What birds eat
Discovering what birds eat really tests your desire to be a birdwatcher-naturalist. The subject, though fascinating, can be extremely complicated and requires enormous patience. However, this should not put you off. There are some comparatively simple things to do. Identifying what plants or animals a bird may be eating may not be too difficult, but the researcher will also want to have an accurate idea about the quantity of each item. As quite often conservation bodies need a scientific evaluation of, for instance, the quantity of food available to birds in the various estuaries around the coast of Britain. They need to know, if barrages are built on the Wash, or if part of Seal Sands are reclaimed, whether there are other estuaries on the east coast of Britain which will be able to supply sufficient food for the displaced birds.
In the 1950s, Dr David Lack and his team at the Edward Grey Institute at Oxford studied the food of tits in nearby Wytham Woods. These studies and the estuarine studies of John Goss-Custard have thrown a new light on the feeding ecology of birds, and whilst the amateur cannot often hope to emulate this work, the published results do show the birdwatcher-naturalist many items which he can look for and record. In spite of the excellence of the work of the professionals there is still much that you can discover for yourself.
Whilst every living creature needs food to keep it alive, a small bird which burns up energy at a furious rate, needs to take about a third of its body weight in food every day to make up for the heat and energy loss. The exact amount depends on the type of food eaten. Large birds need less food because their heat loss is less; being larger creatures they have less surface area in relation to their volume than small birds. At certain times of the year and for certain activities they need to take in extra food. Some need to accumulate extra fat before they migrate and fly long distances; while some males feed their mates before they lay their eggs. Young Manx Shearwaters and similar species put on an enormous amount of fat before they are deserted by their parents and have to learn to fend for themselves.
Birds have managed to exploit almost every habitat and every niche as a hunting ground; they have not been able to dig more than a little way into the ground in search of food, but at sea Long-tailed Ducks regularly dive to 20 metres and can reach 60 metres, but information of the depth at which other diving birds feed is rather limited. Perhaps here is an opportunity for scuba-diving birdwatchers to collect knowledge. Birds hunt in the sky as high as there are insects and other creatures in a sufficient quantity to make it worth their while. To be able to exploit all these niches birds have developed within a general form a huge variety of beak shapes, legs and wing structures. Adaptation to different food sources has probably affected their structure more than any other factor, even the urgent need to escape from predators.
The type of food a bird will eat is influenced by a number of factors, of which the most important is the instinctive hunting methods of the genus. For instance, nearly all flycatchers eat flies and other insects. Most finches eat both animal and vegetable food while crows and gulls are. Generally omnivorous. The second component in the feeding habits are the characteristic specific habits that give a species a measure of ecological isolation. Here, for instance, the Spotted Flycatcher eats insects, chiefly flies, but rarely worms and berries. Although the food of the Pied Flycatchers is also chiefly insects, including flies, it seems to have a wider range of insect food, in particular the Small Pearl-bordered Fritillary Clossiana selena. Some species become highly specific in their eating habits, the slender-billed form of the Nutcracker, although a member of the usually omnivorous crow family, specializes in the seeds of the Arolla Pine Pinus cembra and the Bhutan Pine Pinus excelsa in the more easterly parts of its range.
The structure of the body, particularly its effect on the way the bird moves, may restrict it to a certain type of habitat; the structure of the beak and talons may localize the hunt and finally local experience of the availability and whereabouts of food items will contribute to the success of the hunt. This local knowledge of the food supply is of great importance and is often not taken into account when people who have been caring for sick or injured birds release them into the wild after several weeks or several months of treatment. This particularly applies to recently cleaned seabirds. We tend to think that the shoals of fish on which the auks feed are everywhere. But they move with the seasons. Auks come into the waters near the breeding cliffs when the sand eels and other small fish are near the surface and most abundant, and from daily contact they get to know in which part of the colony’s home waters the fish are to be found. Once a seabird is removed from the sea for any great length of time it will lose touch with the fishing flocks and be considerably handicapped in finding sufficient food when released.
