Irruptive Migration Research Articles

Community Science Reveals High Diversity of Nectaring Plants Visited by Painted Lady Butterflies (Lepidoptera: Nymphalidae) in California Sage Scrub.

California's sage scrub habitats support a diversity of nectar and host plants for migrating and resident populations of painted lady butterflies (Vanessa cardui) throughout all seasons. North America experiences spring V. cardui migrations involving butterflies totaling in the millions in some years. These irruptive years are thought to be driven by winter weather patterns at breeding grounds near the US-Mexico border and due to their irregularity, it is difficult to study floral resource use along the migration route. Here we used the community science platform iNaturalist to quantify patterns in V. cardui nectar resource use in sage scrub over time and space during irruptive and nonirruptive years. We identified over 329 different nectaring plant species of varying functional types (72% native to California) visited by adult V. cardui, 195 of which had not been previously identified as known nectar plants for V. cardui. Vanessa cardui butterflies were observed in similar locations regardless of whether an irruptive migration occurred, indicating the presence of either sparse migrants or resident populations across California. Moreover, irruptive years were positively correlated with warmer and wetter local conditions at observation locations. Our results provide new insights into patterns of floral resource use by North American V. cardui by harnessing the power of community science data and while highlighting the factors associated with its North American migration.

Community science reveals biennial irruptive migration in the White-breasted Nuthatch (Sitta carolinensis)

Dunn, E., A. Dreelin, P. Heveran, L. Goodrich, D. Potter, A. Florea, B. Ewald, and J. Gyekis. 2022. Community science reveals biennial irruptive migration in the White-breasted Nuthatch (Sitta carolinensis). Journal of Field Ornithology 93(2):2. https://doi.org/10.5751/JFO-00091-930202

Which birds participate in mass concentrations of Bramblings Fringilla montifringilla?\u2014Ring recoveries, biometry, age and sex composition

The Brambling Fringilla montifringilla combines several special features of migration and wintering: differential migration according to age and sex groups, large differences in winter densities as a response to food availability, and flocking behaviour as a response to local mass fructification of the beech Fagus sp. resulting in roosts of several million birds. This study examines (a) whether Bramblings participating in mass concentrations originate from different, or additional, breeding grounds compared with birds present in normal winters, and (b) how differential migration according to age and sex groups combines with irruptive migration and with congregations in mass roosts, i.e., whether the benefits from mass concentrations differ between age and sex groups. Wing biometry (wing length, lengths of outer primaries, and wing pointedness) did not vary geographically, as shown by museum skins, and was therefore not helpful to reveal differences in origin between birds of mass concentrations and normal winters. The extent of the post-juvenile moult was smaller in mass concentration birds and suggests that slightly later-born juveniles, i.e., born further east or north, participate in mass concentrations. Ringing recoveries could not demonstrate a difference in breeding range origin of Bramblings which participate in mass concentrations versus Bramblings in normal winters in Switzerland. However, this may be due to insufficient recoveries of ringed birds. A rough estimate suggests that the number of Bramblings migrating SW on a broad front across Europe could be sufficient to form mass concentrations of up to 10 million birds which are halted by a large area of beech mast. Across Europe, the proportion of juvenile males decreased towards south (i.e., with increasing migration distance) in autumn migrants and wintering birds, while adult females showed the opposite pattern, and adult males and juvenile females were intermediate. Mass concentrations in Switzerland were composed of significantly more adults, especially adult males, and fewer juveniles, particularly juvenile females, while the sex–age composition outside mass concentrations was similar to normal winters. Adults and females likely benefit from wintering further south where snow is less likely to cover food of this ground-feeding finch. On the other hand, the costs of migration are probably substantial in terms of direct costs or indirectly in terms of reaching the breeding grounds, so that juvenile males prefer to winter closer to the breeding area than adults. Feeding in large flocks in beech mast areas incurs the high risk of being cut off from this food by snowfall. Adult males are apparently best adapted to this risk, and hence predominate in mass concentrations.

Pulsed resources at tundra breeding sites affect winter irruptions at temperate latitudes of a top predator, the snowy owl.

