Seasonal effects drive North Atlantic vagrancy of Purple Gallinules
Warmer Summers and Drier Winters Correlate with More Winter Vagrant Purple Gallinules (Porphyrio martinicus) in the North Atlantic Region. Farnsworth, A., La Sorte, F.A. & Iliff. M.J. 2015. The Wilson Journal of Ornithology. DOI: 10.1676/14-086.1 View
Vagrant birds are the subjects of books, films, and countless journeys by tens of thousands of birders (or twitchers). But these wandering birds far afield of their typical ranges of occurrence may be more than intriguing patterns or simple flights of fancy, perhaps yielding valuable insights about birds’ responses to environmental change. There are many reasons that vagrants may appear far from home, some intrinsic (such as innate navigation abilities) and others extrinsic (such as environmental conditions). If correlations between patterns of vagrancy and climate exist, projected changes in climate could translate into changes in the distribution of this species that result from continued and increasing patterns of vagrancy.
In our recently published study we correlated vagrancy records with climate and environmental conditions and population trends in the center of Purple Gallinule (Porphyrio martinicus) winter distribution. Purple Gallinule is a champion, dispersing vagrant. This species has a propensity to appear far afield from its typical haunts in tropical and subtropical wetlands of the Americas and Caribbean. Vagrant Purple Gallinules have been recorded well outside their normal range during all times of year, including records from northeastern and western North America, North Atlantic islands and western Europe, South Africa, South Atlantic islands, and Pacific islands. Most Purple Gallinule vagrants occur from April to October; however, a much smaller number of records represent a more enigmatic vagrancy pattern that occurs from November to February. Such records represent an unusual seasonal pulse of vagrants, when few or no migratory or regular dispersive movements occur for many of the most likely populations of origin for these birds.
The Scope of Our Study
We gathered occurrence information for Purple Gallinules in the North Atlantic region from 1 November 1957 to 28 February 2014 from eBird (Figure 1, Sullivan et al. 2014). We chose this temporal window, in part, because we believe that certain intrinsic factors potentially causing vagrancy during migration periods are minimized at this time of the year, because the birds are known to be generally sedentary. We established 32 degrees N latitude as the northern limit of 95% of all records, defining the remaining 5% of records occurring north of this location we defined as vagrants and then creating a standardized index of the number of extralimital records per season. We compiled 77 occurrences of vagrant Purple Gallinules during this seasonal window and examined how those occurrences correlated with environmental conditions and population trends.
Figure 1 eBird records of Purple Gallinule (Porphyrio martinicus) from 1 November 1957 to 28 February 2014. Our study focused on those records north of 32 degrees N latitude (approximately Crossing the southern reaches of South Carolina, USA). Each of the blue balloons represents a record during this period. Note the distribution of records in Florida, USA, the location of the closest regular wintering population to our study area. These records are part of a much larger set that compose eBird’s observation database, the largest citizen science database on biodiversity.
We acquired drought and average temperature data from the National Centers for Environmental Information for every month from 1957-2014 (see Figure 2 for a sample of these data) and precipitation data from the International Research Institute for Climate and Society at Columbia University. We choose several areas from which to gather temperature and drought data for analysis that encompass the center of winter distributions for Purple Gallinule in Florida, the closest regular wintering area to the North Atlantic region. We also included data from eastern Mexico, northern Central America and the Caribbean, since substantial numbers of Purple Gallinules occupy these areas.
Figure 2 A sample graphic from the National Centers for Environmental Information showing average monthly temperatures in Florida for December 1957-2014. These data are invaluable for the kinds of analysis we present in our manuscript, and they are freely available for download and analysis.
Purple Gallinule Population Trends
Although information about population trends of Purple Gallinule is generally lacking, the North American Breeding Bird Survey (BBS) data for 1966-2012 suggest annual declines. We used survey-wide data (i.e., as opposed to much less reliable Florida only data) for Purple Gallinules from 1966-2012, generating a metric for annual changes in population trends by subtracting consecutive years’ trend values.
