Raising a brood in the unknown

The Botanic Garden is an unusual habitat for woodland birds. Species which once lived in the English wildwood are now breeding in a landscape of diverse and exotic trees and shrubs. A recent collaboration has explored how the breeding success of blue tits and great tits in the Botanic Garden compares with local woodland and hedgerows.
This collaboration, led by Dr Shelley Hinsley from the Centre for Ecology and Hydrology with Anglia Ruskin University, Aberdeen University and Cardiff University, followed the fortunes of birds using nest boxes at sites as varied as Monks Wood near Huntingdon, Bute Park in Cardiff, a reclaimed gravel pit at Godmanchester, and here in the Botanic Garden. Dr Nancy Harrison, of Anglia Ruskin University, led the team working here, measuring breeding success in terms of clutch size, chick weights, and fledging success. The energy expended by breeding adults has also been measured daily (to establish the birds’ field metabolism).

It has been found that blue tits and great tits struggle to raise chicks in highly patchy habitats. Both at Bute Park and the Botanic Garden, chick weights were low and the adults were found to work harder for much lower breeding success than in natural woodland. But the surprise was that birds in the Botanic Garden were working even harder than birds breeding in other fragmented habitats – even those in scrappy, scrubby habitats that do not look nearly as good.

Perhaps the problem isn’t only the physical gaps in the tree canopy caused by the distances between trees, but also ‘functional’ gaps in the form of exotic trees and shrubs which offer little reward for the birds.

Julia Mackenzie, working with Nancy Harrison, is trying to understand the impact of exotic trees and shrubs on the tit population. She is comparing a patchy habitat at Wicken Fen (dominated by native plants), and the Botanic Garden. Her results are startling: see the photos of great tit chicks of the same age from the study sites. Very few great tits raise chicks to fledging, and these are often underweight, with an uncertain future. Blue tits do much better, raising more chicks to healthy fledging weights, but rarely more than half the total number they raise in woodland. Exotic trees perhaps represent a greater challenge for the birds than structural gaps with no trees at all. Gaps with no trees represent unambiguous problems for foraging birds. It may be that birds are not making good decisions in a mosaic of exotic trees and shrubs, spending time searching for food where there is none.

The most interesting part of the study now is in the detail, which may lead to advice on the best trees and shrubs to plant in our own gardens in the interest of birds. For three years, Julia has been following colour ringed birds in the Garden, seeking to evaluate whether their decisions when foraging can be related to their breeding success, or failure. As a result we are learning, in minute detail, which parts of the Botanic Garden are good for birds.

The Botanic Garden is not only managed for plants, but for biodiversity. Decisions as to whether to maintain old trees laden with ivy, to leave dead wood on the ground, to keep large expanses of ground unmown – all can assist birds in finding food for their chicks. The most successful birds use ‘habitat hotspots’ within the Garden, and by June the fading cow parsley (Anthriscus sylvestris) is full of fledglings. There are also some surprises; one of the most successful blue tits (gauged by producing fledglings which survive their first winter) spends much of his time in the pruned hawthorn hedge of the Systematics Beds. This hedge in no way resembles a natural woodland habitat, but hawthorn is a woodland scrub, and this sculptured hedge qualifies as blue tit habitat. The project in the Botanic Garden is providing surprising insights into how garden birds perceive our urban green spaces.

Dr Nancy Harrison, Anglia Ruskin University