Irruptive migration is mostly observed in species specialized on pulsed resources and is thought to be a response to unpredictable changes in food supply. We assessed two alternative hypotheses to explain the periodic winter irruptions of snowy owls Bubo scandiacus every 3-5years in temperate North America: (a) the lack-of-food hypothesis, which states that a crash in small mammal abundance on the Arctic breeding grounds forces owls to move out of the tundra massively to search for food in winter; (b) the breeding-success hypothesis, which states that high abundance of tundra small mammals during the summer allows for high production of young, thus increasing the pool of migrants moving south the following winter. We modeled winter irruptions of snowy owls in relation to summer food resources and geographic location. Winter abundance of owls was obtained from citizen-based surveys from 1994 to 2011 and summer abundance of small mammals was collected in summer at two distant sites in Canada: Bylot Island, NU (eastern High Arctic) and Daring Lake, NWT (central Low Arctic). Winter owl abundance was positively related to prey abundance during the previous summer at both sites and tended to decrease from western to eastern temperate North America. Irruptive migration of snowy owls was therefore best explained by the breeding success hypothesis and was apparently caused by large-scale summer variations in food. Our results, combined with previous findings, suggest that the main determinants of irruptive migration may be species specific even in a guild of apparently similar species.

Complex migration and breeding strategies in an elusive bird species illuminated by genetic and isotopic markers

Unlike the annual bi‐directional movements of over 200 bird species within the Palaearctic–Afrotropical region, irregular movements such as irruptive migration with a low degree of philopatry are reported for a variety of species depending on highly seasonal and unpredictable resources. These flexible movements allow for itinerant breeding – consecutive breeding attempts in two or more geographically different regions during the same annual reproductive cycle. In order to illuminate migratory and breeding strategies of the erratic wetland species Baillon's crake Zapornia pusilla across the W‐Palaearctic–Afrotropical region, we used a set of six DNA microsatellites as well as δ2Hf values of individuals sampled at one African and four European breeding sites. We investigated the degree of genetic population structure within and among different sites and assigned individuals’ feathers of unknown origin to their probable moulting (hence breeding) site using a likelihood approach.We found three genetic clusters, differentiating into one ‘European’ and two ‘African’ populations. Connectivity between the sampling sites was probable as genetic ‘African’ individuals were found in breeding conditions in Europe and vice versa. Likewise, assigned moulting locations based on δ2H isoscapes suggested trans‐continental movements as well as moulting and possibly breeding by the same individual both in African and European breeding grounds. Both isotopic and genetic data reveal the Baillon's crake pursue a complex migration and breeding strategy, allowing as well for irruptive movements and itinerant breeding across the W‐Palaearctic–Afrotropical region. However, a better knowledge about the species’ distribution as well as a more comprehensive data set, including samples from the southern and eastern boundaries of the distribution area would be necessary to improve the spatial resolution to the precision required to unambiguously infer migration directions and extent of exchange between African and European breeding grounds.

Nutritional stress and body condition in the Great Gray Owl (Strix nebulosa) during winter irruptive migrations

The largest irruptive migration of the Great Gray Owl ( Strix nebulosa Forster, 1772) recorded since 1831 occurred in Minnesota, USA, during the winter of 2004–2005. We tested the hypothesis that morphometric indicators of nutritional stress covary with stable isotope signatures in a sample of 265 owls killed by vehicle collisions. The ratio of carbon to nitrogen in muscle (C/Nmuscle) was shown to be a reliable proxy of nutritional stress. δ13C values for liver and muscle were significantly higher in owls in poor condition, reflecting the depletion of lipid reserves in fasting individuals. On the other hand, δ15N values for liver and muscle were marginally lower or unchanged in owls in poor condition. Stomachs of emaciated owls were less likely to contain prey, implying that many nutritionally stressed individuals were too weak to hunt and were near the tipping point of irreversible fasts. In a broader context, sexual differences in the correlative relationships between stable isotope signatures, C/N, and body condition suggest that the consequences of reversed sexual size dimorphism extend to physiological processes during the nonbreeding season.

