The Birds at my Table

by Darryl Jones

  This was a major revelation for us and had both exciting and sobering consequences for our scientific investigations into the foraging ecology of suburban magpies. Our first important—and entirely unexpected— finding came through those careful observations of nestling feeding. As the chicks grew and their apparent hunger became audibly and visibly apparent even to us on the ground beneath, we assumed that the fran-tic and exhausted parents would take advantage of all that free food available nearby. Certainly the adults were visiting the feeding tables more frequently as the breeding season wore on. As explained previously, however, we found that almost all the food supplied to the nestling was natural invertebrate foods.7 Even though they could so easily have ferried bill-loads of sausage and ground beef from the closest feeding tray, most of the baby food was the natural stuff. The human-provided foods were obviously being consumed by the adults, but even for them, it typically made up only a small proportion of their overall diet. This observation too, was unexpected: with so much easily accessible human food around, why didn’t the birds simply forsake foraging for natural items altogether? These were important questions to our limited understanding of how the species was utilizing the available resources.

  And our understanding was undoubtedly limited. Although we had made excellent progress in quantifying the productivity (in terms of measures such as worms per square meter) of different lawn types, sports fields, and farm pastures, and had gathered an enormous amount of data on foraging efficiency and chick provisioning,8 the discovery that many of our birds were exploiting a considerable but unknown source of food entirely additional to their regular diet meant that our estimates of feeding resources were clearly incomplete and inaccurate. Attempting to understand the role of this new—“supplementary”—food source in the lives of our birds was now unavoidable. I had only been superficially aware of the many experimental studies into these issues, but now I really needed to find out more. Indeed, for reasons that will become progressively clearer, understanding the influence of bird feeding really starts with this type of research.

  Food Is Fundamental

  There are few requirements for life as undeniably essential or as influential as food in the lives of all animals. Food is the key to both survival and reproduction. Changes in its availability, quality, and the balance of its components can alter every aspect of their lives. The centrality of food can be seen in the extraordinarily detailed research being conducted into the nutrition of the animals entirely dependent on humans for all their needs. The food requirements of our pets and domesticated species are now understood in minute detail, allowing sophisticated and somewhat disturbing manipulation of such things as muscle development, milk composition, and fertility rates, all via their feed trays.

  For wild animals, we know far less. This is unremarkable: there are a lot of species and they all have different diets. Unlike the economic motivation that propels commercial animal production, the reasons for attempting to learn more about the role of food for wildlife are more likely to be driven by scientific curiosity. Pragmatically, that usually means a lot less funding. Nonetheless, researchers throughout the world have undertaken a vast number of studies on a bewildering array of species from tadpoles to polar bears, though a large effort has been focused on birds. The cumulative findings of these studies have fundamentally changed our knowledge of the roles that food plays, providing many insights into its function in the ecology and behavior of wild animals. As we will see, these apparently basic biological findings have been applied to many practical problems such as wildlife management and the conservation of threatened species.

  Adding a Little Something

  In one of the first comprehensive reviews of the research on supplementary feeding, the Canadian biologist Stan Boutin gets straight to the point: “No one would question that individuals and populations are ultimately limited by food supply.”9 The obvious difficulty is how to assess this food supply accurately. Even if we know what a species is supposed to eat, dealing with the daily and seasonal changes in availability, while accounting for competition and interactions between individuals, makes the task extremely complicated and logistically challenging. Most of the observational studies that attempt to account for overall food supply for a species readily acknowledge the limitations and unavoidable compromises.

  The main alternative approach to straightforward observations is to intentionally manipulate the food available to an animal, usually by the provision of a suitable food that is extra to what would naturally be eaten. The key point is that this provisioning is additional, or “supplementary” to the background food resource. Some describe this as a feeding “subsidy,” something supplied that, by definition, shifts the food supply to above that of natural levels. From a scientific perspective, such manipulations are often part of experiments designed to test hypotheses and provide potential explanations about the way food affects populations of wild animals. One of the most fruitful and enduring sources of such hypotheses was the research conducted by the famous British ornithologist David Lack, which he collated in two now classic books, The Natural Regulation of Animal Populations (1954) and Ecological Adaptations for Breeding in Birds (1968). Among many influential explanations for the cycles and dynamics seen in wild species was his suggestion that the fluctuations in the sizes of populations of many small animals were strongly reliant on the food supply available during the winter, the most taxing season. Lack’s ideas have influenced generations of ornithologists, but the impact of Adaptations was especially pronounced. In 1969, the British Ecological Society brought together researchers to discuss the role of food for animal populations, with one outcome being an increasing awareness of the need for manipulation of food supplies in order to test ideas.10 One of the powerful conclusions of this important meeting was that observations alone, no matter how detailed, cannot provide the explanatory power of a well-designed experiment.

