Lack of insects in cities limits breeding success of urban birds
Researchers at the University of Pannonia, Hungary and the University of Sheffield, UK, found that providing high quality supplementary food to urban great tits, in the form of nutritionally enriched mealworms, can dramatically boost their breeding success.
“Urban nestlings had considerably higher survival chance and gained an extra two grams of body mass when provided with an insect-rich diet, an increase of 15% compared to the weight of chicks that didn’t receive extra food. This is a substantial difference.” said Dr Gábor Seress, lead author of the research. "This greater body mass when leaving the nest may increase the chicks’ chance of surviving to spring and breeding themselves".
Reduced breeding success in urban bird populations is well documented but this study is the first to show that insect-rich supplementary food during nestling development largely mitigates these habitat differences.
Dr Seress said: “Given the popularity of year-round bird feeding and the abundance of anthropogenic food sources in cities it might seem unlikely that urban birds have limited food. But quantity is not quality. Most songbirds require an insect-rich diet to successfully raise many and vigorous young, and urban areas generally support fewer insects than more natural habitats, especially caterpillars, which are key components of the optimal nestling diet for many species.”
The authors say that artificially providing insect-rich food for birds in cities may not be the best solution. “Instead of directly supplying high-quality bird food to enhance urban birds’ breeding success, we believe that management activities that aim to increase the abundance of insects in the birds’ environment, would be more effective.” said Dr Seress. "Insects are the cornerstone of healthy and complex ecosystems and it is clear that we need more in our cities"
Increasing insect populations in cities is no easy task. The authors highlight that most urban green spaces are often highly managed which can reduce insect abundance. Modifying how green spaces are managed and encouraging practices like planting trees is likely to benefit both insect-eating birds as well as people.
In the experiment, the researchers studied great tits in nest boxes at urban and forest sites in Hungary, 2017. At both sites there were broods that did not receive supplementary food to act as controls.
For the supplementary fed broods, the researchers provided nutritionally enhanced mealworms throughout brood rearing period on a daily basis, adjusting the amount in accordance to the brood size to meet 40-50% of food requirements. When nestlings were 15 days old (a few days from leaving the nest) the researchers recorded the size, weight and survival rate of chicks.
While the findings demonstrate that providing high quality additional food can boost breeding success, it is unclear to what extent this could increase population size and stability, further work is needed to explore this.
Further research into the reduced abundance of insects in cities is also needed. Dr Karl Evans, co-author of the research, said "This is essential in order to improve the habitat quality of urban environments for nesting birds”.