City Parks Flip From Winter Refuges to Summer Ovens for Birds, Three-Year Acoustic Study Finds

In one of the most ambitious year-round listening campaigns ever mounted in a subtropical megacity, researchers have discovered that the same patch of urban greenery can be a lifeline for birds in winter and a source of heat stress in summer. By deploying automated sound recorders in three very different Shanghai green spaces and pairing the resulting acoustic data with satellite-derived land surface temperatures, a team led by Yiran Zhang of the Shanghai Urban Forest Ecosystem Observation Station has shown that the ecological value of urban green infrastructure is not a fixed property of a park, but something that flips with the seasons. The study, published in Environmental and Sustainability Indicators, offers a rare community-level view of how thermal seasonality reshapes which birds are present, where they concentrate, and when cities become hostile to them.

The technical heart of the project is passive acoustic monitoring, or PAM, a technique that replaces human observers with weatherproof recorders that capture the soundscape continuously. Between January 2023 and December 2025, Wildlife Acoustics Song Meter SM4 units mounted 1.5 meters above the ground at each site recorded for one minute every ten minutes, day and night. After a phase-aware resampling procedure that corrected archived files deviating from the intended schedule, the team assembled 436,775 unique one-minute recordings, of which 483 complete site-days, each containing all 144 expected recording bins, entered the final analysis. That volume of data would be unmanageable without machine learning, so the researchers turned to BirdNET-Analyzer, a deep-learning classifier trained to identify bird vocalizations, to convert raw audio into detection records.

Because automated classifiers make mistakes, the team layered a series of safeguards on top of the raw output. Detections were first filtered against the 2025 regional checklist curated by the Bird Records Committee of the Shanghai Wild Bird Society, removing geographically implausible species. Confidence thresholds were then evaluated from 0.10 to 0.70; at the lenient 0.10 cutoff, 479,343 records covering 212 species survived, while a stricter 0.30 threshold retained 127,256 records and 172 species. The researchers chose 0.30 as a practical compromise between reliability and taxonomic completeness, then excluded any species with five or fewer total detections to weed out isolated misidentifications, leaving 135 species. A random manual quality check across all seasons and eight common species achieved an overall accuracy of 88.7 percent, providing reassurance that the common post-filtered detections could be trusted.

The three monitoring sites were deliberately chosen to span Shanghai’s urbanization gradient. Zhongshan Park, embedded in the dense urban core, sits amid high nighttime light intensity and heavy human presence. Jinhai Wetland Park occupies a suburban position with intermediate context, while the ecological forest belt on Chongming Island represents a peri-urban setting with the tallest trees, the highest tree density at 2,141 stems per hectare, and the strongest vegetation signal as measured by the normalized difference vegetation index. Cumulative acoustic richness over three years proved remarkably similar across the gradient, with 131 species detected at Chongming, 128 at Jinhai, and 127 at Zhongshan Park, a finding that on its own might suggest urbanization matters little. The seasonal breakdown told a very different story.

Monthly richness followed a consistent annual cycle at all three sites, peaking in spring, dipping through summer, and recovering in autumn and winter. But the peri-urban Chongming site, despite its lush vegetation, consistently recorded fewer species than the suburban and urban sites during the cooler months. Community composition analysis using Jaccard dissimilarity and principal coordinates ordination confirmed that site and season both significantly shaped assemblages, with PERMANOVA tests attributing roughly 16 percent of compositional variation to site and 14 percent to season. Most striking were the seasonal centroid trajectories: Chongming’s community traveled the longest distance through ordination space across the year, 0.738 units, followed by Jinhai at 0.614, while the urban park’s community moved only 0.296 units, suggesting the city-center assemblage is comparatively stable while wilder sites undergo dramatic seasonal reassembly.

To connect these patterns to temperature, the team fitted generalized additive models, a flexible statistical framework capable of capturing nonlinear relationships. Daily land surface temperature came from the MODIS MOD11A1 satellite product at one-kilometer resolution, averaged across valid daytime and nighttime observations, while background meteorology came from the ERA5-Land reanalysis dataset. After screening candidate predictors for multicollinearity using generalized variance inflation factors and concurvity diagnostics, and comparing all 64 possible predictor subsets by AIC, the final quasi-Poisson model retained land surface temperature as a smooth thin-plate spline alongside wind speed and shortwave radiation, with site, season, and year as categorical effects. Fitted to 483 complete site-days, the model explained 61.02 percent of the deviance in daily acoustic species richness.

