Wildlife and eclipses
Next month, a total solar eclipse will briefly plunge the South Pacific, Chile and Argentina into darkness, but how will wildlife react to this astronomical event?
A total solar eclipse is an exciting event for us humans, but how does the rest of the animal kingdom react?
There is no more powerful nor more fleeting a sight in nature than a total eclipse of the sun. For humans standing in exactly the right place, this is the only time when the sun’s outer atmosphere, the ice-white corona, can be seen with the naked eye. It can be an amazing, emotional spectacle, but how do animals and insects experience it?
On 2 July 2019, a total solar eclipse will plunge a 145–193km track of the South Pacific, northern Chile and Argentina into blackness for just over two minutes. Witnesses to the rapidly darkening landscape during a total solar eclipse have always observed wildlife, though, mostly, it’s been anecdotal. As the sky turns twilightlike, stories abound of birds going to roost, dairy cows returning to the barn, croaking frogs, flying bats and crickets making a
Colonial orb-weaving spiders began taking down their webs at the moment of totality.
cacophony of noise. Such reactions suggest a change in behaviour but, until recently, there’s been little measurable science.
That all changed on 21 August 2017 when a total solar eclipse swooped across the USA. It was a rare chance to study the effect of totality (when the sun is obscured) across a large landmass and, since the USA has much advanced and consistent weather and radar data, scientists had far more information than usual, to help them design their studies.
Back to black
An eclipse is not like a sunrise or sunset, which occur before and after prolonged twilight. A total solar eclipse – which takes place roughly once every 16 months – brings sudden darkness, along with reduced temperature and wind speed. The profound gloom lasts as little as a few seconds and as long as seven and a half minutes – depending on the observer’s exact location – but it’s often a once-in-a-lifetime opportunity for researchers to see how their chosen field of study is affected by the event.
There is an assumption that totality is mistaken for sunset by wildlife, but the most significant study from August 2017’s eclipse turned that on its head. The paper ‘Aeroecology of a solar eclipse’ describes an experiment using a US-wide radar network to investigate the reaction to totality by flying insects and birds. Data from 143 Doppler radar sites was used, eight of them were within the path of totality.
“We were lucky, in that the radar scans only every five minutes, but most of our sites experienced a scan during totality,” explains Cecilia Nilsson of the Cornell Laboratory of Ornithology, based at New York’s Cornell University. Nilsson was hoping to ascertain whether the decrease in light and temperature would see a spike in activity normally associated with sunset.
“That’s definitely not what happened,” Nilsson says. August is a special time of year, since it’s when many North American bird species are migrating or getting ready to migrate. “That was the main motivation for us to do this study in the first place,” adds Nilsson, “because we know that, in August, there is normally a lot of activity after sunset, when birds and insects take to the air.” Many species of diurnal birds migrate at night. “Suddenly, within the season where birds are tuned into migrating, there would be weird light and darkness in the day.”
The birds did not start migrating during totality. In fact, the data suggests that biological activity decreased shortly before the sudden darkness. “It’s perhaps not that surprising, because totality is a lot shorter a cue than a sunset, but probably the birds understood that it wasn’t sunset,”
says Nilsson. It’s difficult to draw too many conclusions, as totality was near midday, which is when insect activity is higher, so the data could be largely an insect signal.
Are birds and insects more sensitive to light than humans? The study suggests so, with activity reducing in advance of totality, at a time of partial eclipse, when humans would struggle to notice any difference.
A previous study of a total solar eclipse in Venezuela, on 26 February 1998, reported that royal terns stopped foraging over water 39 minutes before totality. They flew inland, and didn’t return for an hour. Frigatebirds and pelicans did the same, 13 minutes before totality, but were back 12 minutes after the sun emerged. It was concluded as being normal behaviour associated with sunset. So, it seems, solar eclipses reduce light levels sufficiently to interrupt normal avian diurnal behaviour, but what about insects?
During a study that was mostly focused on the movement of bees, researchers from the University of Missouri recorded the ambient temperature at 11 sites across the USA within the path of totality on 21 August 2017.
The study found that temperature changes during the eclipse had little impact on bee activity, but that bees stopped flying during totality. That’s, perhaps, not surprising – honeybees navigate using polarised light, so their buzzing rate should be expected to alter with changes in light intensity. However, the scientists also recorded that bees were more active during the partial phases of the eclipse, shortly before and after the brief totality.
Such detailed studies remain unusual. In 1994, researchers looked at the effects of totality on colonial orb-weaving spiders in Veracruz, Mexico. They observed no effect whatsoever until the moment of totality, when the spiders began taking down webs – most rebuilt them after totality ended.
Many diurnal species, such as these ring-tailed lemurs, appear to become confused by the untimely darkness of a total solar eclipse.
Left to right: the sun’s corona; brown pelicans were seen flying inland before totality; studies have looked at how eclipses impact chimps; a partial solar eclipse.