Monarch butterflies cross the Appalachians from the west to recolonize the east coast of North America

Each spring, millions of monarch butterflies (Danaus plexippus) migrate from overwintering sites in Mexico to recolonize eastern North America. However, few monarchs are found along the east coast of the USA until mid-summer. Brower (Brower, L. P. 1996 J. Exp. Biol. 199, 93–103.) proposed that east coast recolonization is accomplished by individuals migrating from the west over the Appalachians, but to date no evidence exists to support this hypothesis. We used hydrogen (δD) and carbon (δ¹³C) stable isotope measurements to estimate natal origins of 90 monarchs sampled from 17 sites along the eastern United States coast. We found the majority of monarchs (88%) originated in the mid-west and Great Lakes regions, providing, to our knowledge, the first direct evidence that second generation monarchs born in June complete a (trans-) longitudinal migration across the Appalachian mountains. The remaining individuals (12%) originated from parents that migrated directly from the Gulf coast during early spring. Our results provide evidence of a west to east longitudinal migration and provide additional rationale for conserving east coast populations by identifying breeding sources.     

Spring remigration of the monarch butterfly, Danaus plexippus (Lepidoptera: Nymphalidae) in north-central Florida: estimating population parameters using mark-recapture

Monarch butterflies [Danaus plexippus) of the eastern North American population migrate each fall from die northern U.S.A. and southern Canada to overwintering sites in Mexico and return the following spring to the southeastern U.S.A. where they lay eggs and then die. The spring remigration is the least studied phase in the annual migration cycle. We therefore conducted a mark-recapture study and examined population recolonization dynamics and residence time in a north-central Florida pasture where the monarch's milkweed host plant (Asclepias humistrata) was abundant. Beginning in late March 1995 two waves of monarchs arrived, their numbers peaked at 71 individuals by mid-April, and the butterflies disappeared in early May. After arriving, the adults remained for 3–5 days, laid eggs and then continued to migrate. We also compared population sizes and arrival times in 1994 and 1996. We found no evidence of a second spring generation, which was also consistent with the deteriorating quality of the A. humistrata plants. Individuals of the new spring generation disappear shordy after eclosion. The arriving population was approximately nine times greater in 1995 than in 1996. Our findings support two recent hypodieses: (1) the bird-like migration of the monarch butterfly in North America evolved with the northward expansion and phenology of milkweeds; and (2) monarchs appear to be migratory throughout their annual cycle of several generations. By lingering for only a short time at each milkweed patch they encounter, the old monarchs returning from Mexico locate the resurgent milkweed flora over an extensive area in the southern states. Then, within less than a month, their fresh offspring continue the migration and exploit the unfolding cornucopia of milkweeds as the spring advances northward. The more we discover about the biology of this insect, the more remarkable is its annual migratory, breeding and overwintering cycle.     

Patterns of parasitism in monarch butterflies during the breeding season in eastern North America

1. Migratory behaviour can result in reduced prevalence of pathogens in host populations. Two hypotheses have been proposed to explain this relationship: (i) ‘migratory escape’, where migrants benefit from escaping pathogen accumulation in contaminated environments; and (ii) ‘migratory culling’, where the selective removal of infected individuals occurs during migration. 2. In the host–parasite system between the monarch butterfly (Danaus plexippus Linn.) and its obligate protozoan parasite Ophryocystis elektroscirrha (OE), there is evidence to support both hypotheses, particularly during the monarchs' autumn migration. However, these processes can operate simultaneously and could vary throughout the monarchs' annual migratory cycle. Assessing the relative strength for each hypothesis has not previously been done. 3. To evaluate both hypotheses, parasite infection prevalence was examined in monarchs sampled in eastern North America during April–September, and stable isotopes (δ²H, δ¹³C) were used to estimate natal origin and infer migration distance. There was stronger support for the migratory escape hypothesis, wherein infection prevalence increased over the breeding season and was higher at southern latitudes, where the breeding season tends to be longer compared with northern latitudes. Little support was found for the migratory culling hypothesis, as infection prevalence was similar whether monarchs travelled shorter or longer distances. 4. These results suggest that migration allows individuals to escape parasites not only during the autumn, as shown in previous work, but during the monarchs' spring and summer movements when they recolonise the breeding range. These results imply a potential fitness advantage to monarchs that migrate further north to exploit parasite‐free habitats.     

