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?

BBC Wildlife Magazine - - Contents - By Jamie Carter

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?

Minute changes

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.

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