October 2018
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.
August 2018
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.
February 2018
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.
October 2017
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.
July 2017
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.
February 2017
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.
August 2016
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.
March 2016
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.
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.
August 2018
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.
February 2018
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.
October 2017
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.
July 2017
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.
February 2017
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.
August 2016
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.
March 2016
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.