by Darryl Jones
Far more prosaically, however, individually marking her chickadees allowed Susan Smith to show that survival over the winter was largely determined by the reliability of the local food supply. Being able to identify known birds confidently at regular intervals throughout the winter and again in the following year allowed careful accounting of the ongoing presence—or sudden disappearance—of each bird. But while Smith could claim that providing additional foods aided chickadee survival through the winter, unfortunately the lack of a “control” site where food was not supplied meant that little could be stated about the relative influence of supplementary foods.
Smith’s studies provided a solid foundation for the next phase of American chickadee supplementary feeding studies. During the 1980s, Margaret Clark Brittingham and Stanley Temple from the University of Wisconsin explicitly addressed the limitations of the earlier work by thoughtful yet straightforward experimental designs.36 In the first of several studies of great importance to our understanding of wild bird feeding, Brittingham and Temple color-banded 576 Black-capped Chickadees in five well-separated study sites in rural Wisconsin.37 Although chickadees are known to be fairly sedentary, remaining close to home year round, it was still remarkable to learn that only two of the almost 600 marked birds were detected more than 2 kilometers from where they were first captured.
Using techniques now familiar to us, chickadees at three of the sites were supplied with a feeder regularly filled with sunflower seeds from October to April, while those at two other sites were offered nothing. This continued for 3 consecutive years except for the crucial variation of stopping the food supply at one feeding site (although, tantalizingly, the empty feeder remained in place) and starting feeding at a site previously without a feeder. As a powerful attempt to understand the impact that the provisioning—and withdrawal—of additional foods may have on the survival of tiny nonmobile birds, this should do it. Their persistence as winter progressed was determined by thorough weekly searches of each site for marked birds and regular captures using mist nets. The detailed information collected allowed monthly survival rates to be calculated as well as year-to-year survival. In addition, astonishingly precise data (to within one-hundredth of a gram) on the body masses of the birds were obtained at the time of capture. Though the procedure is not simple, the painstaking efforts required to handle such a small yet spirited creature were well worth it. Perhaps surprisingly, small birds such a chickadees gain weight over the course of a day as they forage and then lose the weight as they use up their meager stores of fat during the night. The difference in mass at dawn may be a much as 10% less than the night before. For birds weighing only about 12 grams (0.4 ounces), these differences may mean the difference between life and death on a long, freezing night.
It was in the body mass data that the first significant result was noted. At the start of the experiment, males and females were on average slightly lighter (0.17 grams, or 0.006 ounces) than later in the winter, but birds with access to supplementary food were just a little (0.13 grams or 0.005 ounces) heavier than the unfed birds. The differences may seem small, but they appeared to translate into significant survival statistics. Chickadees supplied with additional food had, on average, a 95% chance of surviving through to the next month and a 70% chance of making it all the way through the winter. For the unfed birds, this compares reasonably well, with a monthly survival probability of 87%, but contrasts starkly with an overwinter survival of only 37%. The benefits of having access to supplementary food is even more obvious when monthly survival for fed and unfed chickadees is compared during severe winter conditions (periods of more than five days per month with minimum temperatures less than –18 °C [–0.4 °F]). Without the supplements, about a third of the chickadees disappeared (presumably died) during these cold snaps, while only 7% of the fed birds succumbed.
While these stark numbers may convey something of the scientific significance of the experiment, it is the author’s observations of the behavior of the birds that we find especially compelling. Recall that chickadees are tiny, continually active birds, quite unable to store sufficient fat to last more than a full day without foraging. Yet Brittingham and Temple observed the birds during a period of prolonged extreme cold when the maximum temperature did not rise above –18 °C (–0.4 °F) and the minimum reached –29 °C (–20.2 °F) for over five consecutive days. Although writing in the objective prose required by scientific publications, the authors cannot disguise the reality of what they observed:
During such periods of extreme cold, the behavior of the chickadees on control sites [the nonfed birds] changed. We could not find most individuals, and the few we located were extremely lethargic. They sat motionless, with feathers fluffed out, facing the sun.38
The researchers speculated that this marked inactivity, so unlike the usual demeanor of the species, was their only remaining physical response to the severe cold. To move would expend energy they did not have; by remaining motionless they were conserving the little reserves they retained. Whether this was a strategy that worked would be measured by the brutal calculus of each bird’s fat reserves, the duration of the extreme conditions, and the availability of suitable nutrition once the weather improved. In the frigid midwinter woods of Wisconsin, the odds would not seem favorable.