  Lack’s ideas were especially linked to two of the most familiar, abundant, and well-studied species in Europe, the Great and Blue Tit. Along with the very similar chickadees of North America, tits (from “titmice,” a general name for all members of the family Paridae) usually do not migrate and so are most vulnerable during the cold months in the temperate zone when winter foods are hard to find. Usefully, these small birds readily take to artificial nest boxes, which greatly assists researchers studying their reproduction. Both tits and chickadees also happily partake of human-supplied foods, another attribute of an ideal study species. Unsurprisingly, almost every aspect of the lives of these confident little birds has been carefully studied, including the role and influence of food. While a great array of species has been included in supplementary feeding experiments around the world, the prominence of tits and chickadees within this research field argues that we should pay them careful attention. That and the fact that these are probably the commonest visitors to bird feeders in the world. I think that we can legitimately nominate the “tit-adees” group as being eminently suitable representatives of the world’s garden birds and spend some time reviewing what researchers have discovered.

  Feeding Tits for Science

  In and around the town of Ghent in Belgium, researchers have been studying the breeding of Great and Blue Tits using nest boxes first erected in 1959. Since those early days the number and diversity of locations with nest boxes have steadily increased until ten study sites containing over 800 nest boxes were being monitored. These sites spanned the general habitats typical of Western Europe: parklands in urban areas, suburbs with various deciduous tree species, and rural lands with mixtures of oak, beech, and pine. In the late 1980s, a group of Belgian scientists led by a young André Dhondt (remember the name) compiled almost two decades of detailed breeding information from these tits.11 The researchers were especially interested in following up an observation made by other European ornithologists: that Great Tits living in city gardens tended to lay their first eggs of the season somewhat earlier than those nesting in nearby
woods. The long-term information from Ghent allowed Dhondt and his colleagues to quantify these differences with some precision: across the sites, Great Tits in the urban parks started breeding an average of more than 10 days earlier than those in nearby rural sites. This was a remarkable difference. The effect was far less clear for Blue Tits, however, with the smaller species being later in the countryside but less pronounced in the other habitat types. The difference between the species and the habitats, the researchers speculated, was almost certainly due to the relative amounts of food being supplied by people; feeders were more often associated with urban gardens than were found outside the towns. Understanding whether this was the case would require experiments. As we will see, these were already under way.

  At about the same time, ecologists working along the much drier and warmer Mediterranean coast of southern France were interested in the reasons influencing the start of breeding in the same two species, Great and Blue Tits.12 Starting the breeding cycle early would appear to have obvious benefits to the parents: it allows more time to raise and fledge the nestlings, and possibly even start another brood. Earlier broods are also more likely to survive compared to later hatchlings, an outcome associated with having more time to find their main baby food of caterpillars, the most important source of natural protein for tits and many other small birds. Counting your chicks before the eggs are even laid is, however, risky. Females must be in a suitable physiological condition following winter before they can start the extremely taxing task of producing a clutch of eggs, and that would appear to be closely tied to the availability of suitable food resources leading up to the breeding period. To ensure that their females were in the best possible condition the French researchers decided to provide additional—supplementary—supplies from midwinter, long before the actual period in which the females started to develop their eggs. In addition, they altered the diet over time: in January, the feeding stations were provisioned with sunflowers and margarine, and a month later, dried insects were added. Then, about a month before the typical laying times, live insect food (mealworms, commonly fed and commercially available beetle larvae) was also supplied, to provide additional protein and energy for females during egg formation. All of these dietary additions stopped with the appearance of the first eggs so as not to influence other phases of the breeding cycle. Another (control) population within the same habitat but without the benefits of all that additional food was also carefully monitored.

  By the end of this 2-year experiment, the local tits had consumed a total of 120 kilograms (265 pounds) of sunflowers, 8 kilograms (17 pounds) of margarine, and 19 kilograms (42 pounds) of mealworms. The main result of all that additional, high-quality food was an advance of about 6 days in the earliest eggs being laid by the Blue Tits in the study population. Not so for the Great Tits, however; there was a slight advance, but this was not statistically different from the dates of the unfed birds nearby. These results from the Great Tits are possibly even more surprising than those from the expected effect seen in the smaller species: despite all the additional foods (and both species really did consume most of what was supplied), the Great Tits did not respond in terms of when to lay. The reasons for such perhaps mixed findings were not understood with clarity but seem to indicate that food resources are only some of the cues that stimulate these birds to start breeding. Ornithologists have long argued that a complex suite of triggers may be involved, including changing day length (photo-period), air temperature, female body condition, innate genetic predispo-sitions, and even the availability of the artificial nest boxes. While these arguments have continued, the general importance of food in the overall breeding equation is clearly acknowledged as one of the main actors in a play with a sizeable cast.

