LET’S TALK ABOUT HUMMINGBIRDS, SEX & FLOWERS

This blog post is provided by María A. Maglianesi and tells the #StoryBehindThePaper for the paper “Behavioural and morphological traits influence sex-specific floral resource use by hummingbirds“, which was recently published in the Journal of Animal Ecology. In their study, they investigate how the differences in behaviour and physical form of male and female hummingbirds, even of the same species, changes their relationship with the plants that they pollinate.

When we think about hummingbirds, the first image that comes to our mind probably is: tiny birds thirsty for nectar that hover untiringly around flowers, flaunting their impressive flight abilities and beautiful plumage in exuberant neotropical forests (let’s look at this video).

Less likely is that we think about how different males and females of the same species could be and why it matters for plants that benefit from these pollinators. In hummingbirds, as in other birds, colourful feathers are more typical in males, which often are the ones that must make an effort to seduce females. For this, nothing better than dazzling the ladies with glittering colours, especially because the first impression is what counts. Interestingly, just as hummingbirds vary in their plumage between the sexes they can also differ in their behaviour and morphology.

The White-eared Hummingbird skilfully manoeuvring to extract nectar from pendulous flowers. (Photo credit: Sergio Rivero Benítez)

Differences between males and females in behaviour (e.g. territoriality) and morphological traits (e.g. bill length and curvature), might lead each sex to exhibit considerably different patterns of floral resource use. For instance, they may differ in the amount of plant species visited with one sex using more diverse plants than the other (broad niche breadth), or in the plant species each uses, with few plant species shared by both sexes (reduced niche overlap). Now, entering to a more formal ground, when hummingbirds do not overlap on their food resources, we refer to this as niche segregation, that is, males and females limit their visits to specific plant species rather than using the same species. In this way, both sexes win because they do not have to compete for nectar at the same flowers. This is a clever strategy that animals perform to cope with scarcity of food resources in their habitats, which can make the difference between life and death. Once it is clear that resource partitioning is a key mechanism in ecological communities, we can go to the next level and dare to say that the more different the sexes of the same hummingbird species are in terms of behaviour and morphology, the more different they should be in their feeding ecology.

In a recent paper published in the Journal of Animal Ecology, we assessed the variation in patterns of floral resource use by males and females of 31 hummingbird species distributed widely across the Americas. For this, we compiled a dataset of plant-hummingbird interactions based on the pollen grains that individuals of each sex carry attached to their bill and plumage. This technique entails quite a bit work, but it is certainly worth it, as we can pinpoint the hummingbird’s diet at the individual level. Once we get the pollen grains, these have to be identified under a light microscope in the lab by comparison with reference collections taken from plants at the study sites.

Collection of pollen grains from the bill of the Purple-throated Mountain-gem using fuchsine-stained gelatine in the Caribbean slopes of Costa Rica and subsequent analysis. (Photo credit: María A. Maglianesi)

In our study, we found striking sex-related differences in patterns of food resource use, where females visited a more diverse set of plant species than males and also showed a reduced niche overlap with them, suggesting a high level of resource partitioning between sexes. Even more remarkable is that hummingbird behaviour and morphological traits were related to the specific plant species visited by each sex. In a nutshell: territorial species and those where males and females are more similar in bill curvature showed a higher niche overlap between sexes.

Some hummingbird species have a territorial behaviour where males are often the ones that defend patches with plenty of flowers, which is a good option to avoid spending energy while searching for food scattered everywhere. In these species, males are usually dominant individuals not allowing others to break into their territories, even females. Thus, females usually have no choice but to visit scattered flowers, which probably makes them end up foraging over larger areas than territorial males. This may explain why females of some hummingbird species have a wider range of dishes compared to males.

In territorial hummingbird species, like the White-bellied Mountain-gem, males defend patches with abundant flowers and they do not hesitate to aggressively chase away any intruder trying to boycott their food security. (Photo credit: Alexander Montero)

Notably, in our research, males and females of the same species overlapped overall just 30% in the plant species they visited and females were even more dissimilar in the plants visited with respect to males than to other females within species. These findings speak for themselves: there is a high level of resource partitioning by sex across hummingbird species. The unavoidable question is: why and how this is related to the other results in our study?

As previous research shows for some specific hummingbird species, differences in resource use between the sexes has been associated with sexual differences in their morphology. This is what we found looking at the bill shape in a set of hummingbird species. Let’s break this down a bit: species where females have a more curved bill overlap less with males in the floral resources that they use than those species where the sexes are more similar in their bill shape. If we were a female hummingbird with a curved bill, why not get nectar from flowers with a corresponding curvature? This may represent a double advantage: we could extract nectar more easily because of a perfect match in bill-flower shape and we would have more resources available because these are not used by our male partners that will prefer straighter flowers. That is, we finally got the point: differences in morphology between the sexes relate to differences in the plants used by them, allowing for a harmonious way of living together. The last finding of our work that is important to highlight is that territorial hummingbirds showed higher resource overlap between sexes than nonterritorial ones. This may reflect that nonterritorial hummingbirds of each sex meet only infrequently while feeding, and thus there is less chance they share the same plant species.

The take home message of our study is that we need to examine plant-pollinator systems by using a higher magnification lens, that is, looking within species and not only among them. This is especially important for hummingbirds because of the noticeable differences in behaviour and morphology between the sexes of the same species, which may explain the variation in their food resources. This variation may be a clever strategy for a healthy coexistence with important implications for plant reproduction and ecological communities.

Author biography

Dr. María A. Maglianesi is a Research Scientist at the Distance State University and Professor at the Technological Institute of Costa Rica. She has a background in community ecology and a strong interest in understanding the key processes that determine species interactions from a network and functional approach. Most of her work focuses on mutualistic plant-pollinator interactions using hummingbirds as study system to analyse the drivers of food resource specialization and trait-matching in tropical communities.

Read the paper

Read the full paper here: Maglianesi, M. A., Maruyama, P. K., Temeles, E. J., Schleuning, M., Zanata, T. B., Sazima, M., Gutiérrez-Zamora, A., Marín-Gómez, O. H., Rosero-Lasprilla, L., Ramírez-Burbano, M. B., Ruffini, A. E., Salamanca-Reyes, J. R., Sazima, I., Nuñez-Rosas, L. E., del Coro Arizmendi, M., Rahbek, C., & Dalsgaard, B. (2022). Behavioural and morphological traits influence sex-specific floral resource use by hummingbirds. Journal of Animal Ecology, 00, 1– 10. https://doi.org/10.1111/1365-2656.13746

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