Since our last update, we completed our 41st and 42nd field seasons in summers 2022 and 2023. During these seasons, Olivia Pletcher transitioned to take over much of the day-to-day operations, coordinating nest checks at different colonies, leaving Charles Brown to focus on observations of foraging birds and collecting data on colony sizes. A major paper appeared in 2022 reporting the degree to which individual cliff swallows tend to occupy colonies of roughly similar size over their lifetimes, an assumption that has been important to much of what we have done over the years.
The summer of 2022 was hot and dry, as had been several of the preceding years, and reproductive success was fairly low. However, 2023 was unlike any recent season: mild and wet. This led to very high reproductive success for almost all birds in all colonies, and most notably the weather was the apparent cause of more late-nesting than we have ever seen in western Nebraska. Dozens of colonies began nesting late after the main group of birds there finished. We typically see this at a few sites in most years, but the late-nesting this season was widespread. Late colonies are usually smaller than earlier colonies, and this held true to some degree in 2023; however, we had one late colony (still feeding babies in mid-August) that was about 1400 nests in size, large by any standard! We think that the mild and wet conditions led to large insect populations, and cliff swallows responded to the greater food abundance with higher reproductive success and more late nesting. We are measuring how selection on colony size fluctuates annually in response to environmental conditions, so having an "extreme" year like 2023 is great for the natural selection study.
Also during 2023, Charles began work on a book on cliff swallows, detailing the history of the project and summarizing many of the findings. It is being done in memoir format, showing how scientific thinking at the time influenced each part of the project, and how one result led to additional questions and more research.
We completed our 40th field season in summer 2021. It was a difficult year in some ways, with the deaths of both Paul Johnsgard and Bernie Beckius during the summer. Paul was an internationally known ornithologist who was instrumental in my selecting Cedar Point as a study site in the first place back in 1981; a memorial piece for him has been uploaded in the Publications section of this web site. Bernie was a life-long rancher who lived near some of our study colonies, and during many of the years we would often see him and wave (and occasionally chat) almost daily. Bernie represented everything that is good about the residents of western Nebraska, some of whom we now count as good friends.
Scientifically, the 2021 field season was in many ways a carbon copy of 2020. The summer was hot and dry, and the birds' reproductive success was once again low in general. We continued to see a hint of later birds doing slightly better, perhaps because they were able to avoid the worst of the summer heat, which in 2021 occurred in the middle of June, surprisingly.
We completed our 39th field season on 8 August 2020 despite limitations on personnel due to pandemic restrictions this summer. Our work on fluctuating selection on colony size continued, and we managed to collect reproductive success data on birds at over 50 colonies. The season was hot and dry, and reproductive success was much lower than in 2019. We believe this reflected lack of food, as flying insects seemed reduced in abundance during the hot and dry conditions. Paradoxically, some of the later colonies active in July this year seemed to do best, partly because the hottest weather of the summer was in June. This may also reflect fluctuating selection on laying date.
The project's latest paper to appear, in Biology Letters, reports that cliff swallows that die during severe weather and that live in larger colonies have smaller brains than birds that succumb due to other causes. This study was made possible by our specimen collection that consists of about 2500 birds that we found dead over the last 38 years, these birds having died during bad weather, hit by cars, or for other reasons. Gigi Wagnon did this analysis as part of her Masters thesis. The results seem to suggest that cliff swallows faced with more unpredictable and challenging environments, such as when food is scarce during bad weather or in small colonies where one cannot rely on other colony members to find food or avoid predators, have an advantage if they have the greater cognitive skills inherent in having larger brains. More frequent spells of bad weather, as predicted under climate change, may select for bigger brains, on average, going forward.
The project lost one of its founders on 24 August 2019, when Mary Bomberger Brown died in Lincoln, Nebraska, after a long illness. Mary was involved in all aspects of field work for the first 25 years and was also responsible for all data entry and management during that time. After she left the research in 2007, she remained an active collaborator on papers that continued to result from the previously collected data. Her expertise on cliff swallows was extensive and will be greatly missed.