Breeding latitude and timing of spring migration in songbirds crossing the Gulf of Mexico

Each spring, millions of songbirds migrate across the Gulf of Mexico on their way to breeding sites in North America. Data from radar and migration monitoring stations have revealed broad patterns in the spatial and temporal course of trans-Gulf migration. Unfortunately, we have limited information on where these birds have previously spent the winter and where they are migrating to breed. Here we measure stable-hydrogen isotopes in feathers (δD_f) to infer the breeding latitude of five species of songbirds – hooded warblers Wilsonia citrina , American redstarts Setophaga ruticilla , black-and-white warblers Mniotilta varia , ovenbirds Seiurus aurocapilla , and northern waterthrushes S. noveboracensis – that were captured at a stopover site along the coast of southwestern Louisiana in spring 2004. Values of δD_f across all species ranged from −163 to −35‰ (n=212), and within most species the range was consistent with the latitudinal extent of known breeding sites in central and eastern North America. Individuals that arrived first along the northern Gulf coast had δD_f values indicative of southerly breeding sites in hooded warblers, American redstarts, black-and-white warblers, and ovenbirds, but no relationship was found between passage timing and δD_f for northern waterthrushes. Our findings suggest that spring passage is often timed to coincide with the emergence of suitable conditions on breeding areas, with southern breeding birds migrating first.     

Lack of genetic differentiation between monarch butterflies with divergent migration destinations

Monarch butterflies are best known for their spectacular annual migration from eastern North America to Mexico. Monarchs also occur in the North American states west of the Rocky Mountains, from where they fly shorter distances to the California Coast. Whether eastern and western North American monarchs form one genetic population or are genetically differentiated remains hotly debated, and resolution of this debate is essential to understand monarch migration patterns and to protect this iconic insect species. We studied the genetic structure of North American migratory monarch populations, as well as nonmigratory populations in Hawaii and New Zealand. Our results show that eastern and western migratory monarchs form one admixed population and that monarchs from Hawaii and New Zealand have genetically diverged from North American butterflies. These findings suggest that eastern and western monarch butterflies maintain their divergent migrations despite genetic mixing. The finding that eastern and western monarchs form one genetic population also suggests that the conservation of overwintering sites in Mexico is crucial for the protection of monarchs in both eastern and western North America.     

Ecological niches in sequential generations of eastern North American monarch butterflies (Lepidoptera: Danaidae): the ecology of migration and likely climate change implications

Eastern North American monarch butterflies (Danaus plexippus L.) show a series of range shifts during their breeding season. Using ecological niche modeling, we studied the environmental context of these shifts by identifying the ecological conditions that monarchs use in successive summer months. Monarchs use a consistent ecological regimen through the summer, but these conditions contrast strikingly with those used during the winter. Hence, monarchs exhibit niche-following among sequential breeding generations but niche-switching between the breeding and overwintering stages of their annual cycle. We projected their breeding ecological niche onto monthly future climate scenarios, which indicated northward shifts, particularly at the northern extreme of their summer movements, over the next 50 yrs; if both monarchs and their milkweed host plants cannot track these changing climates, monarchs could lose distributional area during critical breeding months.     

Regional climate on the breeding grounds predicts variation in the natal origin of monarch butterflies overwintering in Mexico over 38 years

