Off-season marketing: how do birds wearing “down jackets” survive the summer?

With the arrival of hot summer, the heavy winter clothes in the wardrobe have been replaced by light summer clothes. When packing winter clothes, have you ever wondered why down jackets can be as light as summer clothes, but still so warm?

1. Why is down so warm?

Down is different from feathers. Down (used in clothing production) grows on the body surface of ducklings and goslings and at the base of the main feathers of ducks and geese. It is fluffy, with soft barbs and slender barbules. Feathers grow on the body surface of ducks and geese. Although they are very light, they have a certain toughness and are not as soft as down.

The basic unit of down is a vascular cell composed of a single or multiple vascular bundles. On the surface of these vascular bundles, there is a layer of cell membrane, which is mainly composed of two hydrophobic macromolecules. The vascular cells formed in this way are curled, so there are a large number of pores in the down, which can store a certain amount of air. At the same time, there is a repulsive force between the feathers in the down fiber structure, which makes the feathers not close together, but keep a certain distance, further improving the ability of down to store air, which is the key to down warmth.

Source: pexels

There are three ways to transfer heat, namely heat conduction, heat convection and heat radiation. Heat conduction transfers heat through mutual contact, such as when we cover our hands with a hot water bottle. Heat convection transfers heat through flowing media, such as the warm air conditioner in the house in winter makes the whole room warm. Heat radiation transfers heat through radiating electromagnetic waves, such as when people bask in the sun to keep warm.

Because flowing air can easily carry away heat, it has a lower thermal conductivity than still air and can act as a barrier to heat transfer. The more still air there is, the better the insulation and warmth. The structure of down enables it to store a large amount of air, so clothes filled with down have excellent warmth retention properties, and this performance will continue to improve as the amount of down filling increases (i.e., the fluffiness increases).

The fluffier the down, the warmer it is

Source: Baidu Encyclopedia

2. How do birds wearing “down jackets” spend the summer?

Birds have feathers, and under the feathers there is down on the surface of their bodies. Birds survive winter after winter with these feathers and down. But when summer comes, these feathers and down will hinder the heat dissipation from the body surface. Aren’t birds afraid of the heat?

Golden crown chrysanthemum with very high "down content"

Source: Baidu Encyclopedia

In fact, most birds cannot withstand excessively high body temperatures, which may cause shock or even death. In addition, birds are often in flight and have a high basal metabolic rate. In the same temperature environment, the temperature felt by birds is higher than that felt by us, so birds do need some special ways to help themselves cool down.

Before we learn about the techniques birds use to keep cool, we must first clarify one fact: not all birds are covered with feathers. In fact, only a few birds are covered with feathers, such as penguins. Penguins mainly live in Antarctica. Considering the extremely cold climate in Antarctica, penguins here do not need to worry too much about whether they will get heat stroke in the summer; penguins living in other areas, such as Galapagos penguins, can "soak" in seawater to cool themselves down.

Source: unsplash

The body surface of most birds is divided into feathered areas and bare areas, that is, areas where feathers grow and areas where feathers do not grow. The bare area is an important channel for birds to dissipate heat. When the temperature rises, birds will lift their feathers to expose the bare area to the air to dissipate some heat. If the temperature continues to rise, birds will release heat by dilating capillaries and increasing blood flow, just like humans. The bare area on the surface of birds is relatively small, so birds will increase blood flow more efficiently.

Feather and bare area of ​​a single-comb common Leghorn rooster

Source: "Feathers: A Miracle of Natural Evolution"

Some birds, such as gulls and herons, can increase blood flow to their exposed legs and feet by as much as 20 times to quickly release heat into the air. Other birds will dangle their legs during flight, also for the purpose of dissipating heat. Some birds even dissipate heat through the bare skin around their eyes and their beaks, so many tropical birds have evolved large beaks to expand the surface area for heat dissipation.

Birds hang their legs down to cool down when they fly

Source: unsplash

Air sacs are sac-like structures connected to the lungs. Birds have nine main air sacs that surround their internal organs and extend into the bones of their legs and wings. When a bird inhales, the gas does not go directly into the lungs, but into the first air sac; when it exhales, the previously inhaled gas enters the lungs. When it inhales again, the first inhaled gas is pushed into the second air sac, and the gas just inhaled enters the first air sac... Repeatedly, the gas inhaled by the bird will continue to flow in its body and take away heat. This efficient respiratory system continuously helps the bird dissipate heat, allowing it to maintain a stable body temperature in hot weather.

Source: Baidu Encyclopedia

We discovered that down can keep birds warm, and from this we began to look for the answer to how birds avoid the heat. Perhaps while studying this issue, scientists have come up with new doubts. This is the mysterious charm of nature and the driving force of human progress.