Sunday, 29 July 2012

Shedding light on the effects of anthropogenic illumination

If you live in an area with lots of artificial lighting, chances are you draw your curtains at night so the brightness doesn't keep you awake. That's not an option available to wildlife, though, and researchers have long wondered whether they suffer as a consequence. Exposure to nighttime light might, for instance, disrupt internal clocks, causing animals to behave in ways that are inappropriate for particular times of day or season.

This possibility was recently examined by researchers in The Netherlands, who installed artificial lights above the nest boxes of breeding great tits (Parus major). They hypothesized that increased exposure to light would cause the birds to forage more, which, in turn, would allow them to provision their young with more food. While this might work out well for the chicks, the increased time and energy expended by the adults could be detrimental to their health over the long term.

(Image courtesy of JYI.)

The researchers performed the light treatment in two shifts. During the first half of the chick-rearing phase (which lasts just over 2 weeks), they exposed half the boxes (n = 26 total) to light and left the other half in darkness; during the second half of the phase, this was reversed, so that previously-lit boxes went dark, while previously dark boxes received illumination. At the beginning of the study, nest boxes were fitted with transponder readers that logged visits from the adults; females had been tagged with transponders at the beginning of the nesting attempt, while males were caught and tagged midway through. Transponder data were used to calculate the beginning, end, and duration of each box visit event. Effects of the feeding regime were gauged by measuring juveniles on days 9 and 16 after hatching.

The effect of artificial light turned out to be unusual, and hard to interpret. Chicks in lit boxes received significantly more parental visits, but only during the second half of the nestling stage, and only from females--though male feeding rates didn't lag far behind. The increases in feeding did not have any impacts on chick size, indicating that parental care didn't translate into faster growth--perhaps because the adults were not able to take the time to forage for the biggest, juiciest prey items before each visit.

(Great tit, Parus major. Image courtesy of Treehugger.)

Interestingly, the presence of artificial light did not cause the birds to start feeding their young earlier in the day, or stop feeding them later; in other words, the illumination wasn't mistaken for an earlier dawn or later sunset. Instead, all the extra light may have tricked the birds into thinking that it was later in the breeding season, when days are longer. This could have prompted them to invest more time and energy into chick care, since "later"-breeding birds can't be certain of having the time to try for a second nest, and therefore want to ensure that their first one turns out well. Alternatively, the researchers hypothesize that chicks in lit areas may beg more, thus prompting the adults to increase feeding rate; however, it is not clear what might prompt this alteration in nestling behavior.

Perhaps the most interesting aspect of the study was the difference between the two parents. Unlike females, males don't spend the night in the box, and so they probably weren't exposed to the artificial increases in light. This means that their extra feeding visits were probably prompted by the upswing in parental care demonstrated by the females--an intriguing insight into the dynamics of mated pairs.

(Great tit nest, with begging chick. Image courtesy of Wikipedia.)

All in all, the study raises almost as many questions as it answers. Might chicks actually benefit from light because they get extra parental care? Does this negatively impact the adults by decreasing the likelihood that they will rear 2 broods per season, or survive from one breeding season to the next? Could artificial light act as an ecological trap? Although this work indicates that light can impact wild birds, more work will be needed to fully understand how, and with what outcome.

Titulaer, M., Spoelstra, K., Lange, C.Y.M.J.G., and Visser, M.E. 2012. Activity patterns during food provisioning are affected by artificial light in free living great tits (Parus major). PLOS ONE 7(5):e37377.

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