The way in which the beaks, legs or wings have been adapted now most affect the selection of food. Herons and egrets have long legs that enable them to wade deeply into lakes, ponds and even the sea to hunt. Wading birds with their long legs, such as the Bar-tailed Godwit, can also wade deeply in water and still probe beneath the surface and into the mud or sand’ with their long sensitive bills. Another wader, the Avocet sweeps the surface of the mud below water from side to side with its awl-shaped bill.
At the other extreme Goldfinches, Redpolls and their relatives have rather short legs. They tend to perch on rather thin and flexible twigs which bend over so that the bird has to hang on to the twig with its short toes, often nearly upside down. Their short legs help to keep the centre of gravity rather low so that there is less strain on the legs. Short-legged birds also use their feet more commonly in holding food than longer-legged birds. Incidentally, the Goldfinch has a rather thin and pointed bill which enables it to probe quite well for the seeds of a number of the compositae family — dandelions and thistles.
Long pointed wings, or high aspect ratio wings as they are called, are the mark of fast ‘pursuit’ birds, while the broad-winged birds are either those which soar in the rising thermals or which live in woodland areas and need the ability to manoeuvre through branches. Included in the first group are long-winged insect-eaters such as Swifts, Swallows and Nightjars which need some speed to pursue their prey which they catch with their mouths in mid-air. Included in this group are also the falcons which use their talons to catch birds and insects, and occasionally rodents.
The evenly balanced arrangement of claws are important in woodpeckers, as it enables them to get a better grip on the bark of trees on which most of them hunt. Their strong bills are also useful when digging holes beneath the bark for insects or their larvae, or hammering at acorns which they wedge in cracks in branches or trunks. Green Woodpeckers have largely deserted the trees and prefer ant nests on the ground into which they insert their long tongue and, with its sticky tip, extract the ants or their pupae.
Waterfowl, particularly those which swim and dive for their food, have developed webs between their toes: some species may only be partially webbed like the Red-necked Phalarope which is essentially a wader which feeds in the water. At the other extreme, the Shag which hardly ever walks any distance on land has webs between all four toes to drive it through the water when it is hunting for fish amongst the sea wrack. Razorbills and Guillemots propel themselves through the water in their search for fish by beating their wings almost as though they were flying. While the Dipper, an underwater thrush, which hunts for its food amongst the stones in swift-flowing streams, holds out its wings in such a way that the flow of water forces it to the bottom.
The range of different feeding methods and the different food taken is extremely varied and a study of them supports the view that closely related species living in the same habitat do not compete for food. Following this point through there is enormous scope for working even in your own garden or some other habitat well-known to you which is utilized by birds for hunting. What, for instance, are the commonest ways of finding food used by the different species? What sort of hunting perches do the Spotted Flycatchers use? What height are the hunting perches? Do they hunt in the trees or just use part of the tree as a look-out? Do the hunting perches change through the breeding season and does a change indicate a different type of prey? When answering the same sort of questions for other species you may have to list the plants birds use and attempt to determine their use as food or hunting perches throughout the season. Does it change? Do birds hunt in the same place or with the same regularity throughout the day?
It is sometimes impossible to do all this in a professional way in which you can completely document the food species of birds but gradually you should be able to build up a picture of the distribution of birds throughout the habitat.
Casual observations can be useful too. In a wood in which I have been working, up to ten Greenfinches may concentrate in October and November upon the hips of the Field Rose Rosa arvensis. By counting the fruit along a ride at regular intervals I was able to see at what speed the hips were eaten. It would have been possible, if I had spared more time from other studies, to have much clearer information upon the proportion of hips eaten by the Greenfinch, Bullfinch and Marsh Tit. The hips of the Dog Rose Rosa canina were not touched by these three species. So on your walks in autumn it is worthwhile examining seed production on all sorts of plants as soon as they begin to ripen and then watch to see if they disappear, and try to discover what is eating them. I also watched three Greenfinches feeding for a time on the seeds of Queen Anne’s Lace Anthriscus sylvestris but again had no time to discover how many days they continued to do this. On my holidays, too, I have been watching Greenfinches and Linnets feeding on the seeds of the Sea Radish Raphanus maritimus, Hedge Mustard Sisymbrium officinale and Fat Hen Chenopodium album.