Rusty Blackbird: Mysteries of a Species in Decline (Euphagus carolinus: Misterios de una Especie en Disminución) Author(s): Russell Greenberg and Steven M. Matsuoka Reviewed work(s): Source: The Condor, Vol. 112, No. 4 (November 2010), pp. 770-777 Published by: University of California Press on behalf of the Cooper Ornithological Society Stable URL: http://www.jstor.org/stable/10.1525/cond.2010.100153 . Accessed: 14/10/2012 15:59
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http://www.jstor.org 16_MS100153.indd 770 SPECIAL SECTION: RANGEWIDE ECOLOGY OF THE DECLINING RUSTY BLACKBIRD reprintInfo.as website, Permissions and Press’s Rights of California University the through content article or reproduce to photocopy for permission requests all The Condor, 4 3 received 30 July 2010;Manuscript 12 2010. accepted September 2010 Society Ornithological Cooper The 112(4):770–777 Condor The E-mail: E-mail: Current address: Boreal Avian Modelling Project, 751 General Services Building, University of Alberta, Edmonton, AB T6G 2H1,Edmonton, of University Canada. 751AvianAlberta, Boreal Project, Building, Modelling Services address: General Current [email protected] bird especial presenta los resultados de las investigaciones del IRBWG—el primero para la especie sobre uso de hábitat hábitat de uso sobre especie la para de la las disminución poblaciones de primero IRBWG—el del Subrayamos la de intrigantes. necesidad investigación para evaluar los roles de los factores en distintos la causa de investigaciones en tejidos de las y Los losdatos elniveles toria. rol de metilmercurio en del elson forestal manejo de éxito reproductivo migra y conectividad muda, de patrones resultados parasitaria, prevalencia reproductivo, éxito los y invierno, de reproductivo presenta sección especial Esta disminución. misteriosa la entender lograr para investigación de esfuerzos desarrollar para inglés) de Trabajo de Internacional Grupo el formó se 2005 el En productivas. grave efecto un en las poblaciones re tenido haber en pueden la ambiental boreal zona global y la contaminación de programas Los de control sospechosa. principal la habitats— invierno—representan de estos de distribución de área degradación el en y pérdida la particularmente tanto lo por año, del largo lo a arbolados humedales de especie Esta depende disminución. la explicar para hipótesis varias propusieron Se siglo. un de más por sostenidamente de abundancia la que sugieren históricos conteos de análisis Los ridional. me boreal zona la en distribución de retracciones y población de disminuciones sugieren regionales censos Los Aves vade que del 5 12% al anual. de aproximado rango un en poblaciones de Conteo el disminución de y tasas altas Aves muestran Reproductivas Navidad de Norteamericano Censo el décadas, cuatro últimas las Durante a lo y elUnidos. delque largo pasa de loseste invierno Estados reproductiva la época durante del Norte América Rusty Blackbird. Rusty of the decline the causing in of factors roles the forof assessment various needed We research outline intriguing. ity. Data on the levels and in the tissues roleof on are success ofmethylmercury reproductive management timber of prevalence, and parasite success, patterns habitat, molt,reproductive breeding winter connectiv and migratory firstthe the section decline. onpresents special This mysterious group’sthe species’ use findings—the research of understanding toward efforts develop research to formed was Group Working Blackbird Rusty International the 2005, In populations. breeding on toll their have may taken zone boreal the in contamination environmental and warming, global disturbance, habitat decades, recent In may have contributed. programs Blackbird-control pect. sus prime a range—is winter the in habitats—particularly of these degradation and loss year, so the throughout wetlands wooded on relies species The cen decline. the a explain to over proposed for have been of hypotheses A number steadily tury. dropping been has Blackbird’sabundance Rusty the that suggest accounts historical of zone. Analyses boreal southern the in retractions range and 5 declines to suggest 12% year. per surveys Regional approximately from ranging decline population of rates high shown have Count Bird Christmas the and Survey Bird Breeding American the North Over the four past decades, States. United the eastern throughout and winters Vol. 112, Number 4, pages 770–777. ISSN 0010-5422, electronic ISSN 1938-5422. ISSN Vol. 770–777. 4, 112,pages 0010-5422, electronic ISSN Number p . DOI: . 10.1525/cond.2010.100153DOI: Key Key words: Resumen. Abstract , winter winter ecology S pecial R pueden haber contribuido. En décadas recientes, la alteración de hábitat, el calentamiento el de calentamiento hábitat, la alteración recientes, En décadas contribuido. haber pueden carolinus E. usty B .
