Photo worth seeing: The breeding season at a glance

barn owls
© Ryan Bourbour/UC Davis

Some bird species, like these barn owls, begin to hatch their young as early as winter. That is why autumn is the perfect time of year to clean the incubators – but spring cleaning should be avoided.

Barn owls are among the relatively common bird species. According to the nature conservation organization NABU, there are also around 30,000 birds of prey in Germany - but the number of possible breeding sites is tending to decrease. This is because barn owls like to nest in tree hollows, old barns and church towers. However, due to increased renovations and increasing development in the landscape, parts of these nesting sites are gradually disappearing. Not only could a decline in barn owl populations impact the ecosystem, farmers value the raptors as a form of natural pest control by feeding on mice and other rodents.

In order to attract more barn owls to the fields, some farmers set up incubators. A team led by Ryan Bourbour from the University of California has now investigated when these should be cleaned. This is necessary because the nestlings leave a lot of dirt in their nest site. "Once the chicks hatch, they feed and live in the incubator," explains co-author Breanna Martinico. The owls' nesting period is relatively long at around 65 days: "That's two months in which they grow up completely in the nest box," says Martinico.

The young barn owls do not grow up as only children: there are typically five to six chicks per nest, which eat four to five rodents per day. In addition to the main food, mice, larger rodents such as ground squirrels and rats are also on the menu. The nutrition of the young is also the main reason why there is so much dirt in the nest: owls eat their food in one go – however, fur and bones cannot be digested. Instead, they are regurgitated as a ball. "They live in there for seven to eight weeks and regurgitate these pellets. As a result, a lot of debris builds up in the incubator over the breeding season and needs to be cleaned out,” explains Bourbour.

However, it was unclear for a long time when exactly the barn owl breeding season begins. To finally answer the question, Bourbour and his team analyzed 96 years of records from the US Bird Banding Laboratory and 39 years of data from the California Birds of Prey Center. The result: On average, the owls lay their eggs around February 20th. Since there are natural fluctuations in both directions, the scientists recommend cleaning the incubators in the months of September to November. “Otherwise the risk of disturbing an already inhabited nest increases. We also want to minimize incidents in the weeks before the eggs are laid,” explains Bourbour. Other bird feeders that are not specifically designed for the barn owl should also be cleaned of dirt and parasites on a regular basis. The same applies here: the best time for this is autumn.

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