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August 2020

Vol. 48, No. 4

Revisiting Bird-Window Collisions in Boston: The First Year of the Avian Collision Team (ACT)

William Freedberg

83 of the 119 casualties from the spring 2019 season of ACT. Photograph courtesy of the author.

Migratory bird numbers in Massachusetts are not what they used to be. Rarer, now, are days with 20 warbler species at Mount Auburn Cemetery, thousands-strong shorebird flocks in Chatham, and 200 dead birds gathered at the base of a single building.

Veteran birders talk fondly of the former two cases, but fewer discuss the bygone days of finding dozens or hundreds of window-struck birds in a morning. But this phenomenon did not go entirely undocumented. In 1974, Bird Observer published Henry Wiggin's article, "Birding at the Prudential Center," with the focus on unexpected patterns in window-collision observations (Wiggin 1974). At the Prudential Center in Boston (the Pru), which was then his office, Wiggin reported several avian mortality events with over 200 casualties and a personal record-setting day on May 4, 1968, with over 700 deceased migrants.

Sometimes befuddling patterns emerged in Wiggin's data. "If a birder were to bird only the Prudential Center," Wiggin wrote, they "might come to some weird conclusions" regarding the relative abundance and timing of migrants. For example, nearly half of Wiggin's 7,000-plus observations were of White-throated Sparrows. More strikingly, the sparrow and finch family (which was then considered taxonomically valid) outnumbered all other birds by seven to one. Lincoln's Sparrows, Orange-crowned Warblers, and Marsh Wrens were unusually frequent, but there were almost no records of diurnal migrants, such as swallows or American Robins. Birdsong at the Pru was rare, and almost all migrants were placidly approachable within a few feet.

Surprisingly, very little has been published on window strikes in Boston since Wiggin's article. Even after collision monitoring programs in Chicago, New York, and other major cities entered their second decade, there was no coordinated, multi-observer effort in Massachusetts until last year. Mass Audubon moved to pilot a monitoring program in 2019 after advocacy for bird-safe building practices ran up against local disbelief. In the absence of recent nearby evidence, many nonbirders were skeptical that this global issue existed in their city. Consequently, the Avian Collision Team (ACT) was born.

ACT was invented as a hybrid between the standardized, rigorous studies normally under the purview of a research institution and the more casual volunteer programs common in most cities. For example, one goal is to clarify how common collisions are on an average block rather than just monitoring at the most collision-prone sites. We select several random sites in addition to buildings already predicted to be strike-prone, and we ask volunteers to record negative data (i.e., where casualties were not found as much as where they were). Volunteers pick a route that they can regularly survey, and we try to ensure that all routes are surveyed during the same early morning period on the same four days of the week.

But data is not the only point of the program. Though we may eventually be able to estimate how many birds die from window collisions annually in Boston, the priority is to raise awareness about the issue, collect stories, and think about solutions at a site-by-site and citywide basis. Volunteer programs have their vulnerabilities and getting clean, noise-free data from a group of mostly casual observers may yet prove to be a pipe dream. Another obstacle, at least for the moment, is the variability between migration seasons, which makes generalizing based on one year of data difficult. Most local birders will remember the fall of 2019 as one of the weakest and strangest fall migrations in memory, which serves to drive home the need for several years of work. Furthermore, Covid-19 made sampling a complete wash for the spring 2020 season and may yet require a total reimagining of the project.

With that said, the initial results from 2019 will be of interest to curious birders. ACT participants recorded 193 strikes across 51 species while monitoring four days a week during spring and fall migration. Just over two-thirds of bird strikes documented so far have been immediately fatal, with the lucky remainder being sent to Tufts Wildlife Clinic for treatment or, sadly, euthanasia.

White-throated Sparrows have been the most prevalent species at nearly 20% of the total so far, with Ovenbirds and Common Yellowthroats being close runners-up. New York's monitoring programs report a similar species composition. ACT volunteers have turned up a few surprises, including six Lincoln's Sparrows, a Virginia Rail, a Belted Kingfisher, and, most notably, a Clay-colored Sparrow and a Nelson's Sparrow found at the same site on the same day. Birds that are weak flyers or that habitually fly low are especially well-represented, as are tree-climbing birds like sapsuckers, nuthatches, and Brown Creepers.

Surprisingly, nearly 50% of the strikes reported by ACT volunteers came from just two sites—one college campus and one city-block-sized glass-clad building. Many buildings have only caused one or two strikes so far, a few have yet to be observed causing any, and a handful punch above their weight, so to speak. But this should not leave any reader with the impression that window strikes, at a regional scale, are concentrated at a handful of buildings. In fact, a nationwide study showed that 80–90% of strikes occurred at one-to-three-story residences (Loss et al. 2014). Even though only one to three strikes per year occur at the average residence, compared to an average of 22 at low-rise commercial or office buildings and 25 at skyscrapers, the sheer number of residences gives them the lion's share of the total.

