External structures at the nest site predict nest's asymmetric architecture in mud-building birds

In the animal kingdom, nests are essential structures and textbook examples of extended phenotypes. However, the relationship between builders, nest traits, and the nest site remains poorly understood. We indirectly examine whether external components in the nest site influence nest building behavior, specifically focusing on their effect on nest architecture. We hypothesized that pre-existing structures at the nest site of horneros (Aves: Furnarius rufus) could influence nest construction, thereby affecting the nests architecture. To test this, we investigated the relationship between nest asymmetry--defined by the side on which the nest opening is located--and external lateral structures using a database of 12,356 nest photographs taken by citizen scientists across the birds entire distribution. We found that in nests built in contact with a lateral structure, birds were more likely to place the nest entrance on the same side as the lateral structure than by chance. When lateral structures were incorporated into the nest, this likelihood increased significantly. Our findings suggest that incorporating pre-existing elements into nests leads to predictable asymmetric architecture, highlighting a strong interaction between birds building behavior and nest site properties. These results underscore the importance of understanding the behavioral mechanisms shaping avian architecture and their plasticity.