The influence of native and non-native trees on provisioning rates in an insectivorous urban bird

Abstract

Non-native trees are common in urban environments all over the world. From a conservation viewpoint, it is essential to understand how these trees are affecting urban breeding birds. Non-native trees house fewer and smaller invertebrates than native trees do and are considered a poor food source for insectivorous birds. Still, studies are lacking on exactly how non-native trees affect birds’ feeding behaviour and overall condition. In this study, we used the great tit (Parus major) to investigate the effects of non-native and native trees on provisioning rates and nestling condition in the urban environment. We performed feeding observations at 21 nests in five city parks in southern Sweden, at two points of the nestling period, day 5 and day 10 post hatch and collected body mass and wing length data of day 14 nestlings. We then compared provisioning rates and nestling condition between different territories based on tree composition and discuss the results in the context of likely food availability. At day 5, provisioning rates were highest in territories rich in non-native trees, although no correlation was found for day 10. The dominant native tree in the parks, the common beech (Fagus sylvatica) had a negative effect on day 10 provisioning rates, but there was no relationship between provisioning rates and pedunculate oaks (Quercus robur) or silver birch (Betula pendula) on either day. We found no evidence to suggest that nestling condition varied with tree composition. The results propose that non-native trees may be a driver behind higher provisioning rates because of smaller and less nutritious invertebrates.

Popular Abstract

Urban birds have to work harder when feeding in exotic trees

Birds in the city live a hard life. Habitat fragmentation, traffic disturbances, air and light pollution and predators such as cats present challenges. Though, towns and cities could also bring opportunities including an abundance of food at bird-feeders, fewer predatory birds and milder temperatures in winter. All these factors play a role in the breeding success and survival of small birds.

In Swedish city parks, you encounter a variety of exotic trees from all over the world, ranging from North America to the Caucasus mountains, to China. People have long considered exotic species to be beautiful, and today they are planted all over cities and suburbs. However, non-native trees could make it harder for birds to find food, because they often support fewer invertebrates.

Native insects and spiders tend to favour native trees over exotic trees. Common Scandinavian trees such as pedunculate oaks and silver birches house plenty of prey for insect-eating birds. Come spring, the great tit, one of our most common birds, can find a lot of caterpillars and spiders here to feed their newly hatched nestlings with.

Until the day they leave their nest, great tit chicks are heavily dependent on parental care. They must be fed frequently to survive. Thus, it is important that food is available close by so as not to leave them hungry for a long time. I wondered how exotic trees might affect this feeding behaviour, so I set out to answer the following questions: How frequently are great tit nestlings fed across different territories, depending on the number of native and exotic trees? Are chicks in nest boxes close to many native trees fed more often? Are they fed less often, when there are more exotic trees? Because of the lack of insects in exotic trees, one could suspect this is the case. Over several weeks, in five parks in Malmö, Sweden, I observed how often great tits fed their baby birds, across different territories.

What turned out to be surprising is that the more exotic trees within a territory, the more often the chicks were being fed. How could this be? One explanation is that exotic trees accommodate lower quality bugs – not just fewer – meaning that, when reliant on exotic trees, great tits must feed their young more often in order for them to get the nutrition they need. Indeed, I found that chicks had the same body weight in both territories with native and exotic trees. This means that great tit parents feeding in exotic trees had to work a lot harder than parents feeding in native trees.

The discovery is important and indicates that birds are affected by the type of vegetation we decide to plant in our cities. Maybe, these findings can lead to a change in park and garden landscaping in the future, as a means to preserve biodiversity. Perhaps, native Scandinavian plants are thriving in our future parks, resembling an environment more similar to that of a natural forest - allowing for a living more amenable to small birds.

Handledare: Hannah Watson & Caroline Isaksson

Examensarbete BIOY01 30 hp i Biologi 2021
Biologiska institutionen, Lunds universitet

• author: Andersson, Martina
• supervisor: Hannah Watson; Caroline Isaksson
• organization: Degree Projects in Biology
• course: BIOY01 20201
• year: 2021
• type: M2 - Bachelor Degree
• subject: Biology and Life Sciences
• language: English
• id: 9059965
• date added to LUP: 2021-06-30 12:06:53
• date last changed: 2021-06-30 12:06:53