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11/25/10 3:31:18 PM RUSTY BLACKBIRD DECLIne 771 SPECIAL SECTION: RANGEWIDE ECOLOGY OF THE DECLINING RUSTY BLACKBIRD look of the ecology and possible factors contributing to the de- INTRODUCTION cline of the Rusty Blackbird. In this commentary, we provide a It is difficult to know the size of the global population of the brief summary of evidence of the Rusty Blackbird’s decline and Rusty Blackbird (Euphagus carolinus). The species is broadly the factors that we and other have hypothesized are contribut- distributed during the breeding season across North Ameri- ing to it (Greenberg and Droege 1999, Greenberg et al. 2011). ca’s vast boreal forest, from New England and the Maritime We then put into a broader ecological perspective the findings Provinces to the tree line in northern Alaska, and it winters of the nine papers included in this special section and end by throughout the southeastern U.S. The only published esti- emphasizing the critical next steps that need to be taken to un- mates, based on inference from the number detected along derstand and reverse the species’ mysterious decline. North American Breeding Bird Survey routes, holds that be- tween 158 000 and 2 million Rusty Blackbirds remain (Rich EVIDENCE OF POPULATION DECLINE et al. 2004, Savignac 2006), suggesting the species is still rea- sonably common. However, the species has experienced a The decline of the Rusty Blackbird, first alluded to by Avery profound decline rangewide, cumulative to a reduction in pop- (1995) and explicitly posited by Greenberg and Droege (1999), ulation size of 88% or more over the past four decades, with generally comes as no surprise to birders and field ornitholo- additional qualitative evidence that this decline has been con- gists familiar with the species—their overwhelming percep- tinuing for a century or more (Greenberg and Droege 1999, tion is that the species is observed much less frequently now Niven et al. 2004, Sauer et al. 2008, Greenberg et al. 2011). than in years or decades past (Greenberg and Droege 1999). The severity and persistence of the decline has recently drawn Because of the inaccessibility of most of its breeding range and attention to the plight of the Rusty Blackbird and placed the myriad problems of surveying the species in the winter (e.g., species on a number of notable lists of conservation concern difficulty of detection, poorly understood movements within including the IUCN’s Red List of Threatened Species (IUCN the season, and annual variation in distribution), databases that 2010), the U.S. Fish and Wildlife Service’s Birds of Conserva- can be used to evaluate the decline are few and less than ideal. tion Concern (U.S. Fish and Wildlife Service 2008), and the The North American Breeding Bird Survey (BBS) and Christ- Committee on the Status of Endangered Wildlife in Canada’s mas Bird Count (CBC) provide the only broad-scale and rea- Species of Special Concern (Savignac 2006). sonably long-term data with which the possibility of a global Despite this attention, the magnitude of decline has decline in the species can be assessed. The BBS trend is –9.3% remained the best-known aspect of the Rusty Blackbird’s per year (95% CI –11.7 to –1.5%) for the period 1966–2008. But ecology, with literature on its natural history and resource this trend is based on data from only 110 routes concentrated in requirements restricted to brief narrative descriptions in av- the southern boreal zone (Sauer and Link 2011). All Bird Con- ifaunal accounts that were first summarized by Bent (1958) servation Regions across the breeding range show evidence of and later updated by Avery (1995) and Savignac (2006). The decline (Sauer et al. 2008), and on the basis of rangewide esti- single published field study designed and focused entirely on mates of annual decline the cumulative decline is 93%. the natural history of this species was conducted nearly a cen- The CBC, although based on a protocol less standardized tury ago by Fred H. Kennard (1920). The Rusty Blackbird es- than the BBS, has advantages of more data and broader cover- caped serious study because of a combination of its breeding age. Over a period 5 years shorter than that of the BBS (winter in remote boreal wetlands, its unassuming behavior, its win- 1965–1966 to 2002–2003), Rusty Blackbirds were recorded tering in forested wetlands difficult of access, and its weak in up to ~1600 count circles per year across the species’ entire association in winter with other blackbirds that have been the winter range. Such extensive coverage is particularly impor- focus of research on pest control in agricultural fields or at tant in the winter as the species’ distribution changes mark- evening roosts (Avery 1995, Greenberg et al. 2011). As a re- edly within and between winters according to the weather sult, even basic information on the Rusty Blackbird’s ecology (Hamel and Ozdenerol 2009). On the basis of the CBC Niven and resource requirements are sorely needed to understand, et al. (2004) estimated the rangewide trend at −5.1% per year let alone reverse, its dramatic decline (Greenberg and Droege (95% credible interval −6.6% to −2.4% per year). The decline 1999, Greenberg et al. 2011). is over the entire winter range and particular strong for the It was this need that prompted both the formation of the In- four Bird Conservation Regions where the species’ relative ternational Rusty Blackbird Working Group in 2005 and the ini- abundance is highest: Central Hardwoods (–6.2%), West Gulf tiation of a series of collaborative research studies by the group’s Coastal Plain (–3.9%), Southeastern Coastal Plain (–5.1%), ten founding members to understand and reverse the species’ and Mississippi Alluvial Valley (–6.5%; Niven et al. 2004). decline (Greenberg 2008, Greenberg et al. 2011). This special Although of slightly smaller magnitude than the trend based section includes nine papers that summarize the initial research on the BBS for the same period, the rangewide decline based findings of various members of the working group, which now on the CBC is considerable and tantamount to a total decline includes over 60 members, and provides the first quantitative of approximately 88% through 2007.
