Why Do Birds Build Nest Walls?
Birds build diverse types of nests—from simple exposed nests to complex structures with protective walls. However, scientists have long puzzled over why some species build nests with walls while others don’t. Previous studies suggested that nest walls help with temperature control or protection from predators, but these explanations do not match the observed difference across species worldwide. In our new study, we reveal a surprisingly simple answer which has long been overlooked—nest walls serve as guardrails to prevent eggs from rolling out.
We propose and test two novel hypotheses: (1) heavy-bird hypothesis predicts that heavier parent birds need walls because they are more likely to shake the nests when they land; (2) light-egg hypothesis predicts that birds with lighter eggs need walls because these eggs roll off more easily. To find out which explanation is correct, we combined phylogenetic comparative analysis of over 4,000 bird species, behavioral experiments with a songbird species and controlled trials with 3D-printed eggs of varying weighs. Our results revealed that it’s all about egg weight, not parent weight, supporting the light-egg hypothesis. Our phylogenetic analysis showed that bird species laying lighter eggs tend to build nests with walls. Behavioral experiments showed that the eggs of lightweight individuals are more likely to roll out of nests without walls. Most critically, our controlled experiments using 3D-printed eggs and artificial nests demonstrated that egg weight, rather than parent body weight, determines egg rolling-off risk and nest walls can significantly reduce the risk.
This study provides the first comprehensive evidence linking egg mass to the evolution of nest architecture. Our findings demonstrate how biological trait evolution can emerge from a simple physical mechanism, resolving one compelling puzzle in bird behavior. When eggs are light enough to be easily displaced, natural selection favors birds that build better guardrails.
We published the results in Proceedings of the Royal Society B. Our collaborative team includes Assistant Professor Chun-Chia Chou from the Department of Life Science, National Taiwan Normal University, and Associate Research Fellows Mao-Ning Tuanmu and Chih-Ming Hung from the Biodiversity Research Center, Academia Sinica.
Article link: https://doi.org/10.1098/rspb.2025.1445
Figure 1. Research approaches. (a) Phylogenetic comparative analyses for the association between bird/egg mass and nest wall structure. (b) Nesting behavior experiments. (c) Incubation experiments manipulating the walls of artificial nests and the weight of artificial. Sketches by Hsiang-Ching Chen.
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