Modelling irruptions and population dynamics of the great spotted woodpecker – joint effects of density and cone crops

Irruptive migrants are partially migrating species showing pronounced temporal variation in the number of migrants. Occasional irruptions are often explained by increased population density (per area) or lack of food. Similarly to population dynamics, these explanations are not mutually exclusive, but probably act in concert. Here we do a parallel analysis on the migration intensity and population dynamics of an irruptive migratory bird species, the great spotted woodpecker Dendrocopos major, in southern Finland. Both analyses include simultaneous effects of breeding density and cone crops of Scots pine Pinus sylvestris and Norway spruce Picea abies. A novel model for statistical analyses of irruptive migration intensity is developed. Population dynamics are investigated using log‐linear state‐space models. Woodpecker migration intensity is well explained by lack of spruce cones and increases proportionally to population density. Population dynamics is most successfully described with a density dependent model, where the natural logarithm of spruce cone availability previous autumn has a strong positive effect on population growth. This is likely to be due to lowered winter mortality and lowered emigration rates resulting from plentiful food availability. Although the species uses pine cones as a food resource in winter, the estimated impacts of pine cone crops on migration intensity and population dynamics are low, probably due to low annual variation in crop comparing to spruce cones. Large irruptions also tend to occur earlier in season, often before cones are a crucial food resource for the woodpeckers, suggesting that the woodpeckers use the amount of maturing cones as a cue for upcoming conditions. Our results show that the ecological role of different resources can differ considerably from the expected pattern based on the proportion of the resources in a species’ diet. Advantages of examining patterns of bird migration, population density and dynamics in unison are apparent.

‘The Fertile Brain and Inventive Power of Man’: Anthropogenic Factors in the Cessation of Springbok Treks and the Disruption of the Karoo Ecosystem, 1865–1908

The demise of springbok treks, the irruptive migration patterns of the species in South Africa's Karoo region, has long been attributed to the rinderpest epizootic understood to have coincided in both time and space with the last of the great springbok treks. This is incorrect. Instead the cessation of springbok treks can be attributed to a variety of anthropogenic factors. This article first examines and then rejects the case for rinderpest, before introducing alternative causal factors such as the increase in livestock and human populations, the effects of fencing and the double impact of hunting and concomitant drought. These factors, it is argued, acted in concert to effectively remove the conditions necessary for springbok treks and thereby end the phenomenon. It is suggested that the local extinction of this phenomenon – a keystone species and process – is an important and heretofore unconsidered element in the decline of the Karoo ecosystem.

'The Fertile Brain and Inventive Power of Man': Anthropogenic Factors in the Cessation of Springbok Treks and the Disruption of the Karoo Ecosystem, 1865–1908

The demise of springbok treks, the irruptive migration patterns of the species in South Africa's Karoo region, has long been attributed to the rinderpest epizootic understood to have coincided in both time and space with the last of the great springbok treks. This is incorrect. Instead the cessation of springbok treks can be attributed to a variety of anthropogenic factors. This article first examines and then rejects the case for rinderpest, before introducing alternative causal factors such as the increase in livestock and human populations, the effects of fencing and the double impact of hunting and concomitant drought. These factors, it is argued, acted in concert to effectively remove the conditions necessary for springbok treks and thereby end the phenomenon. It is suggested that the local extinction of this phenomenon – a keystone species and process – is an important and heretofore unconsidered element in the decline of the Karoo ecosystem.

Annual Migration Density and Stopover Patterns of Northern Saw-whet Owls (Aegolius acadicus)

Abstract Northern Saw-whet Owls (Aegolius acadicus) exhibit large fluctuations in annual number of individuals migrating in eastern United States. Underlying large differences in the magnitude of the migration may be important density-dependent effects on body condition and stopover patterns. We investigated such effects using data from saw-whet owls that were trapped and banded during autumn migration at the southern tip of the Delmarva Peninsula (near Cape Charles, Virginia) from 1994 to 2000. Irruptive migration events occurred in 1995 and 1999 when 1,002 and 700 owls were captured, respectively. Capture totals ranged from 22 to 105 owls during five nonirruptive years. Irruptive migration years were dominated by immature owls (82% of captures) and were characterized by low recapture rates (13%) and shorter minimum stopover lengths (median = 5 nights) for all owls. Body masses and body-condition index scores were significantly lower for both immature and adult age classes during years when owl density was very high. During such years, a smaller percentage of owls elected to stopover and, among those that did, individuals in good condition departed sooner than individuals in poor condition. Conversely, during nonirruptive years, adults constituted 67% of captures, recapture rates were nearly 3× higher (35%), and stopover lengths were twice as long (median = 10 nights). Recapture rates were significantly higher for adults (43%) than for immature owls (23%) and initial body condition of adults was positively correlated with their length of stopover. During light migration years, conditions may be conducive for extended residency, particularly for adult owls in good condition. Our results suggest that density-dependence is an important mechanism driving migration and stopover patterns of saw-whet owls.