All in all, that same study—authored by scientists at the Smithsonian Institution and the U.S. Fish and Wildlife Service—estimated that window collisions kill between 365 and 988 million birds annually.

There is plenty of debate about whether these deaths are additive or compensatory— that is, whether window strikes actually contribute to population declines or whether they remove "extra" individuals that would fail to reproduce because of some other limiting factor such as habitat destruction. Indeed, one paper (Arnold & Zink 2011) showed that the most collision-prone species—such as Wiggin's White-throated Sparrows—were not the species exhibiting the most dramatic continent-wide declines. But this observation could easily be confounded by the intensification or reduction of other stressors at the species level. Even if other factors drive the species' overall trend, window strikes can still have an additive impact.

The estimate of nearly one billion annual fatal collisions from Loss et al. may seem high, but the numbers check out with reports from monitoring programs in most cities, as does preliminary data from ACT. While our current numbers may seem high, looking at historical data like Henry Wiggin's draws attention to how much worse the problem may have been in past decades.

ACT has not recorded any of the avian mass mortality events that Wiggin reported in his 1974 Bird Observer article, either at the Prudential Center or elsewhere. In fact, the most active morning for strikes at the Pru in 2019 yielded five birds, and the highest total anywhere else was only eleven birds. At least at the Pru, this may be explained by the significant changes to the building's interior courtyards. The North Garden is completely gone, as are the moats at the bases of the courtyard windows where Wiggin observed hundreds of drowned window-stunned birds. The lighting has changed too, and light pollution in the city has increased, making the Pru less of an isolated beacon for migrants. Indeed, window collisions may have simply become more diffuse throughout the city as more glass façades have gone up. One pattern beginning to emerge in our data—and already backed up by long-term data in other cities—is that more exposed buildings draw more strikes. Glass-clad buildings surrounded by dense development are much less hazardous than similar-looking buildings that stand alone in a grassy field or facing open habitat.

But it is also worth considering a darker possibility—that there are that many fewer migrants in the skies than in the 1970s. Other reports of building strikes, some much older than Wiggin's observations, contain some truly shocking numbers. Colonel Tassin at the Statue of Liberty counted 1,400 birds killed by the Statue's torch one night in 1888; another night, he counted 50 rails (Wilson 1888). To be sure, scientists have thoroughly documented declines of about 30% across all North American birds in the past fifty years (Rosenberg et al. 2019), but declines of that magnitude would still not fully explain some of the more extreme historical reports of window collisions.

Birders interested in mitigating window collisions at their homes or offices have several options. The most popular options are not necessarily the best. Individual stick-on window decals, commonly sold in the shape of a hawk's silhouette, do nothing unless they are affixed in a regular pattern with no gaps greater than two inches by four inches. Birds will aim around a decal to try to access what they see in a window reflection, and the silhouette of an inexplicably stationary hawk will do little to stop them. Decals or window tape that reflect only UV light—clear to us, but opaque to birds—are a great option when placed no more than two inches apart, and there are a number of companies (e.g., CollideEscape) producing them. Anything else that breaks up reflections in a window works well, from installing screens that also shield incoming birds from the hard glass surface to drawing with soap suds or dry-erase markers. There are more expensive options for commercial buildings, including specialty UV-coated glass brands such as OrniLux or AviProtek. Finally, though it may seem counterintuitive, moving existing birdfeeders to within 1.5 feet of a window can help protect birds from collisions by preventing incoming birds from picking up the speed required for a strike to result in a fatality.

The ACT welcomes inquiries. If you are interested in volunteering with ACT, want to report window-struck birds, or have general questions, contact Will Freedberg at


  • Arnold, T. W. and R. M. Zink. 2011. Collision mortality has no discernible effect on population trends of North American birds. PLOS ONE ٦(٩):e24708. doi: 10.1371/journal.pone.0024708.
  • Loss, S. R., T. Will, S. S. Loss, and P.P. Marra. 2014. Bird-building collisions in the United States: estimates of annual mortality and species vulnerability. The Condor ١١٦ (1):8–23.
  • Rosenberg, K. V., A. M. Dokter, P. J. Blancher, J. R. Sauer, A. C. Smith, A. P. Smith, J. C. Stanton, A. Panjabi, L. Helft, M. Parr, and P. Marra. 2019. Decline of the North American Avifauna. Science 366:120–124.
  • Wiggin, H. T. 1974. Birding at the Prudential Center. Bird Observer 2 (5):136-140.
  • Wilson, V. T. 1888. Killed By Liberty's Torch. The Oologist's Exchange, 1:1(1).

William Freedberg writes, conducts field surveys, and coordinates citizen science programs for the Bird Conservation department at Mass Audubon. He coordinates the Avian Collision Team (ACT). He lives near Boston and spends several weeks each year leading tours in the neotropics.

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