Unless there was a feeder nearby. With access to an unending supply of sunflower seeds, the behavior of the chickadees lucky enough to live near such a bonanza was shockingly different from that of their hard-pressed and unfed colleagues. Despite the severity of the prolonged bitter conditions, “[fed] birds continued to use the feeders normally.”39 The contrast could hardly be clearer: the additional food made a very real life and death difference to these birds.
These are strong findings, but are not dissimilar to other winter-feeding studies on similar species. For example, overwinter survival of two species we have not yet mentioned, Crested Tits and Willow Tits, studied in Sweden was almost double that of birds without access to supplementary foods.40 It should be noted, however, that some studies that have investigated the effect of additional foods on winter survival have not reported such clear findings.41 Indeed, some experiments report no influence on survival or on any of a long list of the other parameters mentioned above. These studies may be just as important as those reporting dramatic effects and point to the realities of scientific field studies: the results rarely play out as simply as expected. This is a theme we will return to at the close of this chapter.
Changes in Behavior
To conclude this selective review of supplementary feeding experiments on tits and chickadees, we turn to the possible influences on perhaps the most malleable and immediate features of their lives, their behavior. It is one thing to measure physical attributes such as egg size and body mass; it’s a very different matter to ask questions about what the birds are actually doing in response to the additional food.
Given the importance of food to the day-to-day survival of these small birds, particularly during tough conditions, it would hardly be surprising to find feeding can change their social interactions. Although an obviously social group, spending almost all their time in loose groups, tits and chickadees tend to forage alone when seeking their natural diet of insects and seeds. They may be close to the rest of their flock, but the type of food they glean means that it is best obtained without the interference of other birds. During periods of natural food abundance, when there is little competition between individuals, the birds are fairly tolerant of one another’s presence. As conditions become more difficult, as occurs with the start of cooler weather, relationships often become less amicable. Wilson looked at such interactions among her Black-capped Chickadees during the harsh winters in Maine when feeders were experimentally introduced.42 In natural situations, the onset of cold conditions led to increasingly more assertive behaviors by individuals with smaller foraging areas being defended against other chickadees. These patches were established throughout the site, with each bird activel
y patrolling what was now a small but private “foraging territory.” Trespassing was strictly policed, each bird the master of its own modest domain.
The introduction of feeders abruptly changed all that. With the sudden arrival of a super-abundant food bonanza at just a few localized spots, the former system of territorial enforcement broke down entirely. For the birds living close to the feeders, whatever perceptions of outrageous good fortune they may have had soon dissolved in the reality of the inevitable influx of all and sundry to the feeder. Although many of the birds initially attempted to maintain their boundaries, the constant crossing by birds drawn to the feeders soon led to territoriality being given up completely. For these small birds, there was simply no way that an individual could successfully defend “their” feeder. In other situations (discussed elsewhere in this book), usually where the species are much larger and more capable of effective defense, the introduction of feeders can lead to a serious esca-lation of territorial behavior.
Another influence of supplementary foods on foraging behavior relates to joining feeding flocks made up of different species. While it is generally rare for most birds to deliberately seek the company of other species, the formation of groups of similarly sized birds of a variety of species has been reported from around the world. These flocks are temporary and are usually made up of groups of insectivorous species that occur together in the same woodland or forest habitat. Although there is much we don’t know about these strange amalgamations, most ornithologists believe that birds join them as a way of improving their ability to find food in times of scarcity. This idea has been tested by researchers working on tits in the UK,43 as well as yet another parid species, the Varied Tit, in Japan.44 In both cases, providing additional foods led to a significant decline in the tendency of birds to join these mixed-species flocks. Interestingly, birds partaking of the supplementary foods were more likely to forage alone when away from the feeder.
One of the most conspicuous behaviors of songbirds such as tits and chickadees is their vigorous singing, particularly during the early stages of the breeding season. As pleasant as these sounds may be to our ears, the functions of these impassioned displays are complex and deadly serious for the (usually) male participants. While female songbirds also vocal-ize, it is almost always the males that produce most of the early morning noise. While we can never be entirely certain of what these vocalizations mean to the birds themselves, countless studies have established that birds use song for a range of purposes but especially to improve their chances of reproduction. Songs can be about territorial defense, the proclamation of identity and occupation of a space, as well as an invitation to mate. Such sweet melodies (to us) may be adamantly “Keep away” (intended for the ears of other males) as well as a vocal résumé of prowess to be assessed by potential female partners. Yes, both of these general functions can operate at the same time, although once the male singer has successfully attracted a mate, the song’s territorial function becomes his key modus operandi.
