Why many birds add fresh plants to their nests remains unclear despite signs of healthier offspring
As they build their nests, many birds are known to interweave dried grasses and other materials with green plants, such as fresh leaves, grass stems, herbs and aromatic leaves. While this behavior is well documented, why many bird species choose to add fresh plants has remained unclear.
Many birds enhance their nests with fresh plants like leaves and herbs, though the reasons behind this behavior remain uncertain. Previous research has suggested this could serve as a courtship signal, protect against parasites, or contribute to the birds' overall health. Recently, researchers at Bielefeld University in Germany analyzed previous studies to better understand the possible benefits of adding fresh plants to nests.
They published their findings in Proceedings of the Royal Society B: Biological Sciences, stating that green nest material may boost birds' fitness, although more research is needed to fully understand its advantages. The investigators were intrigued by this behavior, as many studies have shown that nests serve purposes beyond simply providing a place for eggs and young to develop.
By synthesizing data from various studies, the team aimed to determine whether green nest material indeed provides measurable benefits to birds and which proposed mechanisms best account for these benefits. The researchers identified 28 studies involving seven bird species and 274 effect sizes. They conducted a systematic review and meta-analysis to examine the effects of adding or removing green material from nests on various outcomes, including parasites, nestling size, physiology, reproductive success, and behavior.
The results indicated that green nest material can improve fitness-related outcomes, particularly morphological measures of nestling condition such as body mass, wing length, and tarsus length. However, the study did not find strong evidence supporting any of the three major hypotheses—courtship, nest-protection, or drug—serving as the primary reason for birds adding fresh plants to their nests.
Additionally, the researchers found that study results were heavily influenced by experimental design, suggesting that the physical properties of the green material itself, rather than its volatile compounds, might contribute to the observed benefits for birds.
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