Birds don’t bring fresh plants to their nests for no reason — scientists have discovered a strange effect

Scientists have now pooled the results of 28 experimental studies, covering seven bird species and 274 individual effect estimates. It turns out that fresh greenery can indeed be beneficial, particularly when it comes to the developmental outcomes of chicks. But the most interesting thing is that the researchers were unable to confirm any of the popular explanations for this behaviour.

Why do birds bring greenery to their nests at all?

Unlike twigs, mud or dry grass, which are needed to build the nest itself, fresh leaves and shoots often serve no obvious structural purpose.

That said, some birds do not choose plants at random. Nests contain fresh grasses, leaves and twigs, including fragrant species containing volatile chemical compounds.

This has given rise to several hypotheses. According to one, the plants serve as a kind of courtship signal: the bird demonstrates the quality of the nest or its own abilities to a potential mate. According to another theory, the plant material protects the nest from parasites and pathogenic microorganisms. Finally, it was suggested that certain plants are capable of directly improving the condition of the chicks.

Until now, individual experiments have yielded conflicting results. Researchers at the University of Bielefeld therefore decided to review the entire body of experimental literature at once.

The scientists combined the results of 28 experiments

Shreya Dimri’s team carried out a systematic review and meta-analysis — statistically combining the results of independent studies to identify an overall effect that might not be apparent in individual studies.

The analysis included 28 experimental studies, of which 26 had been published and two remained unpublished. In total, they covered seven species of birds and yielded 274 effect estimates.

This is significantly more robust than conventional observations. In the experiments examined, the researchers deliberately added or removed green material from nests and then compared the results. This design allows for a much more reliable assessment of the influence of the plant material itself than a simple correlation between the amount of greenery and the birds’ condition.

The researchers took into account the chicks’ weight and size, physiological indicators, reproductive success, behaviour, and the number of parasites and pathogens.

The most noticeable effect was observed in chicks

Overall, the meta-analysis showed that fresh plant material can have a positive effect on indicators related to birds’ fitness. In evolutionary biology, ‘fitness’ is understood not simply as health, but as characteristics that are potentially linked to survival and successful reproduction.

The scientists obtained the most pronounced positive results for the chicks’ morphological indicators — for example, body mass and the dimensions of various body parts. It was this group of indicators that proved to be most sensitive to the presence of fresh greenery.

However, it cannot be concluded from this that chicks in such nests are necessarily ‘healthier’ in general or have a higher chance of surviving to adulthood. The meta-analysis shows an advantage in terms of specific measurable characteristics, but does not prove a universal increase in survival rates.

The ‘natural first-aid kit’ theory has not been confirmed

At first glance, the chemical properties of plants seem to offer the most logical explanation.

It is known that some birds prefer fragrant species that release volatile compounds. Researchers therefore hypothesised that such vegetation might act as a natural repellent or a defence against microorganisms and parasites. Previously, a link had indeed been found in certain species between the vegetation in the nest and the number of parasites.

However, a new meta-analysis has failed to find convincing overall support for this theory.

The researchers did not find reliable evidence that fresh foliage systematically reduces the number of parasites or pathogens. Furthermore, experiments comparing fragrant plants with non-fragrant ones did not reveal a clear advantage for the former.

This casts doubt on the appealing idea that birds primarily line their nests with these distinctive plant-based remedies.

There is an effect, but its cause remains a mystery

No other main hypothesis emerged as having a decisive advantage either.

The authors did not find sufficiently convincing data to identify either mate attraction, nest protection from parasites, or the supposed ‘medicinal’ effect of plants on offspring as the main explanation.

However, it turned out that the results were heavily influenced by the design of the experiment itself — for example, exactly which nests and types of plant material the scientists compared. It was the study design that accounted for a significant proportion of the differences between the observed effects.

Therefore, one possibility remains: the issue may lie not so much in the chemical composition of the plants as in their physical properties.

Fresh leaves and grasses are capable of altering the humidity and temperature inside the nest, as well as its structure, cleanliness or the conditions in which the chicks develop. However, these mechanisms have so far been far less thoroughly investigated and remain mere hypotheses.

Why has it proved so difficult to unravel this behaviour?

One of the main problems is the limited scientific evidence.

Although scientists have compiled 28 studies, these covered just seven species. A significant proportion of the data was obtained from a few well-studied birds — in particular, blue tits, common starlings and white-throated starlings. It is therefore unclear whether the observed effect would work in the same way for the hundreds of other species that also use green material.

Furthermore, the results of different experiments varied considerably. This means that one cannot speak of a simple, universal rule such as ‘fresh grass improves the condition of chicks’.

It is quite possible that, in different birds, the same behaviour—at first glance—arose for different reasons: in one species, the plants help regulate the microclimate; in another, they form part of a courtship display; and in a third, they do indeed have a chemical effect.

Scientists will now have to break the behaviour down into its constituent parts

To uncover the mechanism, the authors propose conducting experiments in which the various properties of the vegetation are tested independently of one another.

For example, one could vary the presence of the freshest plant material and the presence of aromatic compounds separately, whilst simultaneously measuring the temperature and humidity of the nest, the number of parasites and pathogens, the chemical composition of the plants, and the development of the chicks.

This will shed more light on whether birds really do utilise plant chemistry, or whether the secret lies in the far more mundane physical properties of fresh leaves.

For now, the findings appear unusual: the repeatedly observed behaviour, on which the birds expend time and energy, is indeed linked to measurable benefits for their offspring — but the mechanism behind these benefits remains unknown.

Source

Study: “Why do birds use green nest material? A systematic review and meta-analysis of experiments”.

Authors: Shreya Dimri, Tuba Rizvi, Julio M. G. Segovia, Meinolf Ottensmann, Alfredo Sánchez-Tójar.

Journal: Proceedings of the Royal Society B: Biological Sciences, Volume 293, 2026. The University of Bielefeld also confirms the publication of the paper in Proceedings B.