The introduction of a feeder into this scenario is likely to have a number of consequences. As we have already described, birds near a rich food source may become far more assertive in their territorial defense as they attempt to repel the inevitable increase in intruders. In plenty of studies, males with territories near feeders physically attacked trespassers and sang more often.45 Fighting and singing are both activities requiring considerable energetic resources and normally take up important time that could be otherwise spent looking for food. With a regular supply close by, males near feeders have the fuel to sing more than those forced to forage naturally. This food-enhanced singing may have real implications because females of both Blue Tits and Black-capped Chickadees have been shown to prefer males that sing earlier in the day and who sing more.46 Indeed, this attraction continues even after pairs have been formed, with Blue Tit offspring not fathered by a female’s mate most likely to be the apparently sexy singer up the road.47 Unsurprisingly then, supplementary feeding has resulted in earlier dawn singing and greater song output in a number of species (including Common Blackbirds in the UK48 and Silvereyes in New Zealand49).
Working within the suburbs of Oslo in Norway, Katja Saggese and colleagues were interested in the possible changes to the vocal behavior of Great Tits associated with supplementary feeding and the possible ongoing long-term effects of feeding.50 These researchers provided ad libitum sunflower seeds and fat balls to their birds and recorded the songs of males either side of dawn, noting the exact time of all relevant events. Great Tits typically start to sing vigorously at least six weeks before egg laying, well before dawn. Would the fed birds start even earlier? Or would the extra food lead to more singing as predicted by previous studies? To the researchers’ considerable surprise, male tits with access to additional foods actually started to sing later than unfed birds. And they did not make up for sleeping in by singing more; their song output was no different from their hungrier colleagues. Again, these are important findings that deserve some consideration. The people who conducted this study were refresh-ingly candid about their results being unexpected, but they did offer some plausible explanations.
Waking well before dawn, by definition, means moving about in virtual darkness, and it has been suggested that attempting to find their insect foods at this time would be very difficult.51 Singing, with the various benefits mentioned above, may actually be a more sensible activity. Birds with access to feeders, on the other hand, would probably be able to feed even in poor light and may be feeding instead of singing. In addition, what of the apparent reproductive advantages of early singing mentioned previously? To ignore aspects such as attracting the attentions of other females suggests that males may be weighing up their options. For example, having access to such a rich and reliable food resource may itself be a powerful inducement. Alternatively, females paired with feeder-owning males may be less inclined to wander. In reality, these are all simply speculations requiring further careful study. Yet again, this nicely executed feeding experiment has delivered intriguing findings that hint at the complexity of the situation. Again we have to conclude that feeding changes things in unexpected ways.
The bulk of information presented in this chapter has focused intentionally on supplementary feeding experiments conducted on tits and chickadees. This somewhat justifies our focus on this group, but there is obviously more to the field than these selective summaries suggest. For example, the significance of supplementary feeding experiments to wildlife management and conservation are such that we devote another chapter to these major themes. To conclude this exploration of supplementary feeding research on garden birds, however, we turn to some key studies of species other than titmice.
Scrutinizing Scrub Jays
One of the more unusual species visiting bird feeders in North America is the Florida Scrub Jay. Although a member of the superabundant and familiar corvid group—the crows, ravens, and (true) magpies—the Florida Scrub Jay is in serious trouble. At home in the sandy, dry scrublands of central Florida, this exceptionally intelligent and highly social species faces habitat losses and degradation due to expanding housing developments, hotels, golf courses, and citrus plantations. As a result, the species has been steadily declining over the past few decades and currently exists in a small number of isolated populations—always a situation that raises the chance of local extinction. Plenty of other species face similar predicaments, and our understanding of their situation is limited. By contrast, we know a lot about the behavior and ecology of Florida Scrub Jays, and our knowledge provides an unusually detailed background for developing sound conservation plans. A critical component of this information is based on numerous supplementary feeding studies.
I described Florida Scrub Jays as “unusual” feeder visitors not just because they are so obviously different from the more typical garden species and their dire conservation status but also because of their remarkable social lives. There cannot be many places in the world where people li
ving in the suburbs play host to a threatened species that also lives in permanent extended families, with all the associated dramas. It was the intriguing communal breeding arrangement of Florida Scrub Jays that led to the pioneering field studies by the late Glen Woolfenden and John Fitzpatrick (now at the Cornell Lab of Ornithology) conducted at the Archbold Biological Station in central Florida. These now famed studies, which commenced in the 1970s, described the remarkably complex social relationships based around the presence and active involvement of individuals other than the breeding pair.52 So-called helpers at the nest, these birds were discovered to be the young from previous years who remained with their parents to assist in the raising of the next batch of young. Woolfenden and Fitzpatrick’s influential research was instrumental in kicking off worldwide interest in the phenomenon now known as “cooperative breeding.”53