Floral nectar plays a crucial role in pollination. While most nectar is colorless, a few plants produce colored nectar to attract pollinators. It has been suggested that colored nectar may serve as an honest signal, helping pollinators associate nectar color with the reward of food, thus enhancing pollination efficiency.

Many flowers that produce colored nectar are primarily pollinated by birds. However, creating pigments can incur additional reproductive costs. Furthermore, animals can easily identify flowers that have already been depleted of nectar and may avoid them, potentially reducing the frequency of visits. This raises an important question: how can plants make their nectar more visually conspicuous without indicating if it has already been consumed, thereby attracting more visits?

To explore this issue, a research team from the Kunming Institute of Botany (KIB) of the Chinese Academy of Sciences conducted studies in South China, with their findings published in the journal Ecology.

The researchers gathered field observations in Wutongshan, Shenzhen, recording visits by birds, bees, butterflies, and bumblebees to the Chinese New Year flower, Enkianthus quinqueflorus, which is common in South China and blooms with pink flowers around the Lunar New Year. At the base of these translucent bell-shaped corollas are five shiny, dark red nectar pouches. Although the nectar itself is colorless, the dark pouches create a strong visual illusion of colored nectar.

In this study, birds accounted for 65% of total visits, visiting flowers more frequently than bees, which made up 34%. Cage experiments revealed that preventing bird visits significantly reduced the fruit set, confirming that birds are the primary and most effective pollinators. The flowers produce ample nectar with a high hexose sugar content, indicating a generalized ornithophilous pollination syndrome.

Reflectance spectrum analysis showed a strong color contrast between the pouches and the nectar from a bird's perspective. To experimentally create the illusion of colored nectar, the researchers used two treatments on cut rose petals: one group received a spot of black ink (true colored nectar mimic), while another was marked with a black dot using a waterproof pen and topped with a drop of clear water (visual mimic). Human participants (n = 20), viewing from a vertical angle that simulated a bird’s view into a flower, judged the two versions to be highly similar. This confirmed that dark backgrounds combined with clear liquids can effectively mimic colored nectar.

Do similar traits appear in other taxa that are also bird-pollinated? The researchers reanalyzed pollination data from 37 published Rhododendron species using generalized linear models.

The team found that bird visitation was positively associated with the presence of dark nectar pouches. In contrast, bees tended to visit species without dark pouches, while bumblebee visitation was not affected.

Phylogenetic evidence suggests that dark nectar pouches evolved independently multiple times within the genus. Notably, colored nectar and dark pouches do not co-occur, and there are currently no reports of colored nectar in the Ericaceae family.

Previous studies indicated that bird-pollinated flowers often lack nectar guides due to the visual acuity of birds. However, dark nectar pouches may serve a similar purpose. Specialist bird-pollinated flowers often have long corolla tubes and lack nectar guides, while generalized bird-pollinated flowers feature open corollas with high color contrast—potentially aiding recognition and visitation by less specialized pollinators.

Dark nectar pouches and "fake colored nectar" may not function as honest signals of reward. The researchers observed birds probing nectarless flowers, indicating that they can be deceived by the mimicry. Producing colored nectar requires simultaneous pigment and nectar production, which is metabolically costly. In contrast, dark nectar pouches may provide a more stable and less resource-intensive visual cue—a functional alternative to colored nectar.

This research's fieldwork received support from the Shatoujiao Forest Farm in Shenzhen. A photo feature of the study was also published in The Bulletin of the Ecological Society of America.

Dark nectar pouches paired with colorless nectar create an illusion of colored nectar (Image by KIB)

A: Ink (true group) B: Water + dark background (mimic group) C: Water (control group)  

(Image by KIB)

Left: Rhododendron cerasinum with dark nectar pouches Right: Melianthus dregeanus with true colored nectar (Image by KIB)