  Although similarly mixed results are standard fare for experiments involving birds in nature, it is still possible to claim that for the majority of supplementary feeding studies of titmice (and indeed, most birds),13 some advance in laying date is a typical and expected outcome. This may suggest that birds breed earlier if they have extra food at critical phases of their lives (such as when they are engaged in egg production). However, it is also important to point out that almost all experimental studies report variable responses with some fed birds breeding much earlier than others. In addition, there is often considerable overlap in the timing of laying dates among the fed and unfed birds.14 This rather typical finding may be due to the supplied food being insufficient in quality or possibly of little importance relative to the feeding opportunities available to the birds naturally. Don’t forget that the foods being provisioned are, by definition, supplementary to the bird’s regular diet. If the supplies of natural foods that the birds use are already high, the additional supplies may be simply less attractive—and less important in the broader scheme of things. This would indicate that there may be some important threshold value of food resources for many birds, which will result in a strong reaction (such as breeding much earlier) for those living below the level, but a much more muted reaction for those living above. In other words, if there is already enough food, birds may use different cues to begin breeding.15

  Does Feeding Change the Timing of Breeding?

  When to start breeding is among the most critical decisions any animal can make, but this is especially important for small birds like tits, which are only likely to have a few reproductive opportunities during their brief lives. As already mentioned, starting early has crucial benefits. For example, hatchlings born earlier in the season tend to grow more rapidly, and being in better condition than later-hatched chicks, they are more likely to survive and breed in the following year.16 In contrast, chicks hatching later in the season are often less vigorous and already disadvantaged as they must compete with well-developed young that fledged before they did. The advantages of early hatching are, however, strongly reliant on their arrival in the nest from the egg coinciding with the peak in caterpillar supply, the primary source of nestling nutrition. This peak is typically measured in days, and so any situation that causes the birds to miss it, such as a period of poor weather conditions—or mistiming by the parents— can have catastrophic consequences. If birds respond to the manipulation of their food resources by breeding too many days earlier than normal, they could find themselves out of synch with the baby food supplies. This may be a legitimate concern for bird feeders: missing the peak by laying too early because the parents have responded to an enhanced food supply could potentially mean that feeding stations actually disadvantage the local birds. If so, this is an issue of fundamental importance.

  Although attitudes—and organizational promotions—as to when to feed are certainly changing (as explored in Chapter 3), most wild bird feeding across the Northern Hemisphere has been typically limited to the winter months. Traditionally, this is probably based on the perception that birds may need some help to survive the tough times but that “it is better that they look after themselves” at other times. Because of the emphasis on investigating the role of food on breeding, a large majority of supplementary feeding experiments have supplied additional food during the early breeding season, and then assessed the effects on the birds’ reproduction that followed. But most of these experiments do not replicate what happens when people feed birds. Does winter feeding actually influence the breeding activities of birds in the following breeding season? Do the effects of feeding carry over into the future?

  Gillian Robb, under the guidance of Stuart Bearhop and Jon Blount, explicitly addressed this question in an important field study of Blue Tits in Northern Ireland.17 This study was also significant in that it took a broad landscape-level approach in an attempt to more closely resemble the way feeders are distributed throughout the landscape. Ten separate woodland blocks already established with nest boxes were liberally supplied with peanuts (a commonly used bird food in this region) throughout the winter in one year but not the following year. Crucially, all feeding stopped at least 6 weeks before the first eggs were laid. This ensu
red that none of the female tits—fed or unfed—was influenced by food supply as they began to form their eggs, although their body condition was undoubtedly enhanced during the year of the study. The results were unexpectedly clear: feeding advanced the average laying date by 2.5 days compared to the nonfeeding sites, and, of particular significance, feeding resulted in more offspring being successfully fledged. Overall, feeding led to about one additional fledgling per nest, a remarkable outcome because this was not associated with any increase in the number of eggs layed or of hatchlings raised, both of which were unaffected. The extra fledglings resulted from their better survival compared to the unfed sites, yet this was obviously not related directly to additional food supply. The researchers put this effect down to the relatively higher body condition of the parents; healthier adults raise more kids. Winter feeding appeared, therefore, to lead to both earlier breeding and better survival of chicks. Given that we already know that feeding also improves the likelihood of survival of adults through the winter, these researchers also mention some potential implications of their work.18 For example, places where winter feeding occurs may be producing both more new birds (offspring) and better surviving older birds (their parents) than places without feeders. This in turn may mean that the areas with reliable feeders are likely to support higher densities of longer-lived birds. This may be regarded as a welcome and possibly unexpected by-product of feeding in winter. For many species, being able to increase their reproductive output is certainly welcome news. But what about the still widespread perspective that we should be leaving the birds to themselves at other times?