Addressing population declines of migratory insects requires linking populations across different portions of the annual cycle and understanding the effects of variation in weather and climate on productivity, recruitment, and patterns of long‐distance movement. We used stable H and C isotopes and geospatial modeling to estimate the natal origin of monarch butterflies (Danaus plexippus) in eastern North America using over 1000 monarchs collected over almost four decades at Mexican overwintering colonies. Multinomial regression was used to ascertain which climate‐related factors best‐predicted temporal variation in natal origin across six breeding regions. The region producing the largest proportion of overwintering monarchs was the US Midwest (mean annual proportion = 0.38; 95% CI: 0.36–0.41) followed by the north‐central (0.17; 0.14–0.18), northeast (0.15; 0.11–0.16), northwest (0.12; 0.12–0.16), southwest (0.11; 0.08–0.12), and southeast (0.08; 0.07–0.11) regions. There was no evidence of directional shifts in the relative contributions of different natal regions over time, which suggests these regions are comprising the same relative proportion of the overwintering population in recent years as in the mid‐1970s. Instead, interannual variation in the proportion of monarchs from each region covaried with climate, as measured by the Southern Oscillation Index and regional‐specific daily maximum temperature and precipitation, which together likely dictate larval development rates and food plant condition. Our results provide the first robust long‐term analysis of predictors of the natal origins of monarchs overwintering in Mexico. Conservation efforts on the breeding grounds focused on the Midwest region will likely have the greatest benefit to eastern North American migratory monarchs, but the population will likely remain sensitive to regional and stochastic weather patterns.     

Associations between host migration and the prevalence of a protozoan parasite in natural populations of adult monarch butterflies

1. Monarch butterflies Danaus plexippus (L.) (Lepidoptera: Nymphalidae) are susceptible to infection by the obligate protozoan parasite Ophryocystis elektroscirrha (McLaughlin and Myers) (Apicomplexa: Neogregarinida). Because monarchs form resident and migratory populations in different parts of the world, this host–parasite system provides the opportunity to examine how variation in parasite prevalence relates to host movement patterns. 2. Parasite prevalence was evaluated using 14 790 adult monarchs captured between 1968 and 1997. Comparison of three populations in North America indicated that parasite prevalence is associated negatively with host dispersal distances. A continuously breeding, nonmigratory population in southern Florida showed high prevalence (over 70% heavily infected). The western population migrates moderate distances to overwintering sites on the Pacific Coast and has intermediate prevalence (30% heavily infected). The eastern migratory population, which travels the longest distance to Mexican overwintering sites, has exhibited less than 8% infection throughout the past 30 years. 3. Variation in parasite loads within North American migratory populations was investigated to determine whether the prevalence of heavy infection and average parasite loads declined during migration or overwintering. Average parasite loads of summer‐breeding adults in western North America decreased with increasing distance from overwintering sites. This suggests that heavily infected monarchs are less likely to remigrate long distances in spring. No differences in the frequency of heavily infected adults were found among eastern or western North American monarchs throughout the overwintering period, however, suggesting that this parasite does not affect overwintering mortality. 4. Changes in the prevalence of monarchs with low parasite loads demonstrate that spore transfer occurs during migration and overwintering, possibly when adult butterflies contact each other as a result of their clustering behaviour. 5. This study of geographical and temporal variation in O. elektroscirrha among populations of D. plexippus demonstrates the potential role of seasonal migration in mediating interactions between hosts and parasites, and suggests several mechanisms through which migratory behaviour may influence parasite prevalence.     

Fueling the fall migration of the monarch butterfly

Monarch butterflies in eastern North America accumulate lipidsduring their fall migration to central Mexico, and use themas their energy source during a 5 month overwintering period.When and where along their migratory journey the butterfliesaccumulate these lipids has implications for the importanceof fall nectar sources in North America. We analyzed the lipidcontent of 765 summer breeding and fall migrant monarch butterfliescollected at 1 nectaring site in central Virginia over 4 years(1998–2001), and compared them with 16 additional publishedand unpublished datasets from other sites, dating back to 1941.Virginia migrants store significantly more lipid than summerbutterflies, and show significant intraseason and between-yearvariation. None of the Virginia samples, and none of the historicalsamples, with one exception, had lipid levels comparable withthose found in migrants that had reached Texas and northernMexico. This evidence suggests that upon reaching Texas, thebutterflies undergo a behavioral shift and spend more time nectaring.The one exceptional sample led us to the discovery that monarchsthat form roosts along their migratory routes have higher lipidcontents than monarchs collected while nectaring at flowers.We propose that for much of their journey monarchs are opportunisticmigrants, and the variation within and between samples reflectsbutterflies' individual experiences. The stored lipids appearto be of less importance as fuel for the butterflies' migrationthan for their survival during their overwintering period, inpart because soaring on favorable winds reduces the energeticcost of flying. The conservation of nectar plants in Texas andnorthern Mexico is crucial to sustaining the monarch's migratoryspectacle, and nectar abundance throughout eastern North Americais also important. As generalists in their selection of nectarsources and nectaring habitats, monarchs are unlikely to beaffected by small changes in plant communities. Agriculturaltransformations of natural communities in the eastern UnitedStates and Great Plains, however, and especially the extensiveplanting of genetically modified herbicide-resistant soybeansand corn, may be changing the availability of nectar for monarchsand other pollinators. This new technology is eliminating virtuallyall forbs in and surrounding agricultural fields, includingthe monarch's larval hostplants (milkweeds) and native and nonnativenectar sources. To evaluate whether changes in nectar availabilityare altering the butterflies' ability to accumulate energy,we recommend that monarchs' lipid contents be assayed annuallyat sites throughout eastern North America.     

