Urban Southern Housewren (Troglodytes aedon musculus) nesting in apparently unsuitable human-made structures: Is it worth it?

Free-living birds in cities interact with humans and human-made objects. Here, we investigated whether nesting in human-made structures that are physically unstable and prone to frequent human intervention benefits urban Southern House Wrens (Troglodytes aedon musculus). First, we describe the behavior of individuals that repeatedly attempted to nest in a motorcycle helmet (an unsuitable structure) based on ad libitum observations and camera trapping. We also reviewed nesting records of this wren throughout Brazilian cities deposited in crowdsourced citizen science platforms, such as Wiki Aves, eBird, and iNaturalist. During our field study in November and December 2019, wrens attempted to build a nest in the helmet for eight days. Each attempt was interrupted by the removal of the helmet. We recorded 103 videos of nesting activity, including three days of high nest-building effort (up to 68 twigs deposited inside the helmet within a 6-hour period) and high territory-defense efforts. Both behaviors were sometimes followed by one of four types of vocalizations (contact call, complete song, incomplete song, or sub-song). We found 372 Southern House Wren nesting records in online citizen science datasets: 100 were in urban areas with 86 nests built on 24 different human-made structures. Most nests (N = 71) were in what we deemed as stable structures (safe from human intervention) and 34 of them (47.8%) likely bred successfully (i.e., fledglings present). Only seven nests were built on unstable and unsafe structures, and four (57.1%) of these had sufficient evidence of successful nesting. Although nesting in unsuitable places in cities is less frequent, their breeding success is comparable with nesting in suitable places. Therefore, the nesting of the Southern House Wren in human-made structures might be of benefit, even if they eventually become ecological traps due the risk of human intervention. Our study adds knowledge about the life history of the Southern House Wren in urban environments.