’s and Nelson’s Sparrows.

Nearly all of the birds had injuries consistent with window or building strikes. Others had already been scavenged by cats and other birds, but all were found in proximity to those that had died as a result of collision. Sites along the northern half of the Rehoboth boardwalk resulted in the greatest number of mortalities. Windows along that stretch showed obvious signs of avian impacts, several with dead songbirds having fallen directly below them. While there were likely far more dead birds than searchers found at the time, between boardwalk residents and maintenance workers removing and discarding some birds and scavengers taking others, the actual number of mortalities is difficult to speculate.

All of the events leading to this mortality event might never be truly understood, but Gonzon ironically pegged it in the larger picture to perfect migration weather patterns. “I’d received word from several sources that migration on Thursday night (Oct. 28) would be historic in terms of the sheer number of birds on the move,” he said. “Given that hundreds of thousands, if not millions, of birds were passing by and descending on us very early Friday morning, and with winds blowing out of the north-northwest, birds coming off Cape May and crossing over to Delaware may have been forced a bit further south. This would result in a landfall at or about Rehoboth Beach.”

As exhausted birds reached the Delaware coast during the predawn period, lights and reflections from buildings and glass could disorient them, resulting in strikes. Because birds fly lower over water as they descend upon reaching land, it’s possible that buildings and glass along the boardwalk were directly in their path. Gonzon also noted that ominously perfect conditions aligned for the unfortunate mortality, and that until those conditions coalesce again, Delaware would likely not see an event like this anytime in the near future.

Unfortunately, there are no easy preventatives for birds striking buildings or glass. Because birds often cannot see glass or comprehend the difference between a flyway and a reflection, bird strikes on glass are implicated as one of the most significant causes of mortality for migrants. Mortality estimates from glass strikes range from a conservative 100 million annually to nearly 1 billion. “However, we can do some things to reduce the impacts of this type of mortality,” Gonzon said. “Decreasing the use of building and window lighting during peak migration periods and using bird-safe glass can help to reduce the number of strikes. The use of exterior window treatments such as pull down shades also can reduce or eliminate reflectivity, and prevent birds from striking.”

Article by Michael Globetti
Public Affairs-Office of the Secretary
Delaware Deparatment of Natural Resources and Environmental Control

Vol. 40, No. 373
Contact Anthony Gonzon, Wildlife Biologist, 302-735-8651, or Michael Globetti, DNREC Public Affairs, 302-739-9902