Do Healthy Monarchs Migrate Farther? Tracking Natal Origins of Parasitized vs. Uninfected Monarch Butterflies Overwintering in Mexico

Long-distance migration can lower parasite prevalence if strenuous journeys remove infected animals from wild populations. We examined wild monarch butterflies (Danaus plexippus) to investigate the potential costs of the protozoan Ophryocystis elektroscirrha on migratory success. We collected monarchs from two wintering sites in central Mexico to compare infection status with hydrogen isotope (δ ²H) measurements as an indicator of latitude of origin at the start of fall migration. On average, uninfected monarchs had lower δ ²H values than parasitized butterflies, indicating that uninfected butterflies originated from more northerly latitudes and travelled farther distances to reach Mexico. Within the infected class, monarchs with higher quantitative spore loads originated from more southerly latitudes, indicating that heavily infected monarchs originating from farther north are less likely to reach Mexico. We ruled out the alternative explanation that lower latitudes give rise to more infected monarchs prior to the onset of migration using citizen science data to examine regional differences in parasite prevalence during the summer breeding season. We also found a positive association between monarch wing area and estimated distance flown. Collectively, these results emphasize that seasonal migrations can help lower infection levels in wild animal populations. Our findings, combined with recent declines in the numbers of migratory monarchs wintering in Mexico and observations of sedentary (winter breeding) monarch populations in the southern U.S., suggest that shifts from migratory to sedentary behavior will likely lead to greater infection prevalence for North American monarchs.     

Fire creates host plant patches for monarch butterflies

Monarch butterflies (Danaus plexippus) depend on the presence of host plants (Asclepias spp.) within their breeding range for reproduction. In the southern Great Plains, Asclepias viridis is a perennial that flowers in May and June, and starts to senesce by August. It is locally abundant and readily used by monarchs as a host plant. We evaluated the effects of summer prescribed fire on A. viridis and the use of A. viridis by monarch butterflies. Summer prescribed fire generated a newly emergent population of A. viridis that was absent in other areas. Pre-migrant monarch butterflies laid eggs on A. viridis in summer burned plots in late August and September, allowing adequate time for a new generation of adult monarchs to emerge and migrate south to their overwintering grounds. Thus, summer prescribed fire may provide host plant patches and/or corridors for pre-migrant monarchs during a time when host plant availability may be limited in other areas.     

A Continent-Wide Migratory Divide in North American Breeding Barn Swallows (Hirundo rustica)

Populations of most North American aerial insectivores have undergone steep population declines over the past 40 years but the relative importance of factors operating on breeding, wintering, or stopover sites remains unknown. We used archival light-level geolocators to track the phenology, movements and winter locations of barn swallows (Hirdundo rustica; n = 27) from populations across North America to determine their migratory connectivity. We identified an east-west continental migratory divide for barn swallows with birds from western regions (Washington State, USA (n = 8) and Saskatchewan, Canada (n = 5)) traveling shorter distances to wintering areas ranging from Oregon to northern Colombia than eastern populations (Ontario (n = 3) and New Brunswick (n = 10), Canada) which wintered in South America south of the Amazon basin. A single swallow from a stable population in Alabama shared a similar migration route to eastern barn swallows but wintered farther north in northeast Brazil indicating a potential leap frog pattern migratory among eastern birds. Six of 9 (67%) birds from the two eastern populations and Alabama underwent a loop migration west of fall migration routes including around the Gulf of Mexico travelling a mean of 2,224 km and 722 km longer on spring migration, respectively. Longer migration distances, including the requirement to cross the Caribbean Sea and Gulf of Mexico and subsequent shorter sedentary wintering periods, may exacerbate declines for populations breeding in northeastern North America.     