The 2019 field season ended on 16 August 2019, and was one of the longest in the history of the project. Cold and very wet weather throughout the Great Plains during May delayed nesting until early June, and also resulted in a mortality event in mid-May when some cliff swallows starved to death during a 4-day period of inclement weather, much like we saw in 2017 (see news entry below). Once nesting began, reproductive success was high across all colonies and virtually all laying dates. Swallows that attempted late nesting in July, which leads to nest failure most years, were successful this year, with many nests with young active until mid-August. Successful late nesting on this scale was unprecedented in the history of the project. We suspect that the very wet conditions in May and June led to unusually abundant food resources that allowed the late birds to be successful. Highly variable conditions such as these from year to year likely drive fluctuating selection in cliff swallows, and one of our main objectives at present is to understand how fluctuating conditions favor different colony sizes and laying dates.
Two publications appeared dealing with natural selection in cliff swallows during severe climatic events. One, in Ecosphere, documented that during the 1996 mass mortality event birds living in the smallest and largest colonies survived better over the 6-day period of bad weather than did swallows residing in intermediate-sized colonies. The results suggest disruptive selection on colony size, favoring the two extreme group sizes. We have rarely seen this form of selection in other contexts. Unusual weather events thus may select for group sizes in novel ways, and such selection can maintain the range of colony sizes we see in nature. The other paper, in the Wilson Journal of Ornithology, looked at two weather-related mortality events in 2004 and 2017, and examined whether natural selection on morphology was the same as in the 1996 catastrophe. In 2004, selection on wing, tail, and skeletal measures was generally the same as in 1996, but the weather event in 2017 showed reverse patterns of selection, with skeletally smaller birds favored. Both papers suggest that selection can fluctuate enough during rare climatic events to influence stasis in both behavior and morphology.
The 2018 field season was an outstanding success, with progress made on four major research themes that included (1) continued collection of data on cliff swallow reproductive success at 47 different colonies (a new record for number of colonies studied!) to examine the importance of fluctuating selection on colony size; (2) continuation of the fumigation experiments to measure the effect of swallow bugs on cliff swallow nesting success, as a comparison to similar data collected in the 1980’s; (3) continuation of personality studies on cliff swallows to measure how risk-taking and boldness varies among birds in different sized colonies; and (4) studies of foraging ecology, both behavior and diet, to investigate how the birds’ social foraging strategies may have changed since the 1980’s. We continue to be amazed at the differences in cliff swallow nesting ecology that we see now compared to the 1980’s. This year was unusual in that it was both warm and wet, a climatic combination that is rare in our study area. The birds probably had better than average food resources, and this may have been the reason why we saw a much larger incidence in late nesting attempts in 2018 than usual. It appears that this year's late nests were also more successful than is typical. We speculate that while late nesting is usually unsuccessful, the occasional "good" year like this one may result in enough fitness gain to select for birds who try to nest late. A number of birds appeared to have done true "second" broods this year, that is, attempted a second clutch in the same nest where they fledged babies earlier in the season.
A paper in the Journal of Evolutionary Biology was published, reporting analyses of survival in primarily nestling birds from the 1980's, in which we found that ectoparasitic swallow bugs apparently drive stabilizing selection on intermediate nestling body mass and brood size in cliff swallows. In the presence of bugs, nestlings reared in the more common brood sizes of 3-4 nestlings and with intermediate measures of body mass were more likely to survive their first year (to return the next summer) than were nestlings reared in larger or smaller broods or ones of heavier mass. In contrast, when bugs were experimentally removed by fumigation, the heaviest nestlings survived best and brood size had no effect on survival. The results are the first to show an effect of an ectoparasite on the evolution of avian brood and body size.
A paper in Animal Behaviour was published, reporting the results of a large-scale analysis of breeding-site fidelity in cliff swallows. We used data on over 45,000 individuals re-caught in successive years to study factors causing birds to return to their previous year's site or disperse elsewhere. Fidelity to the past year's site is affected by multiple variables, but the principal conclusion was that birds were more likely to return to last year's colony site if the colony there was similar in size to the previous year's, and if the site had been fumigated to remove swallow bugs.
