Tweets over Traffic: Noise Pollution in the Seattle Arboretum

By Phoebe Berghout, Aquatic & Fisheries Science and ESRM ‘26

An oasis lies amongst Seattle’s urban landscape. The Washington Park Arboretum houses a diverse menagerie of 40,000 plant species that provide a lush urban forest habitat for over 170 species of birds. With over 90 years as a popular Seattle park, the Arboretum saw over ¾ of a million visitors in 2022. A large portion of this visitation comes from the city’s bird watchers, who flock to see year-round icons and special seasonal species. The park has 230 acres for birds and birders to enjoy, stretching all the way down to Lake Washington’s Union Bay, where it encompasses Marsh and Foster Islands. But if you’ve ever journeyed to the North side of the Arboretum, binoculars in hand, you likely have noticed the heavy drum of traffic from the 520 bridge.

The Washington Park Arboretum Trail looking to Marsh Island (Image credit: Joe Mable).

Male and female wood ducks at Foster Island (Image credit: Phoebe Berghout).

Traffic’s addition of low frequency environmental noise to the soundscape impacts a pivotal part of the avian life cycle – nesting. Greater noise disrupts stress hormone levels in adult females and oxidative stress in nestlings. Female birds carrying these increased physiological burdens may be less able to provide for their young. Oxidative stress in nestlings can decrease body size and delay fledgling behaviors. Delays in nesting phenology are also recorded amongst species that communicate at lower frequencies as these birds become challenged to pick out other birds’ calls in the soundscape. Species like the varied thrush, American bittern, or Hermit thrush, common in the Arboretum, may struggle to find mates and begin the nesting process.

Dark-eyed junco nest (Image credit: National Parks Gallery).

But it’s not just low-frequency callers that bear the burden of noise pollution. The local dark-eyed junco calls at relatively high frequencies and a recent study found urban juncos have increased their pitch compared to forest-dwelling populations. The arboretum is a vital nesting region for Seattle birds. Careful management of the area provides an abundance of preferential native flora for nesting, along with snags of native trees, and significant subsets of crucial marshland habitat.

While our understanding of noise pollution’s impact on the Arboretum’s birds is still minimal, managers can begin to hypothesize the severity of threats posed to avian communities in this precious local landscape. The 520 bridge has been a major restoration project for the past several years. Replacement of onramps, changes in runoff infrastructure, and other associated restoration work have helped improve physical aspects of habitat for avian communities. But aside from the increased elevation of the bridge, little improvement has been made for the habitat’s soundscape. Emerging noise mitigation infrastructure has highlighted the success of vegetation barriers, soundproofing materials incorporated in buildings, and noise-reducing asphalt, a type of road surface designed to absorb and reduce road noise. Additionally, reduced speed limits can improve the noise pollution present in a soundscape. These strategies should be further explored to reduce the impact of 520 traffic.