External structures at the nest‐site predict asymmetric architecture of nests in a mud‐building bird

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 structures in the nest‐site influence nest‐building behaviour, specifically focusing on their effect on nest architecture. We hypothesized that pre‐existing structures at the nest‐site of Rufous Horneros Furnarius rufus could influence nest construction, thereby affecting nest 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 12 504 nest photographs taken by citizen scientists across the species' entire breeding range. We found that in nests built in contact with a lateral structure, birds were more likely than expected by chance to place the nest entrance on the same side as the lateral structure. When lateral structures were incorporated into the nest (i.e. when the structures replace parts of 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 behaviour and nest‐site properties. These results underscore the importance of understanding the behavioural mechanisms shaping avian nest architecture and their plasticity.