Relationships between climate and Purple Gallinule vagrancy
Average temperature anomalies showed significant correlations with patterns of records, with warmer late summer temperatures in particular in Florida and Puerto Rico correlating with more North Atlantic vagrants. Drier conditions in eastern Mexico, especially during winter, showed similar significant relationships. Other variables, generally suggestive of higher temperatures and drier conditions, exhibited near significant relationships. Annual change in population trend did not show a significant correlation with numbers of winter vagrants, although a positive correlation suggests annual population increase may correlate with more vagrant records. One principal component analysis showed a near significant relationship with vagrant records, with the component second loadings strongly representing patterns of late summer temperatures in Puerto Rico.
A Rosy outlook for the Researcher and the Twitcher
Projections for warmer and drier conditions in Subtropical and Tropical western hemisphere suggest an increasing pattern of winter, North Atlantic vagrancy in Purple Gallinule from November to February. Furthermore, Strong cyclonic systems that are almost certainly responsible for transporting Purple Gallinules far afield, are projected to increase in prevalence and intensity. An exceptional pulse of records in the winter of 2013-2014 (11 records) probably relates to the pattern of numerous strong cyclonic storms originating in the Gulf of Mexico during that winter in addition to warmer conditions in Florida and drier conditions in Mexico; similar pulses of records in 1978-1979 and 1986-1987 (seven records each) probably originate from similar causes. Clearly, this is fodder for future research.
Does this mean UK twitchers should plan to scour a nearby wetland in late autumn and winter? (The answer is invariably yes!). Could other species exhibit similar patterns in vagrancy? Paint-billed Crake (Neocrex erythrops), Spotted Rail (Pardirallus maculatus), and Azure Gallinule (Porphyrula flavirostris) of presumably South American origin have occurred in the November to February window north of 32 degrees N in North America. The same may be true for Least Bittern (Ixobrychus exilis), a species that also inhabits wetland habitats; recent unseasonal records from northeastern North America (for example this record) were not lingering birds from summer or fall, and may represent an incipient analogous pattern to watch.
Figure 3 From left to right, Paint-billed Crake (Neocrex erythrops), Spotted Rail (Pardirallus maculatus), and Azure Gallinule (Porphyrula flavirostris). These species have occurred far out of range, albeit with much lower frequencies of occurrence than Purple Gallinule.
Purple Gallinule, and other species prone to frequent and noticeable vagrancy, may represent model species for these correlative investigations. And with additional research on intrinsic and extrinsic factors, such as those in our study, it may be possible to identify the complete cycle of causality operating in vagrancy, finally linking what, when and where with why and how vagrants arrive where they do.
References and further reading
Farnsworth, A., La Sorte, F. A. & Iliff M. J. 2015. Warmer Summers and Drier Winters Correlate with More Winter Vagrant Purple Gallinules (Porphyrio martinicus) in the North Atlantic Region. The Wilson Journal of Ornithology 127, no. 4: 582-592. View
Purple Gallinule Vagrancy in the North Atlantic, November 2013 – February 2014. BirdCast View
Purple Gallinule. All About Birds View
Distribution of Purple Gallinule from November through February. eBird View
Sullivan, B. L., J. L. Aycrigg, J. H. Barry, R. E. Bonney, N. Bruns, C. B. Cooper, T. Damoulas, A. A. Dhondt, T. Dietterich, A. Farnsworth, D. Fink, J. W. Fitzpatrick, T. Fredericks, J. Gerbracht, C. Gomes, W. M. Hochachka, M. J. Iliff, C. Lagoze, F. A. La Sorte, M. Merrifield, W. Morris, T. B. Phillips, M. Reynolds, A. D. Rodewald, K. V. Rosenberg, N. M. Trautmann, A. Wiggins, D. W. Winkler, W.-K. Wong, C. L. Wood, J. Yu, and S. Kelling. 2014. The eBird enterprise: an integrated approach to development and application of citizen science. Biological Conservation 169:31–40. View
Featured image: Purple Gallinule. Wakodahatchee Wetlands, Palm Beach, Florida, USA © Chris Wood/Macaulay Library. 24 April 2004. eBird S29250505
Figure 3: Paint-billed Crake Mustelirallus erythrops © Devin Griffiths/Macaulay Library. 4 Jul 2013. eBird S23580496; Spotted Rail Pardirallus maculatus © Jose Antonio Robles Martinez/Macaulay Library. 1 Apr 2016. eBird S28688255; Azure Gallinule Porphyrio flavirostris © Joel Martinez/Macaulay Library. 12 Oct 2015. eBird S25395412
Author photo: © Philip Montgomery
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