1. Why does the sword-billed hummingbird have a bill longer than its body?
The sword-billed hummingbird’s bill is approximately 8 to 12 cm (3 to 4.7 inches) long. Because the body is only about 13 to 14 cm (5 to 5.5 inches), this bird is considered the only one with a bill longer than its body when the tail is excluded [1]. The exact length varies depending on the source.
In the Andes mountains of South America, where this bird lives, there are passionflower relatives with flowers shaped like long, narrow tubes. Some of these tubes, where nectar collects, are 6 to 14 cm (2.4 to 5.5 inches) long, while the sword-billed hummingbird’s bill reaches a maximum of about 11 cm (4.3 inches) [2]. A 2014 study suggests that these flowers rely on the sword-billed hummingbird to transport pollen because of the position of their reproductive parts and a tendency not to set fruit with their own pollen [2]. However, not all passionflowers are exclusive to this bird; some relatives in the same group are pollinated by bats instead of birds.
2. What happens to long-tubed flowers like passionflowers when these birds are scarce?
One species of passionflower, *Passiflora mixta*, has flower tubes that are 7 to 15 cm (2.8 to 5.9 inches) long, with a very narrow section for holding nectar. Field observations have reported that in areas where sword-billed hummingbirds are few, it becomes difficult for these flowers to produce seeds [3]. If the birds do not come, the flowers struggle to reproduce.
When did this connection begin? Molecular data estimates suggest that the sword-billed hummingbird’s lineage split about 11.6 million years ago, while this group of flowers appeared about 10.7 million years ago [4]. These estimates are close, though they come with a range of uncertainty. On the other hand, this dependence is not permanent. The same research indicates that some species later escaped this extreme specialization, switching to pollination by bats or birds with shorter bills [4].
3. Do male and female purple-throated caribs feed from different flowers?
The purple-throated carib (*Eulampis jugularis*) lives on the Caribbean island of St. Lucia. The female’s bill is, on average, about 20% longer and about 40% more curved than the male’s [5]. Although males are about 25% heavier, the difference is not simply about size. Males primarily use *Heliconia caribaea*, a flower that is short and slightly curved, while females primarily use *Heliconia bihai*, which is long and strongly curved [5].
The flowers also show variation. In places where *Heliconia caribaea* is scarce, *Heliconia bihai* flowers have been observed to split into two types: a red-green type matching the male’s bill and a green type matching the female’s [5]. This is considered one piece of evidence for coevolution between birds and plants, though it was observed in specific locations and may not happen everywhere.
4. Do hummingbird bill lengths match flower lengths across the Americas?
The sword-billed hummingbird and the purple-throated carib are prominent examples. But does this trend hold up when we look at a wider range? A 2021 study examined 93 networks of birds and plants across North and South America, involving 181 hummingbird species and 1,256 plant species [6]. The study found that bill length and flower tube length changed together in all regions and for most lineages [6].
Groups with many long-billed lineages visited longer flowers and had more restricted diets [7]. In Caribbean islands, flowers were found to be shorter, and birds had broader diets [6]. However, the paper notes that bill shape does not always predict how specialized a bird’s diet is. Matching shapes and food selection are not the same thing.
5. Is a hummingbird's tongue a straw or a trap for nectar?
For about 180 years, it was thought that hummingbird tongues sucked up nectar like a straw through capillary action. A 2011 study showed that the tongue works like a "trap" to collect nectar [8]. The tongue splits into two tips with fine hairs called lamellae. When these touch nectar, the hairs spread out. When the tongue is pulled back, surface tension causes the hairs to curl and close, trapping the nectar inside [8].
This research used high-speed cameras filming at 500 frames per second to observe 30 live birds from 10 species. The same phenomenon was seen in 20 dead birds, showing that the tongue’s structure closes without the bird using energy [9]. However, later studies have proposed other theories, such as an elastic micro-pump mechanism, so it is too early to say that one single mechanism explains everything.
6. How can hummingbirds see colors that humans cannot?
Hummingbird eyes have four types of cone cells for distinguishing color, while humans have three. Because of this, it is said that hummingbirds can see ultraviolet light, which is invisible to humans [10]. A 2020 study in Colorado reported that wild broad-tailed hummingbirds chose feeding sites based on "non-spectral colors" not found in rainbows [10]. These colors are mixtures, such as ultraviolet with green or ultraviolet with red.
Researchers estimated that about 30 to 35% of flower and feather colors appear as non-spectral colors to birds. This figure combines both flowers and feathers, rather than just flowers [10].
7. How can you investigate hummingbirds and flowers yourself?
You can try observing nature to see these connections. When walking, look for flowers with tube shapes. Do not pick the flowers; instead, place a ruler nearby to estimate the length and observe which insects visit for a while. Be careful of bees and do not enter private gardens.
You can also look in encyclopedias or museum materials for examples where bill length and flower tube length match, and compare them to the examples in this article. Finally, try summarizing the difference between the "capillary action theory" and the "trap" theory of the hummingbird tongue in your own words [8].