Why Are So Many Autumn and Winter Berries Red?
― Red Is a "Eat Me" Sign Meant Only for Birds
Rowan, pyracantha, nandina, coral ardisia. When the leaves begin to fall, red berries stand out in gardens and parks. That red is no accident. Plants are thought to have chosen birds as their seed carriers, and to decorate their fruit in the colour that catches a bird's eye best.
It is a November morning. A hedge of pyracantha is packed with red berries. A brown-eared bulbul lands and swallows them whole, one after another.
A few weeks later, most of the berries are gone. On a nearby wall and bench lie bird droppings with small seeds mixed in.
The tree cannot move. Yet its offspring, the seeds, have quietly set off on a journey far away.
There are two main reasons for red berries
Birds are very good at telling colours apart. They pick out red against green leaves with ease. Many mammals, such as dogs and raccoon dogs, are thought to have trouble telling red from green.
Birds have no teeth. They swallow berries whole, digest only the flesh, and drop the seeds elsewhere with their droppings. Seedlings do better away from the parent tree.
So a red berry is a device that combines two jobs: "call birds with a bright colour" and "get the seeds carried far without damaging them". Let's look at each in turn.
Red looks sharp and clear only to birds
At the back of the eye sit several kinds of colour-sensing cells. Humans have three kinds. Most birds are thought to have four, and can even sense ultraviolet light, which we cannot see.
Many mammals, such as dogs, cats, deer and raccoon dogs, are thought to have only two kinds. To such an eye, red and green both look like similar dull colours, so leaves and berries are hard to tell apart. See Figure 1.
Small, red or black, with no hard shell and a faint smell: berries carried by birds are thought to share these traits. By contrast, berries carried by mammals tend to be large, dull brown or green, and often attract animals by smell.
Birds are carriers that don't chew
Mammals bite fruit with their teeth, and may crush the seeds too. Birds have no teeth, and most small birds swallow berries whole. Only the flesh is digested in the gut, and the hard seeds come out intact.
While the seeds are inside, the bird keeps flying about. So the seeds land away from the parent tree. Right under the parent it is shady, and insects and diseases that favour that same tree gather there, so seedlings are thought to struggle.
So how far can seeds travel? Use the slider in Figure 2 to change how long the seeds stay inside the bird.
A maximum of over ten kilometres is only an upper limit, for a bird that never rests. Real birds stop here and there as they feed, so most seeds are thought to land within a few tens to a few hundred metres of the parent tree. Even so, that is far better than landing right underneath it.
Along a fence or under a power line on vacant land, you sometimes see young nandina or mulberry trees standing in a row. Birds often perch there. Seeds in droppings left while the birds rested are thought to have sprouted right on the spot.
Capsaicin, the hot compound in chilli peppers, is a strong irritant to mammals' tongues. Birds, however, are thought to barely feel it. Some researchers think this is a device that keeps off seed-crunching mammals and leaves the fruit to seed-swallowing birds.
Why are they common in autumn and winter?
Autumn is when winter birds such as thrushes arrive from the north. Birds that live here all year, such as bulbuls and white-eyes, also eat more berries in winter, when insects become scarce. More birds are eating fruit, at a time when food is short, and that is just when the trees ripen their berries.
It is also thought to help that berries stay on the branches after the leaves fall. Once the green leaves are gone, the red berries stand out even more. Note that some red berries are not safe for people to eat. Even if birds eat them, please don't put them in your mouth.
Summary
Red berries are common in autumn and winter because red stands out most to the birds that carry the seeds. Birds have no teeth, so they don't damage the seeds, and as they fly about they drop them far from the parent tree. Unable to move, the tree hires its carriers with colour.
A red berry is a sign to birds: "Eat me."
The payment is the flesh, and the delivery address is some distant patch of ground the tree will never see.
How flower colour attracts insects is covered in "Why Are Flowers So Colourful?", and the colours of leaves in "Why Do Autumn Leaves Turn Red and Yellow?". For fruit carried by mammals and birds that bury it and forget it, like acorns, see "Why Do Acorns Have Good Years and Bad Years?".
- In a garden or park, pick one tree with red berries (pyracantha, rowan, nandina, Japanese holly, etc.) and count the berries on one branch.
- Count the berries on the same branch once a week, and note which birds visit. See whether any week shows a sharp drop.
- Look under nearby walls and wires for bird droppings with small seeds in them, or for young trees growing in a line.
Just observe: please don't eat the berries or take them home. Some trees lose their berries early, while others keep them until late winter. Recording that difference can lead to interesting discoveries.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from lower secondary science to university courses
- Lower sec.Covered in lower secondary school science
- Upper sec.Covered in upper secondary school biology
- Upper sec.+Advanced upper secondary material, or textbook sidebars
- UniversityUniversity-level specialist courses (ecology, evolutionary ecology), not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Lower sec.Terms: this phenomenon has a name
- Seed dispersal: plants moving their seeds away from the parent. Wind, water and animals act as carriers.
