Examples of functional plasticity in one of the birds studied, the rose-ringed parakeet (Psittacula krameri). A. Predation of Prunus dulcis seeds. B. Pulping of Citrus × sinensis fruit. C. Dispersal of Morus alba seeds via endozoochory and epizoochory. D. Dispersal of Phoenix dactylifera seeds via stomatocory. Photos: Dailos Hernández-Brito

A new study led by the Doñana Biological Station - CSIC and the Pablo de Olavide University reveals that the interactions between birds and the plants they feed on are much more complex and variable than previously thought. The research, published in Communications Biology (Nature Portfolio), shows that birds do not play a single ecological role in nature, but that this can oscillate along a continuum between antagonism (seed predation) and mutualism (seed dispersal). This gives rise to dispersal mechanisms that cannot be explained solely by traditional approaches focusing on the physical characteristics of fruits and seeds.

To better understand these processes, the researchers conducted a comprehensive study of the feeding behaviour of 25 bird species in human-modified environments in Seville, Spain. “Based on our previous experience in forest ecosystems, such as tropical rainforests, we knew that observing these interactions in detail would be much more difficult there. Therefore, we chose urban and rural environments, where systematic and detailed monitoring of individuals during feeding periods is more accessible,” explains Fernando Hiraldo, co-author of the study and researcher at the Doñana Biological Station.

After a full year of observations, the team recorded more than 21,900 bird–fruit interactions. One of the most striking findings was that 96% of the bird species displayed multiple functional roles, alternating between seed predation, pulp consumption, and seed dispersal.

“These processes depend on multiple factors acting together, such as fruit size relative to bird size, fruit type, ripeness, and feeding strategy,” says Dailos Hernández-Brito, lead author of the study. “For example, seeds that are normally dispersed after ingestion can also be transported by birds traditionally categorized as seed predators, especially when fruits are large, lack pulp, or are still unripe,” he adds.

Another important finding is that 50% of the plant species exhibited multiple seed dispersal pathways, in which different mechanisms coexisted, including endozoochory (seed ingestion), stomatochory (transport of propagules to perches), and epizoochory (seed attachment to feathers after feeding). “This multi-phase dispersal process, known as polychory, has received little attention until now,” emphasizes Hernández-Brito. “As a result, the role of certain animals has been underestimated, and dispersal mechanisms in plants without fleshy fruits have often been overlooked,” adds Hiraldo.

The study also highlights variability at the individual level. Researchers found that both the antagonism–mutualism continuum and polychory are not exceptional phenomena but rather frequent processes. “At the plant level, these patterns appeared in approximately 50% and 38% of the individuals observed, respectively,” explains Martina Carrete, co-author of the study and professor at Pablo de Olavide University. “They were more common when birds fed for longer periods on small, fleshy fruits and when plants attracted a greater diversity of consumers, underscoring the importance of context in these ecological processes,” she adds.

This study provides strong evidence that plant–bird interactions are highly dynamic and strongly dependent on context. These findings challenge traditional classifications that assign fixed roles to species and question approaches based solely on morphological traits to define seed dispersal modes.

Publication Details: The study was conducted by Dailos Hernández-Brito, Fernando Hiraldo, Jaume Izquierdo-Palma, José L. Tella, and Martina Carrete.

Participating Institutions: Doñana Biological Station (CSIC), Seville, Spain; Department of Physical, Chemical and Natural Systems, Pablo de Olavide University, Seville, Spain.

Corresponding Author: Dailos Hernández-Brito (dailoshb@gmail.com)

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Instituciones Participantes: Estación Biológica de Doñana (CSIC), Sevilla, España. Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Sevilla, España

Contacto (Para Correspondencia): Dailos Hernández-Brito (dailoshb@gmail.com)