16_MS100153.indd 771 11/25/10 3:31:18 PM 772 RUSSELL GREENBERG AND STEVEN M. MATSUOKA
The Rusty Blackbird’s downward turn appears to be may further degrade habitat suitability by exposing Rusty rooted in historical changes of North America’s avifauna, as Blackbirds to increased predation and increased competition our qualitative analysis of descriptions of the species’ abun- with the Common Grackle (Quiscalus quiscula) and other dance since the late 19th century implies the species had been species of blackbirds. declining before the advent of the BBS. In 56% of state or pro- Historically, boreal wetlands were less affected by human vincial ornithological overviews published before 1920 the activities than were forested wetlands across the species’ win- Rusty Blackbird was considered to be very common to abun- ter range. Even so, many species that breed primarily in boreal dant. This percentage declined to 19% in accounts published wetlands, including the Horned Grebe (Podiceps auritus), from 1920 to 1950 and to only 6.7% in those published from Lesser Scaup (Aythya affinis), White-winged and Surf Sco- 1951 to 1997. These and similar data suggest that a long and ters (Melanitta fusca and M. perspicillata), Lesser Yellowlegs severe decline may have haunted populations well before the (Tringa flavipes), and Solitary Sandpiper (T. solitaria), have recent 88–98% decline estimated from the BBS and CBC experienced some of the highest rates of decline among North (Greenberg and Droege 1999). In addition to the long-term American birds (Austin et al. 2000, Hanna 2004, North Amer- decline, recent surveys in central and western Canada suggest ica Waterfowl Management Plan Committee 2004, Sauer et a range contraction from areas traditionally occupied in the al. 2008, U.S. Fish and Wildlife Service 2009, Greenberg et al. south (K. Hobson, S. Van Wilgenburg, J. Morrisette, and A. 2011). Declines of the Rusty Blackbird and other boreal wet- Smith, unpubl. data), and recent work in Maine indicates that land species may be associated with exploitation and develop- the species’ breeding range in that state has contracted 65–160 ment of natural resources, which have accelerated in recent km northward over the past century (Powell 2008). Limited decades, particularly in the southern and eastern portions of data from more remote northern regions of the boreal forest the boreal zone. Agricultural development, peat production, suggest that range retractions or general declines have not been timber harvests, oil and gas exploration and development, and as pronounced (Machtans et al. 2007, Cadman et al. 2009). reservoir formation have contributed both to large-scale di- rect losses of boreal wetlands and profound changes in hydrol- ogy (Greenberg and Doege 1999, Savignac 2006, Greenberg POSSIBLE REASONS FOR DECLINE et al. 2011). Apparent range retractions at the southern edge of Reasons for the Rusty Blackbird’s sharp downturn in num- the boreal forest may provide prima facie evidence that global bers largely have remained a mystery. Anthropogenic habi- warming is taking its toll, although climate change is also dry- tat change is often a primary cause of songbirds’ population ing wetlands in the northern boreal forest (Klein et al. 2005, declines (Terborgh 1989). Unlike other blackbirds, the Rusty Riordan et al. 2006), drying that appears to affect the chemis- Blackbird depends on wooded wetlands throughout the year try of water and abundance of macroinvertebrates (Corcoran and is therefore vulnerable to the many forces that affect wet- et al. 2009). lands throughout North America. It has faced a continuous de- Other environmental factors of extrinsic origin are affect- cline of potential habitat since European settlement especially ing boreal wetlands negatively. An increase in atmospheric in winter, with as much as 75–80% of bottomland hardwood deposition of industrial pollution, particularly in the eastern forests in the United States converted to agriculture. More re- breeding range, may affect trophic resources through an in- cent losses have been the result of logging or conversions to sidious combination of lower pH and higher load of mercury. pine plantations or urban development (Hefner and Brown In acidic wetlands, mercury is readily converted to its toxic 1984, Hefner et al. 1994, Twedt and Loesch 1999, Hamel et form, methylmercury. In eggs