THE NATURAL FOODS AVAILABLE WITHIN GARDENS
Invertebrates
Many of the birds making use of garden feeding stations also feed on invertebrates; for some garden birds, such as Wren, it is the insects and spiders found within the garden that are the food of choice. Because species like Wren usually ignore the food provided at bird tables and in hanging feeders, their presence in the garden can be easily overlooked. We don’t know a great deal about how the features present within gardens and across the wider urban environment shape the community of invertebrates that is found there. While there are some obvious relationships, others are masked by the complexity of the many microhabitats present and the influence of components within the wider landscape. Work carried out as part of the Biodiversity in Urban Gardens project (BUGS) underlines the difficulties in seeking to identify relationships between invertebrate abundance and particular features (Smith et al., 2006a).
The plant species growing within gardens will have a shaping influence on many of the invertebrate species present, not least because the plants provide many of the feeding opportunities that such invertebrates require. A typical garden flora will almost certainly comprise a mixture of native and non-native species, and the latter may prove unsuitable for insects because of their chemical composition or structural characteristics. Things may not be as bad as you might assume, however, since the chemistry of individual plant species tends to be fairly similar within a family and many of the non-native plants introduced into UK gardens belong to families that also contain a native component. The BUGS project revealed that of the 1,166 plant species recorded in 61 Sheffield gardens, 70 per cent were non-native; however, at the family level, just 36 per cent of the families were alien in origin (Smith et al., 2006b).
FIG 23. While some hoverfly species avoid being predated by mimicking less palatable species, other garden-visiting flies may be taken by Spotted Flycatchers and other bird species. (Mike Toms)
Despite this, there is evidence from work carried out in the suburban landscapes of southeastern Pennsylvania, US, that native planting can be better for both invertebrates and the birds that feed on them (Burghardt et al., 2008). Karin Burghardt and colleagues looked at a number of avian and lepidopteran community measures across paired suburban properties, one property in each pair being planted with entirely native plants and the other with a conventional suburban mix of natives and non-natives. The native-planted properties were found to support significantly more caterpillars and range of caterpillar species and had a significantly greater abundance, species richness and diversity of birds; they also had more breeding pairs of native bird species.
The structure of plants can be an issue where the cultivars presented in garden centres and plant catalogues are divergent from native forms. For example, the ‘double cultivars’ of certain flower forms, which prove popular with gardeners because of their extended flowering season and novel appearance, are less suitable for nectar-feeding insects and they also set less seed. If flower selection and form reduces opportunities for invertebrates within the garden, then it will also reduce opportunities for insect-eating birds. One consequence of this can be seen in the generally lower productivity of tit species within the urban environment. Great Tits and Blue Tits feed their young on small caterpillars and appear to struggle to find enough of these in many urban and suburban gardens (Cowie & Hinsley, 1987).
FIG 24. The larvae of craneflies and other soil-dwelling invertebrates taken from garden lawns are important for Starlings and other species. (Jill Pakenham)
It is also important to recognise the widespread use of insecticides and related compounds in gardens, since these have the potential to both reduce prey availability and to enter the food chain, where problems may then occur. More widely within the built environment there is the risk from pollution, such as with heavy metals (see Chapter 4), which can also impact on the availability of invertebrates and alter the composition of the prey communities available to foraging birds (Eeva et al., 2005).
Fruits, seeds and nectar
Garden plants can also provide food for visiting birds more directly, through their fruits and seeds; in fact, many plants rely on birds to act as dispersal agents for their seeds, offering a nutritious fleshy fruit to attract the bird to take the seed. For example, the natural foods taken by garden-wintering Blackcaps include the berries of Cotoneaster, Honeysuckle Lonicera, Holly Ilex aquifolium, Mistletoe Viscum album and Sea-buckthorn Hippophae rhamnoides. Blackcaps have also been reported feeding on the seeds of Daisy Bush Brachyglottis greyi (Hardy, 1978). The different fruits and seeds become available at different times of the year, though predominantly from October through to January, and this can alter the shape of the bird community visiting gardens. The movement into gardens of wintering Redwing, Fieldfare and Waxwing may reflect the availability of favoured berries in gardens and their scarcity elsewhere. It has been noted, for example, that the timing of hedgerow cutting within the UK’s arable landscapes may remove large quantities of the berry standing crop from the wider landscape, leaving gardens as an important resource (Croxton & Sparks, 2004).
Some of these berries are favoured over others and in some cases a long fruiting season, such as in Holly, can suggest that the berry isn’t particularly favoured by visiting birds, only being eaten once other options are no longer available. In addition to the differences seen between different plant species, the nutritional characteristics of individual fruits may also vary with season. In many berries, the water content of the pulp decreases as the season progresses, while the average lipid content increases. Berry colour may be used as a signal, alerting berry-eating birds to the reward on offer, and there is evidence that birds may select fruits of a particular colour because of their nutritional value. Some bird species appear to select fruits with a high anthocyanin content; anthocyanin is an antioxidant and berries rich in this pigment tend to be black in colour or ultraviolet reflecting. Work on Blackcaps indicates that individuals actively select for anthocyanins in their diet and that they use fruit colour as an honest signal of anthocyanin rewards when foraging (Schaefer et al., 2007).
The consequences of the variation seen in fruits leads to species-specific preferences within the bird species that feed upon them. A series of studies by David and Barbara Snow has highlighted some of these preferences. Mistle Thrush, for example, was found to favour sloes over haws, while the preference was reversed in both Redwing and Fieldfare. Song Thrush Turdus philomelos showed a clear preference for Yew Taxus baccata, Elder Sambucus nigra and Guelder Rose Viburnum opulus, and the apparent avoidance of rosehips, while Blackbird was found to be fairly catholic in its tastes (Snow & Snow, 1988). Blackcaps make use of smaller berries in gardens, such as those of Cotoneaster conspicua (Fitzpatrick, 1996a), reflecting their almost entirely frugivorous diet (when fruit is available) within the Mediterranean wintering area (Jordano & Herrera, 1981), although it is interesting to note the importance of supplementary foods presented at garden feeding stations highlighted by Plummer et al. (2015) and explored later in this chapter.
FIG 25. Berry-producing