Annual Migration Density and Stopover Patterns of Northern Saw-whet Owls (Aegolius acadicus)

Abstract Northern Saw-whet Owls (Aegolius acadicus) exhibit large fluctuations in annual number of individuals migrating in eastern United States. Underlying large differences in the magnitude of the migration may be important density-dependent effects on body condition and stopover patterns. We investigated such effects using data from saw-whet owls that were trapped and banded during autumn migration at the southern tip of the Delmarva Peninsula (near Cape Charles, Virginia) from 1994 to 2000. Irruptive migration events occurred in 1995 and 1999 when 1,002 and 700 owls were captured, respectively. Capture totals ranged from 22 to 105 owls during five nonirruptive years. Irruptive migration years were dominated by immature owls (82% of captures) and were characterized by low recapture rates (13%) and shorter minimum stopover lengths (median = 5 nights) for all owls. Body masses and body-condition index scores were significantly lower for both immature and adult age classes during years when owl density w...

Irruptive Migration of Common Redpolls

We do not know whether the movements of irruptive migrants are fundamentally different from more conventional migration of birds to their wintering areas because irruptive migration has never been thoroughly described for any species on a continent-wide scale. We use data from a citizen-based monitoring program, Project FeederWatch, to describe the patterns of movements of redpolls (principally Carduelis flammea) across North America in 1993-1994, the winter of a major irruptive migration. Although redpolls moved into new areas during this irruptive migration, the normal wintering range was not completely abandoned. In fact, redpolls were more prevalent in the southern part of their normal wintering range in an irruption year than was typical in non-irruption years. There also was no indication that the majority of the North American population of redpolls moved continuously through the winter: although redpolls peaked in prevalence at different times across the continent, their spring departure was relatively synchronous across the entire continent. Group sizes were typically smaller the later redpolls arrived in a region, also suggesting that the entire redpoll population did not move continuously. In contrast to a non-irruptive migrant, the American Tree Sparrow (Spizella arborea), redpolls arrived at feeders in a given region later but reached peak densities more quickly. Our results suggest that the irruptive migration of redpolls is more allied to conventional winter migration than to nomadism.

Storms affecting Crested TitParus cristatus social organization

In January 1990 and November 1993, a series of heavy storms dramatically reduced the number of locally-surviving Crested Tits in a pine forest near Antwerp (Belgium). The number of individually-marked residents, resighted immediately after the storms, was 2 of 29 (7 %; 1990) and 12 of 78 (15 %; 1993). Both in 1990 and 1993, a number of residents was resighted in adjacent areas, and I argue that their choice to return to the former territory or settle in a vacant one depended on the costs and benefits of homing behaviour in relation to the timing of the storm and territoriality. Since in 1990, all resighted birds were found scattered in a direction perpendicular to the main wind direction, I suggest that storms cause forced dispersal, similar to irruptive migration, which could be triggered by hormone secretion.

Speciation or irruption: the significance of the Darwin finches

Abstract Criticism is made of the conventional use of the Darwin finches in teaching. An alternative, ‘irruption’, theory is presented, to explain the occurrence of the finches in the Galapagos Islands. This postulates that all the species we know today arrived in the archipelago as a mass irruptive migration movement from the north-east. The theory is explained by reference to the European crossbills. In a discussion of the educational role of the theory, it is suggested that we need to make more effort to encourage the creative and critical faculties in advanced work in schools and at undergraduate level. The occasional presentation of alternative theories, and the discussion of their evaluation, are recommended as means to achieving these aims.