of the Common Grackle, meth- al. 2009). Gross loss of wooded wetlands in the southeastern ylmercury kills the embryo at relatively low concentrations United States would be consistent with a severe long-term de- (Heinz et al. 2009). Given the Rusty Blackbird’s preference cline in any species that depends on these habitats. Indeed, for feeding on predatory aquatic invertebrates and small fish since the 1950s regional declines in both the Rusty Black- (Beal 1900, Martin et al. 1951), its risk of accumulating meth- bird’s abundance and area of forested wetlands are roughly ylmercury may be higher than that of other passerines. equivalent in the species’ two primary wintering areas, the Habitat loss and degradation on the breeding and win- Mississippi Alluvial Valley and the Southern Coastal Plain ter range, climate change, and contaminant exposure are the (Greenberg and Droege 1999, Hamel et al. 2009). Conversion usual suspects for the decline of a migratory bird. Given our of bottomland forests to agriculture has progressed from drier general ignorance about the population dynamics of the Rusty to wetter sites over time; therefore, habitat conversion in the Blackbird, however, researchers need to be open to other pos- last half of the 20th century may have had a particularly det- sibilities. For example, in the 1970s, already declining pop- rimental effect because the species generally winters in wet- ulations may have been particularly hard-hit by large-scale ter sites (Hamel et al. 2009). Across the winter range many pest-control programs that focused on reducing the numbers wooded wetlands and bottomland forests now exist as small of blackbirds and European Starlings (Sturnus vulgaris) that SPECIAL SECTION: RANGEWIDE ECOLOGY OF THE DECLINING RUSTY BLACKBIRD RUSTY DECLINING THE OF ECOLOGY RANGEWIDE SECTION: SPECIAL fragments (Twedt and Loesch 1999), and such fragmentation congregated at large evening roosts in several states within
16_MS100153.indd 772 11/25/10 3:31:19 PM RUSTY BLACKBIRD DECLIne 773 SPECIAL SECTION: RANGEWIDE ECOLOGY OF THE DECLINING RUSTY BLACKBIRD the Rusty Blackbird’s winter range (Heisterberg et al. 1987, small stunted conifers, primarily spruce (Picea spp.), for nest Dolbeer et al. 1997). Rusty Blackbirds were generally found sites is selective and leads to relatively high rates of nest sur- in only small numbers in these large mixed-species roosts and vival when nests are placed in undisturbed wetlands in Alaska thus are thought not to have been greatly affected by these and New England. In New England, however, when Rusty control programs (Avery 1995, Greenberg et al. 2011), but Blackbirds nested in young spruce or balsam firs Abies( bal- negative effects may be cumulative. Such large-scale control samea) regenerated from recent logging of wetlands or adja- programs have ceased, but continued small-scale programs cent uplands, they experienced a nearly 70% reduction in nest to control blackbird populations may have long-term effects survival because of high nest predation. Thus, buffers from during the nonbreeding season. The importance of disease in logging around wetlands ought to be explored as a conserva- regulating bird populations has become increasingly appar- tion measure to protect nesting Rusty Blackbirds from these ent (Marra et al. 2004, LaDeau et al. 2008) and should not be negative consequences (Powell et al. 2010b). Average rates of overlooked as a factor contributing to the Rusty Blackbird’s nest success in Alaska (56%; Matsuoka et al. 2010b) and New decline. The dramatic increases in many raptor populations England (62%; Powell 2008) are not at chronically low levels during the post-DDT era (Sauer et al. 2008) may also be ham- that could be linked to long-term declines, so factors reducing pering the recovery of the Rusty Blackbird. survival of adults or juveniles are likely limiting population growth (Matsuoka et al. 2010b). Luscier et al. (2010) conducted the first formal surveys IMPORTANT FINDINGS OF THE SPECIAL targeting wintering Rusty Blackbirds and modeled occupancy SECTION to show that birds wintering in the lower Mississippi Alluvial The first task of the International Rusty Blackbird Working Valley are commonly found in a variety of forested wetlands Group in 2005 was to identify and implement critical studies but often are observed in adjacent agricultural fields, where to