- Endozoochory (dispersal by being eaten): animals eat the fruit and scatter the seeds in their droppings. Most red berries use this method.
- Cone cells: colour-sensing cells in the retina of the eye. The number of types sets the range of colours an animal can tell apart.
Lower sec.Upper sec.Checking with a formula: how far can a seed be carried?
From the time between a bird eating a berry and passing the seed, and from its flying speed, we estimate an upper limit on the distance.
| In symbols | L ＝ v × t |
| In words | Distance the seed is carried ＝ bird's flying speed × time the seed stays in the bird |
| Where it comes from | It is the relation "distance ＝ speed × time" for something moving at constant speed. It gives an upper limit, assuming the bird flies straight without resting. |
| Symbol | Meaning and unit |
| L | Distance the seed is carried (m) |
| v | Bird's flying speed (m/s) |
| t | Time from eating the berry to passing the seed (s) |
| Normal flying speed of a small bird | About 10 m per second |
| Time before the seed comes out, for a small fruit-eating bird | Roughly tens of minutes (30 minutes here) |
| 1 minute | 60 seconds |
| Convert 30 minutes to seconds | 30 × 60 ＝ 1800 s |
| Distance the seed can be carried | 10 × 1800 ＝ 18000 m |
| Convert to kilometres | 18000 ÷ 1000 ＝ 18 km |
If the bird flew without resting, a seed could travel up to about 18 km from the parent tree. In reality birds rest on perches, so most seeds are thought to land within tens to hundreds of metres. Even so, that is a distance far beyond what a berry simply falling straight down could manage.
Upper sec.Upper sec.+How colour is seen, and the give-and-take between plants and animals
Upper sec.Each type of cone in the retina responds best to a different wavelength of light. Humans have three types, and most birds are thought to have four, including one for ultraviolet. Bird cones contain coloured oil droplets, which are thought to sharpen colour discrimination further.
Upper sec.+The plant pays a "reward" in the form of flesh, and the animal carries the seeds. A relationship in which different species both benefit like this is called mutualism. It has the same form as the relationship between flowers and insects.
UniversityDispersal syndromes and the Janzen–Connell hypothesis
The tendency for fruit traits to line up by carrier is called a dispersal syndrome. Bird-dispersed fruits tend to be red or black, small and faintly scented. The reason seedlings survive better away from the parent is explained by the Janzen–Connell hypothesis: insects and pathogens specialised on that tree concentrate near it. The distribution of distances at which seeds land is described by a function called a dispersal kernel.
📖 Derivations and further reading: Seed dispersal (Wikipedia, Japanese) ／ Colour vision (Wikipedia, Japanese)
ResearchWhat is still not clear
- Are autumn leaf colours a "signpost" for fruit? One idea is that coloured leaves act as flags telling birds where fruit is. However, clear evidence has not yet been found.
- What does it mean when berries last until late winter? Some berries are eaten quickly and others stay long. Whether lingering berries are a strategy to attract birds when other food runs out, or are simply disliked, is still debated.
- Red or black: which is better? Many bird-dispersed fruits are also black. How much the colour difference matters, and to which birds, is not yet well understood.
So the contents of this article, too, are "an explanation within what is known today". Field observations and experiments on the meaning of colour are still going on.
Links to the textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Lower sec. | Science: "Kinds of plants", "Speed" | How seeds are carried; distance ＝ speed × time |
| Upper sec. | Biology: "Animal responses and behaviour (vision)" | Types of cone cells and colour vision |
| Upper sec.+ | Biology: "Ecosystems and relationships among organisms" | Dispersal by being eaten as mutualism |
| University | Ecology, evolutionary ecology | Dispersal syndromes, Janzen–Connell hypothesis, dispersal kernel |
| Research | Field studies of seed dispersal | The leaf-signpost idea; the meaning of berries that last into winter |
| ― | Links to daily life | Watching berries in gardens and parks; young trees in a line under wires |
- Seed dispersal ― Wikipedia (Japanese)
- Colour vision ― Wikipedia (Japanese)
- Capsaicin ― Wikipedia (Japanese)
- Tewksbury, J. J. & Nabhan, G. P. (2001) Directed deterrence by capsaicin in chilies. Nature 412, 403–404.
- Janzen, D. H. (1970) Herbivores and the number of tree species in tropical forests. The American Naturalist 104, 501–528.
- Stiles, E. W. (1982) Fruit flags: two hypotheses. The American Naturalist 120, 500–509.
※This article is a general-audience science explainer. The numbers given are rough guides to help you understand the mechanism. A bird's flying speed and the time a seed stays inside it vary greatly with the species and the situation. Please do not eat berries found outdoors.