The presence of non-native berry-producing shrubs in gardens, while potentially a valuable food resource for visiting birds, brings with it a possible conservation issue. Since birds are the main dispersal agent for the seeds held within berries, they are a potential route by which non-native species may become more widely established within the wider countryside (Greenberg & Walter, 2010).
It is not just the berries that birds seek; some, such as Greenfinch, are after the seeds themselves. This can prompt plants to incorporate toxic compounds into the seed coat or its lining. Although it is the absence within the wider countryside of the seeds of larger shrubs and trees that can drive birds to garden feeding stations (see later in this chapter), it is important to note that some of our garden birds specialise in the seeds of smaller plants. The Goldfinch, for example, specialises in seeds of the Compositae family, particularly the thistles and dandelions and it is this habit that probably first brought them into gardens to feed on ornamental thistles, teasel, lavender and cornflower (Glue, 1996). Maddock (1988) reports how, from the winter of 1983/84, a few Goldfinches would visit a suburban Oxfordshire garden to feed on lavender and dry teasel heads. Their interest was maintained by brushing the teasel heads with Niger. Over the following winter, up to two dozen Goldfinches again visited the teasel head before turning to a mix of Niger, canary seed and millet provided at a garden feeding station. Important berry- and seed-producing plants for birds are shown in Table 4.
TABLE 4. Suitable plants for providing berries and seeds for garden birds. Adapted from Toms et al., (2008).
On occasion, garden birds have been reported stealing nectar from flowering plants, the latter typically exotic species whose flowers show features used to attract birds as pollinators (Búrquez, 1989; Proctor et al., 1996). Reports from within the UK have included Blue Tit – feeding from Crown Imperial Fritillaria imperialis (Thompson et al., 1996); Blackcap – feeding from Mahonia (Harrup, 1998); and Blackcap feeding from Kniphofia. In addition, the behaviour is widely recognised in a number of the warbler species migrating through the Mediterranean region (Cecere et al., 2011). Nectar feeding has also been recorded for a number of European plants (see the review by Ford, 1985), including those in the genera Rhamnus, Ferula, Acer, Crataegus, Ribes and Salix.
Nectar feeding is thought to be a widespread behaviour in UK Blue Tits, with the species recorded feeding from a variety of flowers across 33 counties within the UK (Fitzpatrick, 1994). There has also been a small amount of more detailed work, examining the contribution that nectar taken from Flowering Currant Ribes sanguineum makes to Blue Tit spring diet (Fitzpatrick, 1994). This revealed that although the nectar was not the preferred food – it was used most when peanuts were unavailable because of competition from other birds – it was a highly profitable food source, contributing up to 50 per cent of the average daily metabolic rates of the Blue Tits studied. The use of nectar sources by suburban populations of predominantly nectar-feeding birds has been examined in Australia, with a view to understanding the contribution that native and exotic shrubs make towards the available nectar resource (French et al., 2005).
FOOD SELECTION BY GARDEN BIRDS
The contribution that food provisioned at garden feeding stations makes to the energetic and nutritional requirements of garden birds varies between seasons and between species. It also varies between the different types of food provided, with some higher in fat or protein content than others, and with differences in the amount of time required to ‘handle’ and process the food. It is difficult to study the feeding preferences of birds out in the field, particularly in a garden setting where individual birds can find alternative feeding opportunities nearby. A small amount of work has been done here in the UK with, for example, Greenwood & Clarke (1991) examining the selection of black-striped sunflower seeds and peanuts – peanuts were the preferred food taken.
One of the biggest studies to have been carried out was that undertaken by Geis (1980) in the United States. The study, carried out between November 1977 and July 1979, made use of a network of volunteers, observing and recording feeding preferences at specially designed feeding stations, capturing details of 179,000 feeding visits. Geis adopted a standard approach, whereby the food types being tested were compared against two ‘standard’ foods commonly provided by householders: black-striped sunflower seed and white proso millet. The results of the work underlined a general pattern of preferences across the bird species visiting and also highlighted the individual preferences of particular species. American Goldfinch, for example, favoured hulled sunflower seeds over Niger, with Niger favoured over oil-type sunflower seeds, black-striped sunflower seeds and white proso millet. In contrast, House Sparrows favoured white proso millet but would feed on almost anything, with the exception of flax and rape seed. Selection of Niger by fine-billed finches is something that has been documented in other studies (Horn et al., 2014).
FIG 26. Large-billed species like House Sparrow are more likely to take larger seeds from a seed mix, the smallest seeds being difficult for them to handle. (John Harding)
The abilities of wild birds to select foodstuffs based on their nutritional characteristics is, as we have just seen in relation to fruits and berries, evident from observational studies; it is also evident from experimental work, though this is more limited. Work on Common Myna Acridotheres tristis has, for example, revealed that urban populations display a strong preference for food with a high protein content, favouring this over foods with high lipid and high carbohydrate contents in field-based trials (Machovsky-Capuska et al., 2016). This work suggested that these urban Common Mynas were protein limited, something reinforced by the way in which the birds competed for access to this food.
Two reportedly individually identifiable Blue Tits, visiting a bird feeder in a suburban Belfast garden during winter, were estimated to have obtained 25.36 per cent and 44.47 per cent of their daily energy requirements from peanuts (Fitzpatrick, 1995); comparable figures from the same study for two Great Tits were 16.41 per cent and 22.2 per cent. Although this study doesn’t adequately deal with questions over how the birds could be recognised as individuals from their plumage characteristics, the work has been used to suggest that these suburban individuals were obtaining a significant component of their daily energy requirement from the food supplied. Better-documented studies, this time on provisioned Black-capped Chickadees Poecile atricapillus in Alaska, found that individuals were obtaining up to 29 per cent of their daily energy requirements from the food supplied (Brittingham & Temple, 1992a; Karasov et al., 1992).
Selection for particular food items might also be shaped by how that food is presented. Garden bird species that would feed within trees and shrubs when foraging in the wider countryside – such as the tits – were some of the first to take advantage of seeds and peanuts presented in hanging feeders and mesh cages. The strong feet of these species enable them to grip hold of small perches or mesh, something that ground-feeding species like Robin, Dunnock and Blackbird would find too challenging. These latter species would be more likely to feed from a bird table or to take food from the ground. Changes in feeder design, most notably in the shape and size of feeder perches, can exert a big influence on which birds are then able to feed. The move from a straight perch to an ‘o’ shaped perch seems to have aided Robins to feed from hanging feeders. The advice to those feeding garden birds is to feed a range of foods in a number of different ways, providing opportunities for the different species and their favoured means and locations for taking food.
FIG 27. Like Robin and Wren, the Dunnock prefers to feed on the ground but it will venture onto bird tables and, occasionally, onto hanging feeders. (John Harding)
HOW AND WHEN BIRDS USE GARDEN FEEDERS
As we will see in the following section, the use of garden feeding stations is influenced by food availability over wider areas, from that available in the nearby countryside