The birds have more difficulty rearing chicks in “man-made” environments filled with non-native trees and shrubs than in more natural landscapes such as woodlands and hedgerows, according to new research.
The study by Anglia Ruskin University and the Centre for Ecology and Hydrology monitored breeding success of blue and great tits in nest boxes in the Botanic Gardens in Cambridge, and compared it with those in nest boxes in more traditional woodland and hedgerow habitats.
Very few great tits raise chicks to fledging in the Cambridge University Botanic Gardens and the young that do survive are often underweight, according to the research.
The researchers also monitored the energy expended by breeding adults, and found they had lower breeding success despite working harder than their country cousins.
The larger physical gaps between trees in parks compared to woodlands and the exotic plants and trees which do not support as much of the insect food the birds eat and need to feed their chicks adversely affected breeding success.
Dr Nancy Harrison, from Anglia Ruskin University, said exotic trees and shrubs were worse than having no plants at all, because the birds expended energy and time searching for food among them with little reward for their effort.
The average weight of an 11-day-old great tit was 14.5g in the Botanic Garden compared to 17.5g for nestlings of the same age in woodlands. It was a similar story for blue tits, with 11-day-old chicks weighing in at an average 9g in the park landscape filled with non-native species, compared to 10.6g in a woodland habitat.
Dr Harrison said that in the woodland habitats the chicks tended to be bright and perky looking and nests could have eight to 12 chicks in them, whereas in the urban park nest boxes, there would often just be one healthy chick.”