Not as the crow flies: a historical explanation for circuitous migration in Swainson's thrush (Catharus ustulatus)

Many migratory songbirds follow circuitous migratory routes instead of taking the shortest path between overwintering and breeding areas. Here, we study the migration patterns in Swainson's thrush (Catharus ustulatus), a neartic-neotropical migrant songbird, using molecular genetic approaches. This species is presently separated into genetically distinct coastal and continental populations that diverged during the Late Pleistocene (as indicated by molecular dating), yet appear to have retained ancestral patterns of migration. Low nucleotide diversity, a star-like haplotype phylogeny and unimodal mismatch distributions all support the hypothesis that both the coastal and the continental populations have undergone recent demographic expansions. Nearctic-neotropical banding and genetic data show nearly complete segregation of migratory routes and of overwintering locations: coastal populations migrate along the Pacific Coast to overwintering sites in Central America and Mexico, whereas continental populations migrate along an eastern route to overwintering sites in Panama and South America. Nearctic-neotropical banding data also show that continental birds north, northwest and east of this migratory divide fly thousands of miles east before turning south. We conclude that circuitous migration in the Swainson's thrush is an artefact of a Late Pleistocene range expansion.     

Linking the continental migratory cycle of the monarch butterfly to understand its population decline

Threats to several of the world's great animal migrations necessitate a research agenda focused on identifying drivers of their population dynamics. The monarch butterfly is an iconic species whose continental migratory population in eastern North America has been declining precipitously. Recent analyses have linked the monarch decline to reduced abundance of milkweed host plants in the USA caused by increased use of genetically modified herbicide‐resistant crops. To identify the most sensitive stages in the monarch's annual multi‐generational migration, and to test the milkweed limitation hypothesis, we analyzed 22 years of citizen science records from four monitoring programs across North America. We analyzed the relationships between butterfly population indices at successive stages of the annual migratory cycle to assess demographic connections and to address the roles of migrant population size versus temporal trends that reflect changes in habitat or resource quality. We find a sharp annual population decline in the first breeding generation in the southern USA, driven by the progressively smaller numbers of spring migrants from the overwintering grounds in Mexico. Monarch populations then build regionally during the summer generations. Contrary to the milkweed limitation hypothesis, we did not find statistically significant temporal trends in stage‐to‐stage population relationships in the mid‐western or northeastern USA. In contrast, there are statistically significant negative temporal trends at the overwintering grounds in Mexico, suggesting that monarch success during the fall migration and re‐establishment strongly contributes to the butterfly decline. Lack of milkweed, the only host plant for monarch butterfly caterpillars, is unlikely to be driving the monarch's population decline. Conservation efforts therefore require additional focus on the later phases in the monarch's annual migratory cycle. We hypothesize that lack of nectar sources, habitat fragmentation, continued degradation at the overwintering sites, or other threats to successful fall migration are critical limiting factors for declining monarchs.     

Diversification of the American bulb-bearing Oxalis (Oxalidaceae): Dispersal to North America and modification of the tristylous breeding system

? Premise of the study: The American bulb-bearing Oxalis (Oxalidaceae) have diverse heterostylous breeding systems and are distributed in mountainous areas from Patagonia to the northeastern United States. To study the evolutionary processes leading to this diversity, we constructed the first molecular phylogeny for the American bulb-bearing Oxalis and used it to infer biogeographic history and breeding system evolution. ? Methods: We used DNA sequence data (nuclear ribosomal internal transcribed spacer, trnL-trnL-trnF, trnT-trnL, and psbJ-petA) to infer phylogenetic history via parsimony, likelihood, and Bayesian analyses. We used Bayes Multistate to infer ancestral geographic distributions at well-supported nodes of the phylogeny. The Shimodaira-Hasegawa (SH) test distinguished among hypotheses of single or multiple transitions from South America to North America, and tristyly to distyly. ? Key results: The American bulb-bearing Oxalis include sampled members of sections Ionoxalis and Pseudobulbosae and are derived from a larger clade that includes members of sections Palmatifoliae, Articulatae, and the African species. The American bulb-bearing Oxalis comprise two clades: one distributed in SE South America and the other in the Andes and North America. An SH test supports multiple dispersals to North America. Most sampled distylous species form a single clade, but at least two other independent distylous lineages are supported by the topologies and SH tests. ? Conclusions: Phylogenetic results suggest the American bulb-bearing Oxalis originated in southern South America, dispersed repeatedly to North America, and had multiple transitions from tristyly to distyly. This study adds to our understanding of biogeographic history and breeding system evolution and provides a foundation for more precise inferences about the study group.     

