Just make lots of noise and we’ll be fine (Image: Joel Jorgensen/Nebraska Game and Parks Commission)

Dense fog can not only ground planes but also birds – just not always.

Eileen Kirsch of US Geological Survey’s Upper Midwest Environmental Sciences Center in La Crosse and her colleagues were observing sandhill cranes in the Horicon wildlife refuge in south-east Wisconsin one November morning. Heavy fog settled, but the cranes still set off from their night roost to reach foraging areas, albeit later than usual.


This provided a rare opportunity to study how birds behave when flying in heavy fog. The cranes flew cautiously, staying close to the roost, and went in circles rather than straight lines.

“They were going every which direction, which we’ve never seen before,” says Kirsch. She thinks they were probably reluctant to fly further than they could see and also wanted to keep the flock together.

They also vocalised much more frequently and loudly than normal. This is common among birds flying in low-visibility conditions, says Graham Martin at the University of Birmingham in the UK. It probably allows them to stay in touch when they cannot be seen.

When there is fog, mist or heavy rain, larger birds such as cranes, ducks and geese usually prefer to stay put until conditions improve. “They can afford not to venture to foraging grounds for a couple of days,” says Martin.

Kirsch thinks hunger and their impending southern migration might have driven the sandhill cranes to flight, despite the poor visibility. “It was very cold, and energetically they needed to eat,” she says.

Night-time challenge

But if some birds avoid flying in fog, how come many other species are happy to fly at night? Kirsch says that most birds that do so typically set off at sunset, when there is enough light available for them to orient themselves.

And like airplane pilots, night-time flyers don’t rely only on vision. They also navigate using acoustic and magnetic cues, as well as the positions of the stars and moon, says Andrew Farnsworth of Cornell University in Ithaca, New York.

But even so, they can get confused. Light pollution is a particular concern for nocturnal flyers like songbirds, which are drawn to artificial light. And if they are dazzled, they also display circling flight behaviour, says Farnsworth.

Fog and darkness aside, many birds are betrayed by their own eyes even in broad daylight, says Martin. Some literally can’t see straight ahead while flying and trying to track prey or predators at the same time. The placement of their eyes means that they have several blind spots that make them prone to crashing, in particular into artificial structures like transmission lines and towers, which they don’t expect to encounter.

Birds that dive for food face the additional challenge of seeing underwater. This can be difficult even for birds with keen sight.

Puffins and guillemots, for example, are not particularly vulnerable to crashing when airborne, but are prone to being entangled on gill nets. Martin and his colleagues have found that this is because the peculiarities of their visual fields mean that they may not be able to see straight ahead in the water.

Journal reference: The Wilson Journal of Ornithology, DOI: 10.1676/wils-127-02-281-288.1