The 2017 field season ended successfully on 22 July. In contrast to last summer, cliff swallow reproductive success across the study area was relatively high, with only a few colonies exhibiting complete failure (like in 2016, for unknown reasons). The work this summer focused on three separate projects: studying reproductive success at different sites as part of our long-term study of fluctuating selection, repeating the 1980's-era fumigation experiments to study potential changes in the birds' tolerance to parasites, and a new study measuring neophobia and risk-taking (as personality components) in cliff swallows. We continued to document changes in foraging ecology since the 1980's, by taking nestling diet samples, quantifying foraging group sizes, and scoring the frequency of food deliveries to nestlings. Evidence is accumulating that the birds' foraging ecology has changed considerably over the last 30 years.
In May 2017, a 4-day period of cold and rainy weather led to another weather-related mortality event among adult cliff swallows, the first since 2004. Mortality was relatively limited. Although we picked up about 50 individuals that had died, estimated colony sizes later in the summer showed no dramatic demographic effect of the kill. We caught some living adults immediately after the cold weather and measured their wings, tails, legs, and bills to compare to those that died. This will reveal whether there was selection on body size during the weather event, and if so, was it in the same direction as we found in 1996. Results have not yet been analyzed.
A nice feature on the cliff swallow project, by Conor Gearin, appeared in issue 4 of The New Territory magazine. Conor's article focused on the 1996 weather kill and its consequences for the rapid evolution of morphology in cliff swallows.
The revision of the 1995 cliff swallow account in the Birds of North America series was published, after an extensive revision and updating. It had been badly out of date, given the many publications on cliff swallows since the first version.
Collaborator Mary Bomberger Brown, along with co-authors Allison Johnson (a former research assistant on this project) and Jonathan Mitchell, published a paper on convergent evolution in social swallows in Ecology and Evolution. The paper reports results of analyses showing that solitary species of swallows show highly variable morphology, while more social swallows show much less absolute variance in morphological traits. The physical traits seen in the social species (such as cliff swallows) lead to a distinctive flight style that increases maneuverability and foraging success and reduces the likelihood of in-flight collisions within large flocks. Selection for these traits has been less intense in solitary species, potentially explaining their more variable morphology.
The summer 2016 nesting season in Nebraska was marked by extensive nest failures at colonies throughout the study area. We are unsure of the cause(s) of the reduced reproductive success. Colonies where some nests were fumigated and others not showed higher success among fumigated nests, consistent with swallow bugs’ being responsible for the nesting failures. Yet at those very sites, bug numbers were low and did not approach the same levels of past years when birds were successful. We think the hotter than normal season may have contributed in part to the nesting failures, as nightly low temperatures were much warmer than usual this summer. This may have affected availability of flying insects, and the reduced food supply (plus additive effects of bugs) may have been enough to depress cliff swallow reproductive success. We predict a demographic consequence of these nesting failures: fewer breeding birds in 2017.
The swallow project had a high-profile publication appear in the Proceedings of the National Academy of Sciences. This paper reported the results of our 30-year mark-recapture study of cliff swallows to show that the annual survival advantages of different colony sizes fluctuated among years. Colony size was under both stabilizing and directional selection in different years and reversals in the sign of directional selection regularly occurred. Directional selection was predicted in part by drought conditions: birds in larger colonies tended to be favored in cooler and wetter years and birds in smaller colonies in hotter and drier years. Oscillating selection on colony size likely reflected annual differences in food availability and the consequent importance of information transfer, and/or the level of ectoparasitism, with the net benefit of sociality varying under these different conditions. Across years, there was no net directional change in selection on colony size. The wide range in cliff swallow group size is probably maintained by fluctuating survival selection and represents the first known case in which fitness advantages of different group sizes regularly oscillate over time in a natural vertebrate population. The results have broad significance because most animal groups vary extensively in size, but why wide group-size variation persists in most natural populations remains unexplained.