The fitted temperature response was decisively nonlinear. The partial effect of land surface temperature rose across cool conditions, peaked at 12.46 degrees Celsius, and then declined, crossing from a positive to a negative effect at 21.69 degrees Celsius. Sensitivity analyses confirmed this was no statistical artifact: models with larger basis dimensions produced transition estimates between 21.60 and 21.97 degrees, an autoregressive mixed model accounting for temporal autocorrelation yielded 21.44 degrees, and leave-one-site-out refits kept the zero-crossing within 21.00 to 22.40 degrees regardless of which site was excluded. The seasonal implications were stark. Winter surface temperatures at the urban park averaged 7.92 degrees Celsius, significantly warmer than the suburban and peri-urban sites, and 27.4 percent of winter site-days there fell within the range where the fitted thermal effect was at least 90 percent of its maximum, compared with only 9.1 percent at Jinhai and 8.3 percent at Chongming. In summer, by contrast, every site’s temperatures sat squarely in the negative portion of the curve, with means of 28.18, 29.98, and 31.94 degrees at Chongming, Jinhai, and Zhongshan respectively.

The migratory component of the avifauna drove most of the seasonal redistribution. Of the 135 retained species, 103 were classified as migratory and only 29 as residents, and migratory richness ranged from 67 to 91 species across site-season combinations, comprising up to 79.6 percent of the classified assemblage. Site-sharing analysis revealed a clear annual rhythm: 70 migratory species were detected at all three sites in spring, a figure that collapsed to 43 in summer, when ten migratory species vanished from all sites entirely, before recovering to 52 in autumn and 62 in winter. Critically, the summer contraction did not shift toward the cooler Chongming site; spatially restricted states appeared across all three locations, indicating that summer heat acted as a broad seasonal bottleneck rather than simply pushing birds toward the coolest refuge. In winter, the most frequent two-site combination was Jinhai with the urban park, a pattern the authors note could reflect either shared thermal conditions or simply similar habitat structure.

The findings carry a pointed message for urban planners betting on greening as a climate solution. The urban park’s winter warmth, combined with predictable food resources and sheltered vegetation, appears consistent with a winter thermal refuge role, echoing evidence that milder cities can lower thermoregulatory costs and improve overwinter survival. Yet the same heat-island effect that buffers winter cold becomes a liability in summer, when extreme temperatures constrain foraging, parental care, nestling growth, and even survival. Notably, even the cooler peri-urban forest failed to prevent the summer decline, suggesting that no single green space in the study provided adequate summer thermal refuge. The authors are careful to frame the 21.69-degree zero-crossing as a local, community-level statistical feature of their fitted model, not a universal physiological threshold or a validated management trigger, and they acknowledge that each urbanization context was represented by a single site, that acoustic richness may partly reflect temperature-dependent changes in calling behavior rather than true occurrence, and that site identity cannot be fully disentangled from vegetation structure and management history.

Even with those caveats, the study points toward a concrete shift in how cities might manage biodiversity under intensifying heat. Rather than uniform annual greening targets, the authors advocate season-specific habitat actions: maintaining continuous canopy shade, shallow water sources, and blue-green cooling features where summer exposure is high, while retaining sheltered vegetated cover and reliable food resources where winter refuge potential matters. They also propose a site-level thermal surveillance framework in which locally monitored surface temperatures and independently validated ecological indicators, rather than fixed seasonal calendars, guide adaptive planning as conditions approach adverse levels. In a megacity of 24.87 million people on the East Asian-Australasian Flyway, where built-up area has nearly doubled since 2000 and habitat quality indices have fallen sharply, the message is that a park’s worth to birds depends on when you ask, and that climate-resilient urban design must account for the full annual cycle of thermal life.

Subject of Research: Seasonal effects of urban land surface temperature on bird community richness and migratory redistribution across green spaces in Shanghai

Article Title: Urban thermal seasonality modulates avian community responses across contrasting green spaces

Article References: Zhang, Y., Zhang, W., Long, Y., Zheng, J., & Yin, S. (2026). Urban thermal seasonality modulates avian community responses across contrasting green spaces. Environmental and Sustainability Indicators, 32, Article 101543. https://doi.org/10.1016/j.indic.2026.101543

Image Credits: AI Generated

DOI: 10.1016/j.indic.2026.101543

Keywords: urban ecology, passive acoustic monitoring, BirdNET, land surface temperature, urban heat island, migratory birds, species richness, green infrastructure, Shanghai, generalized additive models, seasonal dynamics, climate adaptation