First reported case of a stacked sedge wren (Cistothorus stellaris) nest in northern Illinois

Birds are masters of construction, weaving intricate homes from grass, mud, and twigs to shelter their young. For many species, finding the right spot to build is a matter of survival, as suitable locations can be scarce in a world where natural landscapes are increasingly broken up by farms and cities. When a bird finishes a nest, it is usually a one-time structure; once the season ends or the eggs hatch, the old home is abandoned. However, a rare behavior known as "nest stacking" challenges this norm. This happens when a bird builds a new nest directly on top of an older one, creating a double-decker or even multi-level structure. Scientists have observed this in a few wetland birds, often suspecting it is a response to a lack of good building sites or a way to save energy. Understanding why and how this happens offers a glimpse into how wildlife adapts to difficult conditions, particularly when their preferred habitats are shrinking or degraded.

In the summer of 2025, researchers working in northeastern Illinois documented a first-of-its-kind event that sheds new light on this unusual behavior. At the Goose Lake Prairie State Natural Area, a team led by Bethany J. Darby and Anastasia A. Rahlin discovered a sedge wren, a small songbird that typically lives in wet meadows, living in a double-decker nest. This was the first time such a structure had ever been recorded for this species. The bird in question was a male in his second year of life, breeding for the first time. He had built a complete, lined nest, and instead of moving to a new spot, he constructed a second, identical nest directly on top of the first one. When the researchers found the structure on June 2, the lower nest was empty, but the upper nest held a clutch of five eggs.

The story of this nest unfolded over several weeks of careful observation. The researchers had been monitoring sedge wrens in the area, tracking their movements and counting nests. They had seen this specific male building a nest in mid-May, surrounded by invasive reed canary grass. By late May, the male had finished a nest, and a female was nearby, but the team could not find that specific nest again a few days later. Then, on June 2, they found the surprise: the male had built a second nest, Nest C, sitting right on top of the original one, Nest A. The lower nest stood 16 centimeters above the ground and was 13 centimeters tall. The new upper nest was 14 centimeters tall, bringing the total height of the structure to 43 centimeters. The upper nest was carefully lined and contained five eggs, while the male and female were seen nearby, tending to the site.

The fate of this unique nest was not a happy one. By June 10, heavy rain had caused the upper nest to sag, though the female was seen carrying fresh lining material, likely trying to repair the damage after the storm. The male was not seen during this check. A week later, on June 17, the researchers returned to find the nest empty and the eggs gone. The nest itself remained intact, with the entrance hole looking exactly as it had before. This specific detail suggested that the eggs had not been taken by a bird or a mammal that would have torn the structure apart. Instead, the evidence pointed to a snake, which could slip into the nest through the opening without damaging the walls. The researchers noted that snakes are known predators of sedge wren eggs in this area.

The discovery raises questions about why a bird would choose to build on top of an old home rather than starting fresh. The researchers suggest that the answer lies in the quality of the territory. The area where this double-decker nest was found was small and fragmented, bordered on all sides by mowed footpaths. Unlike the large, continuous wet meadows these birds prefer, this patch was dominated by invasive plants, which can make a habitat less suitable for nesting. The male who built the nest was young and breeding for the first time, meaning he might have been less competitive than older, more experienced birds. In a crowded or degraded environment, a young bird might struggle to find a prime spot. Building on an existing structure may have been the only viable option to stay in the territory without wasting the energy required to find a new site and build from scratch.

While the stacked nest did not result in a successful hatch, the observation provides valuable data for scientists studying how birds cope with changing environments. The sedge wren is a species that relies on wet meadows, a habitat that has disappeared by more than 90 percent in Illinois over the last century due to development and agriculture. As these natural spaces shrink and become fragmented, birds face increasing pressure to adapt. This single instance of nest stacking suggests that habitat limitations can drive birds to try unusual strategies. The researchers emphasize that this is just one example, and more observations are needed to understand if this behavior is a common response to poor habitat or a rare anomaly. For now, the double-decker nest at Goose Lake stands as a quiet testament to the challenges wildlife faces in a world where their homes are becoming harder to find.