Animals use invisible light layers in the sky for navigation

The air above our heads is busy. Birds, bats, and insects sort themselves into altitude layers that work like lanes, guided by physics, weather, and the presence of other animals. In a recent study, experts have proposed a simple way to see the sky layers as habitat, not as a void.

That framework explains why an animal might fly a few hundred feet up on one day, then many thousands the next. Conditions change fast, so choices change too.

Cecilia Nilsson of Lund University (LU) led the study, with collaborators in the Netherlands and the United States. Her team connects animal behavior with the vertical structure of the atmosphere.

Sky layers as living space

Scientists now treat the air as a living environment with food, competitors, and risk. This idea has a name: aeroecology, the study of how animals interact with and live within the atmosphere.

The authors use the notion of an aerial niche, the specific range of altitudes and conditions that a flying species occupies. This describes where a species can operate and where it is usually found.

The first part is set by physiology and flight mechanics, while the second is narrowed by predators, prey, and human structures.

Who flies where in the sky layers

Slow flying insects usually stay low, where winds are calmer and temperatures are steadier. Many bats work the middle levels, where they can chase faster insects without spending too much energy.

Birds often rule the higher altitudes. They make use of thermals – upward currents of warm air that lift gliders and soaring animals.

The tailwinds also help: they move move in the same direction an animal is flying, thus supporting long flights with less effort. Exceptions exist, and the mix can flip with weather or season.

“We want to encourage more studies that map which species use different altitudes, and under what conditions,” said Nilsson. Consistent altitude maps would let researchers test predictions and refine models.

How scientists study sky layers

Weather radar is a system that uses radio waves to detect objects and measure their distance and movement. This can track the density, speed, and direction of flying animals at many heights. Radar networks reveal flows of birds flying in layers across regions, night after night.

On-board sensors add detail. GPS, altimeters (instruments that measure altitude based on air pressure), and barometric loggers (small devices that record changes in air pressure to infer height), can all record the ups and downs of individual flights across full migrations.

Why altitude choices change

Winds aloft can speed a journey or stop it in its tracks. Temperature and air pressure change with height, so the cost of flying shifts as animals climb or descend.

Large insect migrations show how strongly animals use favorable winds. Over southern Britain, a long-term study, estimated that roughly 3.5 trillion insects, about 3,200 tons of biomass, cross the skies each year, and larger insects choose winds that move them in useful directions.

How humans affect the sky

Structures reach into the same space that animals use. A national estimate, indicates that between about 365 million and 988 million birds die each year in the United States after hitting buildings, with most deaths occurring due to low- and mid-rise structures.

Turbines pose a different problem for bats that move across continents. Population modeling research, found that mortality at wind facilities could drive steep declines in a widely distributed migratory species over the next 50 years if nothing changes.

Cities and energy impact

If the sky is habitat, then height matters in planning. Bird safe glass, better lighting schedules, and careful siting of tall structures reduce deaths without waiting for new technology.

Wind farms can cut bat fatalities by adjusting rotor speeds during high risk nights. Pairing curtailment with smart forecasts that include likely flight altitudes can protect wildlife and keep energy production steady.

Layers of the sky

The lowest slice of the troposphere, the layer of the atmosphere closest to Earth where weather occurs, is shaped by friction with the surface, so wind speeds usually increase from the ground up through the first few hundred feet.

This zone includes the flight boundary layer, the region of air close enough to the ground where insects can still control their flight despite wind.

At night, a nocturnal boundary layer, a stable layer of cooler air that forms after sunset, often develops above the surface. Winds there can become smooth and fast, and animals that match their flight to those layers can travel far while saving energy.

Body and behavior limits

Body design sets limits. Wing loading (the ratio of body weight to wing area) and aspect ratio (the relationship between wing length and width), determine how high an animal can go and how much it pays to stay there.

Behavior narrows the choices. Predation risk, prey distributions, and the need for information can hold animals to a band of air that is safer or more profitable, even if it is not the cheapest place to fly.

Altitude maps for conservation

Altitude maps help match solutions to local problems. A city may need bird-friendly codes for mid-rise corridors, while a migration bottleneck may call for targeted lights-out nights.

Energy planners can place tall structures outside peak flight layers during critical periods. Accurate altitude data make that practical rather than speculative.

The framework treats altitude as a dimension that deserves the same attention as latitude and longitude. It pushes studies to report how height use shifts by time of day, season, and life stage.

“We hope that our framework will be a tool for both researchers and decision-makers,” concluded Nilsson. The focus is on practical steps that lower risk and keep the sky alive.

The study is published in the journal Trends in Ecology & Evolution.

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