Birds that feed on fruits play a key role in nature, participating in seed dispersal and forest regeneration. Therefore, it is important to know which birds can disperse which fruits, even more so in a present where human activity profoundly transforms terrestrial ecosystems. Traditionally, scientists have believed that the size of birds' beaks limits which fruits they can consume, according to the hypothesis of size correspondence. This postulates that, if a fruit is larger than the bird's beak, it is almost impossible for the bird to eat it. However, a recent study carried out by the Institute of Regional Ecology (with the participation of Tobias Rojas, Pedro Blendinger and Giselle Mangini) challenges this simplified view.
The researchers analysed more than 800 interactions between birds and fruits from different regions of the world, mainly from the cloud forests of the Yungas and the Atlantic Forest in South America. They classified the birds into two groups based on their feeding strategy, swallowers (birds that ingest the fruit whole) and mashers (birds that process the fruit with their beaks before eating it). They found that the relative size between the bird's beak and the fruit had a smaller impact than expected on fruit consumption. The swallowers showed a slight preference for fruits smaller than their beaks, but even consumed some larger ones. In contrast, the mashers were not affected by the size of the fruit: they can process fruits even if they are larger than their beaks. This indicates that the ability to manipulate fruits is more important than simple beak size. For example, a soft fruit or one with few seeds can be easily consumed by a bird with a smaller beak if it has the ability to mash it.
The results suggest that size correspondence is not a decisive criterion for understanding bird-plant interactions. Factors such as fruit texture, its chemical composition and the birds' feeding strategies also play a key role. This study highlights the complexity of bird-plant relationships and the need to consider multiple factors to better understand ecosystems and conserve their functioning. In practical terms, this study highlights the importance of preserving both swallowing and masher frugivores to improve conservation and recovery strategies for forested environments, since both groups of birds contribute to seed dispersal, albeit in different ways.
