The Birds at my Table

by Darryl Jones

  As we have already discussed, the “winter-only” attitude is being challenged vigorously. Proponents of year-round feeding point to the apparent conservation benefits of studies such as that described above as why feeding should be continuous. Here we need to ask whether supplementary feeding experiments can shed light on this particular issue, given the proliferation of year-round feeding.

  The best study to address this question was published under a title of commendable clarity: “Does food supplementation really enhance productivity of breeding birds?”19 The question is fundamental to the conservation claims at the heart of continuous-feeding arguments and a test of the largely accepted assumption that more feeding leads to more chicks. Focusing on Great and Blue Tits using nest boxes, the experiment offered three feeding menus over a 3-year cycle but with the foods used swapping between sites so that the birds had a different diet each year. The key element of this experiment was 500-gram (17-ounce) blocks of protein-rich peanut cake continuously available from about a month before egg laying until up to 2 months after the end of fledging. Depending on the site and year, the birds received either peanut cake, peanut cake with mealworms added after the nestlings had hatched, or nothing (the control). While not strictly “year-round,” the food was supplied during all phases of the breeding cycle, far longer than any other study on tits. Conducted by Tim Harrison, Jim Reynolds, and Graham Martin, the study took place in woodland sites in Worcestershire, central England.20 Given the quantity and quality of the additional food being provided, the researchers were bold but justified in expecting big things: they anticipated that the fed birds would have earlier breeding, larger clutches, shorter incubation periods, enhanced hatching successes, and more chicks per nest. While these predictions tended to be a little optimistic (the effect of additional food has been quite mixed for clutch size, incubation period, hatching success, and brood size in a wide variety of species), very few studies had been as ambitious in the scale of provisioning. As a corollary, few studies were of such potential significance.

  As would be reasonably expected by now, the date of laying was earlier and the length of incubation lower in the fed populations of Great and Blue Tits. Advanced laying dates are now a fairly common finding in experiments of this kind, although reduced incubation times are less so. What was not at all expected was the impact of prolonged supplementary feeding on the key components of reproduction: eggs, hatching success, and resulting brood size. Entirely contrary to the predictions, clutch size and brood size in both species were actually less for the fed birds compared to the unfed. Similarly, the proportion of eggs successfully hatching was lower in the fed Blue Tits, although there was no difference in the Great Tits. Thus, in relation to the question raised in the title of this study, the unavoidable answer was actually: “No, supplementation did not really enhance the productivity of the tits,” although this conclusion relates only to this particular experiment. Nonetheless, the findings are reliable and sound and require careful consideration.

  The results are also potentially alarming and sobering. They were certainly not anticipated by the experienced research team that conducted the experiment. Although one other supplementary feeding study also reported a decline in the number of eggs laid, it was a very different situation: a far larger, long-lived waterbird (the American Coot)—hardly an ideal comparison.21 Where additional foods have affected clutch size, the result tended to be more eggs, but this is one component of breeding that appears to be difficult to alter. Most feeding studies found no change in clutch size, a finding that is even more pronounced for tits, with well over half of supplementary feeding experiments reporting no change in the number of eggs laid.22

  What was far more significant in the Harrison study was the decline in hatching success in Blue Tits and brood size in both species associated with the provision of additional foods. These are unique findings among hundreds of supplementary feeding studies. This means that the extra peanut cake somehow led to a reduction in the proportion of eggs in the clutch producing live chicks compared to the unfed nests. The productivity of bird nests may be assessed at two critical intervals: first, at the time the hatchlings emerge from their eggs (“hatching success”), and finally, as young leave the nest as fledglings (“fledging success”). Between these two key milestones, plenty can go wrong. Starvation, predation, weather, and other factors will normally lead to a difference in the number of chicks that hatch and the number of fledglings that leave the nest. What is important for overall breeding productivity is the proportion of fledglings that survive, and that starts with the size of the brood. To have lower numbers of eggs and then fewer hatchlings is a major early blow, especially if this is due to the parents partaking of what should be of clear benefit to breeding.

  It is important to remind ourselves that these are findings so far limited to one study in a single location over a limited time period. Nonetheless, the research was intentionally designed to assess the possible impact of more or less continuous feeding, as is increasingly the case in urban landscapes. It is striking, therefore, that some of the key findings of the Harrison study—earlier laying dates, smaller clutches, and reduced brood sizes—are remarkably similar to the breeding attributes of Great and Blue Tits living in typical urban environments in Britain. Compared to birds in largely rural landscapes, urban tits lay fewer eggs earlier and raise significantly smaller broods.23 Furthermore, in a comprehensive review of the reproductive parameters of many bird species—including tits—living in urban areas, advanced laying and lower clutch sizes were found to be typical compared to birds living in nonurban areas,24 a result attributed primarily to the greater availability of human-provided foods. This adds just that much more substance to the significance of the study we have been discussing, as well as raising some real concerns.

  What about the Composition of the Food?

