Steven and Krista Latta’s article on the population decline of the common nighthawk (Chordeiles minor) is less an article about the nighthawk and more about the American Crow (Corvus brachyrhynchos). Noting hte aforementioned decline in the population of the insectivore, migratory nighthawks, the authors attempt to find a causal link between this decline and predation of nighthawk eggs by crows. Since the crows’ population has recently ballooned in American urban areas, the authors believe this may be a “non-mutually exclusive” cause of the nighthawks’ decline.¹
To test this hypothesis, the authors devised a method that utilized artificial nests of egg clutches distributed on different university roofs, each with a different roofing (stone, large or small gravel, etc.). These were compared against another set of experimental egg clutches distributed in a rural area where, the authors hypothesized, crows would be less likely to eat the eggs. In order to reduce the chance that the crows would memorize the locations of the artificial nests, the researchers placed each new experimental set “on a different rooftop, and a rooftop was only revisited after a minimum 7-day interval between trials” in the urban areas, and moved the clutch locations in the rural area, a forest preserve, without repetition.² They watched each clutch through camera feeds for 72 hours before being inspected. If their eggs were missing, they were considered to be predated.
At the end of their experiment, they found that none of the rural ground clutches had lost eggs to predation, while 44.7% of the urban nests suffered plundering over a 72-hour span.³ Moreover, every act of predation caught on camera was the act of a crow. From these results, the authors draw the conclusion that crows “could limit reproductive success of nighthawks in urban environments.”⁴ These results are certainly striking, and, within the confines of the author’s experimentation, the significant difference in predation between rural and urban areas, with their differing levels of crow activity, they suggest there is some veracity to their claims. However, there are also some significant gaps and inadequacies in their method and in their structural assumptions that inhibit the usefulness of the study.
First, as the author’s admit, the use of artificial nests and surrogate quail eggs is a useful expediency but is nonetheless a sub-optimal compromise method. Because the experiment here concerned a species that lays its eggs on the ground without much nest structure, there could be less of a discrepancy between predation of quail eggs and real eggs than in other studies using similar methods. However, as the authors also admit, their artificial nests lacked any parental defence, which could skew results, especially in areas where the eggs are highly visible and exposed.⁵ This flaw is not necessarily fatal to their conclusions, however, because rural predation was far, far lower than urban predation despite the use of false eggs. The relative difference could, therefore, be considered more significant than the absolute accuracy of the predation rates they found.
Another difficulty with the study is a minor discrepancy in the authors’ presentation of their methods between the urban and rural settings. While the false nest sites for the eggs at the university are described in great detail, down to the density and size of the gravel lining the rooftops, the rural sites are simply presented as being “on the ground.”⁶ Though there is a note that the sites chosen were in accordance with an encyclopedia of American birds, the reader would benefit from a short description of what those sites might be, since they might be quite physically distinct from exposed urban rooftops. This is not so much a methodological as a presentation error, but it leads to an inadequate understanding on the part of (this) reader, which could be remedied in only a few words.
More importantly, the previous flaw points to a more structural problem with the experiment: the lack of a precise knowledge of the differences in crow behaviour in rural and urban areas. The entire premise of the article is that crows are more active and more numerous in urban centres, which is the reason they were identified as a potential cause for population declines in the first place.⁷ When the authors admit that their article is unable to resolve the question of why they saw such a discrepancy in predation between urban and rural areas, it shows the weakness of one of the core assumptions of the study. As Latta and Latta assert, “Differences in abundance of crows between urban and rural sites may also play a role in determining predation rates, though we expected at least some predation in rural areas given the common occurrence of crows there.”⁸
Without more precise knowledge of the differences in crow populations and behavioural patterns in the rural and urban areas under investigation, the authors can only speculate that the urban crow population is higher or more active than the rural. Relative abundance of other food sources and competition from other predators in urban or rural areas and the the greater ease of finding and predating eggs on rooftop nests could all be factors driving the differences the authors see. Because of this, although the results the authors obtained show a strong contrast between the two ecological zones, they do not give the reader a clear grasp of why this might be nor whether crow behaviour is directly causative of this difference.
To conclude, none of these flaws strip away the entire worth of the study, but the inadequacy of their method and some of the vagueness of their assumptions prevents them from deriving effective results from their data. Simply knowing that nighthawks face much higher predator pressure from crows in urban areas is certainly worthwhile. But there is missed potential here. On a broader level, though, this study illustrates the logistical and logical challenges of posing effective ecological questions that can be answered using a set time and budget. And as urban ecologies grow larger and larger, it pays great dividends to understand the nonhuman life-cycles and energy dynamics that pulse within the city. It also reminds us to be mindful and caring for every environment rather than only ones that we can designate as “wild,” since the wild is often happening right above our heads. Figuratively and literally.
1. Steven C. Latta and Krista N. Latta, “Do Urban American Crows (Corvus brachyrhynchos) Contribute to Population Declines of the Common Nighthawk (Chordeiles minor)?” The Wilson Journal of Ornithology, no. 127, vol. 3:529.
2. Ibid, 530.
3. Ibid, 531.
5. Ibid, 532.
6. Ibid, 530.
7. Ibid, 529.
8. Ibid, 532.