Genomic evidence for gene flow between monarchs with divergent migratory phenotypes and flight performance

Monarch butterflies are known for their spectacular annual migration in eastern North America, with millions of monarchs flying up to 4,500 km to overwintering sites in central Mexico. Monarchs also live west of the Rocky Mountains, where they travel shorter distances to overwinter along the Pacific Coast. It is often assumed that eastern and western monarchs form distinct evolutionary units, but genomic studies to support this notion are lacking. We used a tethered flight mill to show that migratory eastern monarchs have greater flight performance than western monarchs, consistent with their greater migratory distances. However, analysing more than 20 million SNPs in 43 monarch genomes, we found no evidence for genomic differentiation between eastern and western monarchs. Genomic analysis also showed identical and low levels of genetic diversity, and demographic analyses indicated similar effective population sizes and ongoing gene flow between eastern and western monarchs. Gene expression analysis of a subset of candidate genes during active flight revealed differential gene expression related to nonmuscular motor activity. Our results demonstrate that eastern and western monarchs maintain migratory differences despite ongoing gene flow, and suggest that migratory differences between eastern and western monarchs are not driven by select major‐effects alleles. Instead, variation in migratory distance and destination may be driven by environmentally induced differential gene expression or by many alleles of small effect.     

Combining genetic markers and stable isotopes to reveal population connectivity and migration patterns in a neotropical migrant,Wilson's warbler (Wilsonia pusilla)

We used results from the analysis of microsatellite DNA variation and hydrogen stable-isotope ratios to characterize the population structure of a neotropical migrant passerine, the Wilson's warbler (Wilsonia pusilla). The resulting information was then used to infer migration patterns and population connectivity between breeding grounds in North America and overwintering areas in Mexico and Central America. The microsatellite data revealed genetic structure across the North American continent; populations in the west were found to significantly differ from the east. Minimal genetic structure was observed among western sites. The lack of isolation by distance and low variance in FST values suggests that gene flow could play an ongoing role in limiting genetic differentiation among sites in the western part of the distribution. However, additional information including estimates of effective population size and the proximity of the population to equilibrium is required before the role of gene flow can be assessed fully. Analysis of isotope data showed a negative relationship between latitude and hydrogen isotope ratios in breeding ground individuals. There was a positive relationship between wintering ground latitude and hydrogen isotope ratios for individuals that were genetically western in origin. This is consistent with a leapfrog pattern of migration, in which genetically western birds from the northernmost breeding areas overwinter at the most southerly locations in Central America. Additionally, isotopic ratios of western birds suggest that coastal breeders overwinter in western Mexico, while western birds from further inland and at high elevations overwinter in eastern Mexico. Using information from both genetic an isotopic approaches will probably be useful for identifying patterns of migration and population connectivity between breeding and overwintering areas, both important issues for conservation efforts, and may also contribute to investigation of the evolution of migration.     