  An obvious next step toward understanding the way supplementary foods may be influencing the breeding of birds is to see how characteristics of the food itself may play a role. At a rather basic level, the two fundamental components of foods are fats and proteins. Although both are essential, protein is especially important in preparing the female’s body for the substantial demands of producing a clutch of eggs. A lack of protein—or, more specifically, particular molecules known as essential amino acids— during the early spring can, potentially, greatly limit the capacity of females to form their eggs, possibly resulting in fewer, smaller, or even poor quality eggs. Although these are common effects associated with seasons with low caterpillar numbers (a main source of natural protein), the same response has been attributed to the consumption of fat-rich but protein-poor foods such as peanut cake.25 Indeed, the relative proportion of fat and protein in many commonly used bird feeder foods is decidedly fat biased; the percentage of fat and protein respectively for peanut cake is 70.5% and 17.1%, peanuts 44.5% and 28.7%, and black sunflower seeds 44.4% and 18.0%.26 If birds are consuming more of these human-provided foods than natural insects, their bodies could be misreading the nutritional cues, with serious implications for breeding activities.

  Fats are crucial as short-term energy sources and are almost certainly an important reason for the improved survival over winter for birds using supplementary foods. Most small songbirds, such as tits, however, are unable to store large, complex molecules such as fat in their bodies for any extended times. Instead, the fat-derived energy from all those suet balls and peanuts is utilized fairly promptly. And while winter feeding really does seem to enhance short-term survival, there is also some evidence that fatty diets generally can have longer-term health impacts. We know this from our own waistlines and heart disease statistics. What if this type of provisioning of birds for the tough times was actually detrimental for the birds we are trying to aid? Before we tackle that big question, a more immediate issue is whether supplying fats in winter affects things later in the year when breeding starts. After all, if fats aren’t stored, perhaps there is little or no “carry ov
er.”27

  These issues are, as usual, more complicated than they appear. For one thing, while macronutrients can’t be stored for later use, a number of much simpler molecules known as micronutrients can be sequestered away and released when needed. This is especially important for reproduction in small birds because they are strongly reliant on the nutrients and energy immediately available as they start preparing for breeding. The most important of these micronutrients are calcium (needed for shells and bones), vitamin E, and a group called “carotenoids” (the source of most reds, yellows, and orange colors in animals and plants that can be stored in the fat reserves).28 These molecules are antioxidants and play important roles in protecting the body from various physiological stresses associated with metabolism. The protective functions are increasingly valuable as the demands on the female’s body build up during egg production. Vitamin E and carotenoids are also known to play critical roles as antioxidants in the developing embryos within the eggs, and the deposition of both micronutrients in the yolk has clear benefits for the subsequent body condition of the hatchlings.

  These somewhat technical details are needed as background to discussing the next crucial supplementary feeding experiment. Undertaken by Kate Plummer and colleagues, this particular study is of fundamental importance because it explored the potential carry-over influences of winter feeding on breeding in Blue Tits, with careful attention to the way that the macronutrients (fat) and micronutrients (vitamin E and other carotenoids) affected egg production.29 In other words, the experiment replicated typical winter feeding of small birds throughout the Northern Hemisphere.30 Again, the strength of this research stems from the clever but simple study design employed. The researchers set up three nest box sites in deciduous woodland in the beautiful countryside of Cornwall in the far southwest of England. Three different menus of supplementary foods were used: fat alone (handmade balls of vegetable fat); fat plus vitamin E (added to the ball at the same concentration as that found in peanuts); and, of course, no additional treats at all (the control) as a comparison. These courses were supplied for the winter period only and were stopped a month before the start of the breeding season, well before the birds had even started to think about breeding. Because the key goal was to assess whether the effects of the different winter-feeding regimes carried over into the following seasons, the diet offered to the birds in each study site was swapped each winter over the 3-year study. If there were clear outcomes, these should show up in the eggs laid in each of the sites according to the diet.

  Following on from our discussion of the ability of birds to store nutrients needed for breeding, we would probably expect that the provision of fat alone—so early in the cycle—would not affect egg production, while the addition of vitamin E would probably be useful. The findings of the experiment were, again, unexpected. First, contrary to other similar studies, there was no clear change in laying date. Nor was the number of eggs produced or their relative size affected by any of the diets. What was thoroughly unexpected, however, was that both the amount of yolk per egg and the level of stored carotenoid were significantly lower for the birds consuming the fat-only diet. These components of the eggs are critically important to the growth and development of the embryo and the health of the chick. The yolk contains all the ingredients needed for the construction and maintenance of the growing offspring within the egg. For this reason, the body condition of the mother bird at the time the eggs are formed appears to be of fundamental importance to the subsequent well-being of her young. Therefore, to find that partaking of a fat-heavy diet in winter appears to result in female tits producing impaired yolks—and potentially leading to lower-quality chicks—is of considerable concern.

  But what happened when vitamin E was added to the fat balls? Although theoretically some benefit of the addition of this antioxidant was expected, the outcome observed was still striking: vitamin E appeared to cancel out the effect of the fat, with yolk size in the groups of birds receiving this diet being no different from that of birds eating a natural diet.31 How might this come about? Although the actual biochemical interactions involved were not studied, enough is known about fat and antioxidants to be able to offer plausible explanations as to how this might work.

