Blackbirds: Previous mating through street lighting

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Light pollution in cities irritates the biorhythm of birds

Light pollution or urban lighting during the night affects the biorhythm and the willingness of birds to mate. "It has long been suspected that the nighttime artificial light in cities can influence plants, animals and people," reports the Max Planck Institute for Ornithology in Radolfzell on Lake Constance. So far, however, only a few “studies that test this influence directly” have been carried out. Therefore, the ornithologists of the Max Planck Institute have now carried out a comprehensive investigation to check "how nighttime city lighting affects blackbirds ()."

Artificial light from street lights, traffic lights, illuminated advertising and home lighting have made the nights in Germany brighter for years. As a result, the rhythm in plants, humans and animals could be severely disturbed. The scientists at the Max Planck Institute for Ornithology have now examined these concerns in a study on blackbirds. They found that birds "that are exposed to low light levels at night, comparable to urban light levels" reach reproductive readiness earlier in the year, start singing earlier, and start to blossom earlier. However, the scientists could not say what consequences these significant changes in the daily and seasonal rhythms have for the future blackbird population.

For "many animals, the seasonal change in the length of the day is one of the most important environmental signals for the control of daily (e.g. sleep-wake cycles) and seasonal rhythms (e.g. breeding season)", report the ornithologists from the Max Planck Institute in Radolfzell. This effect is used, for example, in laying batteries, where "egg production is increased by changing the length of the day with the help of artificial lighting". Animals in cities also experience extreme lighting conditions through artificial light. Here, consequences for the biorhythm are a logical consequence, but these have so far only been insufficiently scientifically examined. In their study, the experts at the Max Planck Institute for Ornithology therefore checked “what influence artificial light has on the daily and seasonal organization of these city animals”.

Nocturnal lighting causes significant changes in the biorhythm. First, the scientists determined what light intensity the blackbirds are actually exposed to during the night. The research team led by Jesko Partecke from the Max Planck Institute for Ornithology caught several blackbirds and equipped them with light sensors that were able to determine "what illuminance levels the birds were exposed to on average at night," reports the institute. According to the study leader, it was found that "the intensities of 0.2 lux were very low - only one thirtieth of what a typical street lamp emits." The researchers then placed "caught city and forest blackbirds over a period of ten months at night illuminance of 0.3 lux. ”Even these low values ​​were sufficient to cause drastic changes in the biorhythm of the birds, according to the Max Planck Institute for Ornithology.

Blackbirds ready for reproduction earlier through night-time lighting The relatively small increase in light intensity during the night was sufficient to allow the gonads of male blackbirds to mature earlier, according to one of the study results. "The results were astonishing: the testicles of birds grew on average almost a month earlier than those of animals that slept in the dark at night," emphasized Jesko Partecke. Measurements of the testosterone values ​​in the blood of the birds have shown that this increases earlier in the course of the season due to night lighting. This also suggests that the blackbirds are more likely to reach their reproductive readiness. Furthermore, the daytime rhythm of the singing activity had become confused by the artificial light. According to the researchers, the birds started singing about an hour earlier in the day. In addition to the indications of premature reproductive behavior, it had also been shown that the blackbirds, which were illuminated at night, started moulting much earlier than the birds on dark nights.

Deviations in the daily and seasonal rhythms "These results clearly show that the artificial light that we find in cities can drastically change the seasonal organization of wild animals," explained study leader Jesko Partecke. However, the researchers are not yet sure why the artificial light in the blackbirds requires early reproductive readiness and whether this is more of an advantage or disadvantage. One explanation for the earlier willingness to reproduce is, for example, that the blackbirds are pretended to have a longer length of day because of the artificial light. The lighting may also make it easier for birds to search for food at night, so that the animals have additional energy available for reproduction. Ultimately, the light "can also affect the metabolism of animals and the changed metabolism can lead to an earlier growth of the gonads," reports the Max Planck Institute for Ornithology. If the birds were exposed to light at night, they were also more active during the day, according to the experts.

Previous fertility an advantage or disadvantage? According to the ornithologists from the Max Planck Institute in Radolfzell, the long-term effect of artificial lighting on the blackbird population has also remained unclear. "Whether the earlier breeding offers the city blackbirds an advantage or is simply an unwanted side effect of the lighting" cannot be determined from the current study results. In fact, it would be theoretically conceivable that "the blackbirds breed in the city earlier in the year due to the artificial light and thus produce more youngsters a year," explained study leader Jesko Partecke. However, this supposed advantage is only effective "if the nestlings have enough suitable feed available". "Otherwise, the early reproductive capacity for blackbirds could turn out to be an evolutionary disadvantage," concluded the Max Planck Institute for Ornithology. (fp)

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