Crosswise migration by Yellow Warblers,Nearctic‐Neotropical passerine migrants

Yellow Warblers (Setophaga petechia) are abundant breeding birds in North America, but their migratory and non‐breeding biology remain poorly understood. Studies where genetic and isotopic techniques were used identified parallel migration systems and longitudinal segregation among eastern‐ and western‐breeding populations of Yellow Warblers in North America, but these techniques have low spatial resolution. During the 2015 breeding season, we tagged male Yellow Warblers breeding in Maine (N = 10) and Wisconsin (N = 10) with light‐level geolocators to elucidate fine‐scale migratory connectivity within the eastern haplotype of this species and determine fall migration timing, routes, and wintering locations. We recovered seven of 20 geolocators (35%), including four in Maine and three in Wisconsin. The mean duration of fall migration was 49 d with departure from breeding areas in late August and early September and arrival in wintering areas in mid‐October. Most individuals crossed the Gulf of Mexico to Central America before completing the final eastward leg of their migration to northern South America. Yellow Warblers breeding in Maine wintered in north‐central Colombia, west of those breeding in Wisconsin that wintered in Venezuela and the border region between Brazil, Colombia, and Venezuela. Our results provide an example of crosswise migration, where the more easterly breeding population wintered farther west than the more westerly breeding population (and vice versa), a seldom‐documented phenomenon in birds. Our results confirm earlier work demonstrating that the eastern haplotype of northern Yellow Warblers winters in northern South America, and provide novel information about migratory strategies, timing, and wintering locations of birds from two different populations.     

Taxonomy and biogeography of the fiddler crabs (Ocypodidae: Genus Uca) of the Atlantic and Gulf coasts of eastern North America

Fifteen species of fiddler crabs are reported for eastern North America between Massachusetts and Quintana Roo, Mexico. Thirteen occur in the United States and 11 in Mexico, with eight in common to the two countries. Of 13 species in the Gulf of Mexico, five are endemic and a sixth is restricted largely to the peninsulas of Florida and Yucatan. The status of U. rapax in the northern Gulf remains to be resolved. Range limits of most species approximate one or the other of two sets of intersecting thermal and geological boundaries that subdivide the Gulf of Mexico along north-south and east-west axes. Species belonging to subgenus Minuca tend to replace one another at the thermally-controlled Carolinian-Caribbean marine biotic boundary across the Florida peninsula and northern Gulf. However, only U. minax of all the North American fiddler crabs exhibits the classical disjunct Carolinian distribution, and this appears basically to reflect the discontinuous distribution of temperate salt marshes that are the habitat of the species. Distributions of species belonging to subgenus Celuca adhere for the most part to the subdivision of the Gulf into western terrigenous and eastern carbonate sedimentary provinces. The northern transition occurs in the vicinity of Apalachee Bay and the southern at Laguna de Terminos. A third distributional pattern is shown by U. subcylindrica , a specialized endemic species of the hypersaline Laguna Madre system of the western Gulf. The level of endemism in the fiddler crabs is relatively high in comparison with that of other marine groups within the Gulf of Mexico. This may be a consequence of the adaptations of fiddler crabs as specialized deposit feeders to regional differences in climatic and edaphic characteristics of a marginally marine upper shore habitat. The distributional patterns of the endemics could prove useful in reconstructing palaeoecological events of evolutionary significance within the Gulf of Mexico.     

Are female monarch butterflies declining in eastern North America? Evidence of a 30-year change in sex ratios at Mexican overwintering sites

Every autumn the entire eastern North American population of monarch butterflies (Danaus plexippus) undergoes a spectacular migration to overwintering sites in the mountains of central Mexico, where they form massive clusters and can number in the millions. Since their discovery, these sites have been extensively studied, and in many of these studies, monarchs were captured and sexes recorded. In a recent effort to compile the sex ratio data from these published records, a surprising trend was found, which appears to show a gradual decline in proportion of females over time. Sex ratio data from 14 collections of monarchs, all spanning 30 years and totaling 69 113 individuals, showed a significant negative correlation between proportion of females and year (r = −0.69, p = 0.007). Between 1976 and 1985, 53 per cent of overwintering monarchs were female, whereas in the last decade, 43 per cent were female. The relationship was significant with and without weighting the analyses by sampling effort. Moreover, analysis of a recent three-year dataset of sex ratios revealed no variation among nine separate colonies, so differences in sampling location did not influence the trend. Additional evidence from autumn migration collections appears to confirm that proportions of females are declining, and also suggests the sex ratio is shifting on breeding grounds. While breeding monarchs face a number of threats, one possibility is an increase in prevalence of the protozoan parasite, Ophryocystis elektroscirrha, which recent evidence shows affects females more so than males. Further study will be needed to determine the exact cause of this trend, but for now it should be monitored closely.