Endless Daylight and Circadian Rhythms: What Birds Tell Us


WC was in downtown Fairbanks in the late evening recently. Tourists were out on the streets, walking around dazed, their sleep cycles completely frazzled by too many time zones and the endless light.

As we approach summer solstice, WC wondered, “How do birds adapt to the endless light?” Whether the birds have migrated from southern climes, or were foolish enough to spend the winter here, the usual cycles of light and dark are gone here in the high summer. How do the birds cope? How does the normal circadian rhythm adapt to endless daylight?

Happily, there’s research going on addressing this very issue. And it turns out that different species handle it in different ways. In a recent study of four different species, all of whom winter outside of Alaska, some kept regular 24 hours schedules, some kept a cycle but it wasn’t tied to the 24 hour day, and some had no discernible cycle. Oh, and it varied by sex and activity, as well as by species.

The study looked at four species.

Lapland Longspur Male

Lapland Longspur Male

The Lapland Longspur, Calcarius lapponicus, keeps a regular schedule, sleeping from about midnight to about 4 AM each short summer night. The technical term is “entrained,” more or less bound to a regular 24 hour cycle. So Lapland Longspurs adapt to the absence of darkness by relying on internal mechanisms, likely melatonin levels, to deal with the absence of environmental cues.

Pectoral Sandpiper

Pectoral Sandpiper

At the other extreme, male Pectoral Sandpipers and female Red Phalaropes were continuously active over a 24 hours day, without long rest phases. The old joke about sleeping when it’s winter? These birds live it. How do they get by without adequate rest? Science can’t answer that question yet.

A lousy photo of a Red Phalarope

A lousy photo of a Red Phalarope

Male red phalaropes and female Pectoral Sandpipers, on the other hand, showed some adherence to the 24 hour cycle. This is less confusing when you remember that among Phalaropes, the sex roles are reversed. The male incubates the eggs while the female goes off looking for another male. So incubation tends to be associated with more “normal” rest patterns.

Semipalmated Sandpipers were different still.

Semipalmated Sandpiper

Semipalmated Sandpiper

During the incubation period, at least, male and female semipalmated sandpipers showed activity patterns with variable period lengths that significantly differed from the normal 24 hour cycle. Interestingly, male and female Semiplamateds share egg incubation.

The authors summarize all this in a spiffy table.

Table 1.

Overview of activity rhythms in four free-living arctic-breeding birds in relation to sex and breeding stage. Activity patterns were classified into three categories based on Lomb–Scargle periodogram analysis: arrhythmic, no periodicity could be detected; entrained, a significant periodicity was detected that did not differ from 24 h; free-running, a significant periodicity was detected that deviated significantly from 24 h. Note that the three shorebird species have precocial young (parental care only includes brooding, attending and defending the young), whereas the Lapland longspur has altricial young (fed by both parents).

species social mating systema parental care patterna sex activity pattern
arrhythmic entrained free-running
semipalmated sandpiper monogamous biparental male pre-incubation incubation
female pre-incubation incubation
pectoral sandpiper polygynous female only male entire season
female pre-incubation incubation
red phalarope polyandrous, sex-role reversed male only male pre-incubation incubation
female entire season
Lapland longspur monogamous, occasional polygyny biparental (female-only incubation) male entire season
female entire season

Steiger et al. conclude they have found:

the first compelling evidence based on behaviour of free-living, arctic-breeding birds for the existence of marked plasticity and substantial diversity in daily activity rhythms among species, between males and females of the same species, and between individuals in different stages of reproduction, from a single site and a single, brief observation period.

Now this was admittedly  small sample size, but if it is borne out by additional research the study suggests that birds, at least, have sleep schedules that are a lot more malleable, a lot more easily shaped by the environment, than had been thought. And there’s no obvious reason why that should stop with birds.

So there may be hope for those bleary-eyed visitors from out of state; indeed, there may be hope for dog-tired WC, who seems to have been living by the sun’s schedule and not his own.

(A hat tip to Mrs. WC who pointed a Science article that discussed this research. )

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