  As explained earlier, small birds cannot store macronutrients such as fat for long periods: it is a useful form of energy that enhances the physiological challenges of surviving the cold months. Nonetheless, access to readily available, high-fat foods in winter may mean the birds are less likely to be obtaining a more diverse natural diet. This may also mean they have fewer antioxidants, at a time when they are internalizing plenty of polyunsaturated fats, just the sort of harmful molecules the vitamin E and other carotenoids can neutralize. Although a fatty diet in winter would not have directly influenced the formation of yolk much later, the birds’ bodies appeared to be still dealing with the oxidative stress, which affected their ability to form egg yolks. By adding an antioxidant, this stress seemed to be reduced.

  Yes, we certainly need to be careful in extrapolating generalization from a single experiment, however elegant. These are results specific to Blue Tits in Cornwall, and the study has yet to be replicated anywhere else or with other species. Equally, however, it would be sensible to learn from such carefully planned and relevant research that not only looked at a popular feeding practice (providing fat in winter) but may also offer a solution (adding an antioxidant). For tits, at least, this may be as simple as throwing some peanuts in with the suet (although, of course, this still needs verification).

  Feeding and Survival

  Finally, we return to the primary question of the relationship between food and survival in these tiny birds, the key to population regulation as suggested by David Lack, and perhaps the fundamental issue associated with supplementary feeding experiments. Indeed, virtually all the features found to be influenced by the addition of food may be evident in the number of birds remaining in the population in the year following the experiment. If, because of the availability of additional food, more eggs are being laid by healthier females, and if the resulting hatchlings are slightly more likely to make it through to fledgling, and if these are more likely to survive the winter, then it follows that there should be more birds than before the food was provided. Yes, theoretically, but there is plenty that can go wrong too.

  As portrayed by Lack, the harsh realities of winter in the Northern Hemisphere’s temperate zone provide the ultimate test of the population that survives to breed in the following spring. As conditions become increasingly tough and foods scarce, competition for what little is available is inevitable; food supplies will pretty much determine survival. Numerous supplementary feeding studies have assessed this directly, providing additional food and monitoring the changes in bird numbers before and after. In an early study conducted near Lund, Sweden, Great Tits at several sites were either provisioned with lots of sunflower seeds (dispensed in large hoppers) or had to make do with completely natural resources.32 The number of pairs were counted in each site over the two following winters and showed dramatic yet dissimilar results. During the first winter of the study, which happened to be particularly severe, populations of tits without additional foods decreased by at least 10%, whereas the number of birds at the fed sites increased by between 20% and 60%, depending on the site. Clearly, supplementary foods were greatly enhancing the ability of the birds to survive the winter. It also greatly improved the chances of young birds hatched the previous spring to remain through the winter and go on to breed. But the story gets more interesting. In the following year, all populations, whether fed or not, increased dramatically, with numbers of tits being between 35% and 94% higher than the previous winter. This spectacular result clearly had little to do with all the seed laboriously provided by the researchers. Rather, natural events that year entirely overwhelmed the influence of careful experimental design. The winter months in southern Sweden that year were distinctly mild with considerably less snow, enabling the
birds much more opportunity to forage naturally. But far more important, that year the local beech mast crop was large, providing an abundance of easily obtained natural food. In such circumstances, to paraphrase the author of this study: “In a good mast year, there was no effect of supplementary food.”33

  By this stage the attentive reader will have noticed, possibly with some affront, that this discussion of feeding experiments, ostensibly covering the titmice group (the parids), has been entirely preoccupied with European tit species. But what of their close North American relatives, the chickadees, and especially the ubiquitous Black-capped Chickadee? Unfortunately, far less supplementary feeding research has been conducted on these extremely popular little birds, a somewhat surprising situation given that they are almost certainly the most abundant species using feeders throughout the United States and Canada. Black-capped Chickadees are especially conspicuous during winter in the more northern parts of their distribution as they are among the smallest birds to remain during winter and consequently flock to feeders in large numbers. The potential value of human-supplied foods during winter for chickadees has long been recognized, but was most famously studied by Susan Smith in the Massachusetts woods during the 1960s. Professor Smith would continue to investigate every aspect of the behavior and ecology of these birds for the next 30 years,34 but it was her first study of overwinter survival that is of particular importance to this discussion.35

  To estimate the impact of winter feeding on birds, it is important to compare the number of birds before and after. For most studies, this involves attempting to count birds visually using standard methods that ensure that the place, duration, and searching effort are the same each time. For anyone who has watched flocks of chickadees (or most small, active birds) swarming around feeders, then sweeping into the nearby foliage before trickling back in dribs and drabs, making a reliable “count” is, shall we say, challenging. One of the main problems is that we don’t know how often the same birds are being recounted. This inevitably makes comparisons rather sketchy, undermining our ability to make sound claims. The significance of Susan Smith’s remarkable research is that her estimates of numbers before and after winter were unusually robust. She was able to identify individuals among the clouds of otherwise anonymous chickadees because most of the birds had been marked with individually colored leg bands (or “rings”). Although color-banding is thoroughly standard practice for much bird research today, Susan Smith’s ability to recognize specific birds was fairly revolutionary at the time.