understand the species’ natural history and factors contrib- they sometimes feed in loose association with other black- uting to its decline across its annual cycle. This special section birds. The species appears to depend on forest wetlands with presents the findings of the majority of the initial recom- open water but may use nearby disturbed sites, possibly to mended studies. Hobson et al. (2010) examined levels of deu- feed on waste grains and weed seeds to supplement its prin- terium in feathers of wintering Rusty Blackbirds and found cipal winter diet of aquatic and terrestrial invertebrates (Beal strong isotopic evidence for separate Mississippi and Atlantic 1900) and acorn masts and pine (Pinus sp.) seeds (Meanley flyways: birds that breed from Alaska through central Canada 1995). Other studies are currently underway in various parts winter in the Mississippi Alluvial Valley, and birds that breed of the winter range to fill critical gaps in our understanding of in eastern Canada winter along the Atlantic Coastal Plain. the species’ winter foraging ecology and the relative impor- These findings allow scientists to now link research, monitor- tance of various habitats for foraging and daytime and eve- ing, and management to the appropriate flyway, an approach ning roosts (N. W. Dias, C. Mettke-Hofmann, and P. Newell; effective in the management of North American waterfowl unpubl. data). (North American Waterfowl Management Plan Committee The cross-seasonal investigation by Edmonds et al. 2004). Hobson et al. (2010) also identified a geographically (2010) shows that methylmercury levels in Rusty Blackbirds restricted and potentially smaller Atlantic flyway population are an order of magnitude greater in breeding than in winter- that appears to be particularly susceptible to the extirpations ing populations, in part as a result of a trophic shift from an observed in New England, the Maritime Provinces, and the omnivorous diet during winter to a diet of predatory inver- southern boreal zone (Powell 2008, Greenberg et al. 2011; tebrates during summer. Methylmercury is higher in birds Maritime Breeding Bird Atlas, unpubl. data). breeding in New England and the Maritime Provinces than The special section updates substantially our understand- in those in breeding in Alaska, likely because of both lower ing of the species’ nesting ecology and habitat requirements pH in eastern wetlands and higher rates of atmospheric de- in te breeding season, topics previously known from brief de- position of mercury from industrial emissions. Barnard et scriptions of sixteen nests found in New England by Kennard al. (2010) found wintering birds in the Mississippi Alluvial (1920). Matsuoka et al. (2010a) and Powell et al. (2010a) re- Valley to have an unexpectedly high prevalence of infections spectively surveyed and radio-tracked breeding Rusty Black- with hematozoa, equivalent to levels found in birds breeding birds and found that the generally low breeding densities in Alaska and Maine. In temperate regions, winter is a time observed over much of the species’ range are linked to spe- when insect vectors are dormant, so the presence of hemato- cialized use of shallow-water habitats for foraging and large zoa in the peripheral circulatory system of birds is typically home ranges, which typically include multiple wetlands used rare but can be brought on by stress. These findings present for foraging and nesting. Matsuoka et al. (2010b) and Powell the intriguing hypothesis that in winter Rusty Blackbirds are et al. (2010b) analyzed data from >450 nests across the breed- prone to parasite and microbial infections because of stress ing range and showed that the species’ widespread use of induced by degradation of winter habitat. Researchers are
16_MS100153.indd 773 11/25/10 3:31:20 PM 774 RUSSELL GREENBERG AND STEVEN M. MATSUOKA
currently testing whether the rate of hematozoan infections or geologic formations that increase wetlands’ pH (i.e., lime- of populations wintering along the Atlantic Coastal Plain is stone) and thereby minimize methylation of mercury into also high (W. H. Barnard and P. Newell, unpubl. data). toxic methylmercury (M. Cadman, pers. comm.). Experi- Mettke-Hofmann et al. (2010) examined patterns of molt, ments that identify concentrations of methylmercury that kill finding that Rusty Blackbirds in fresh basic plumage in their Rusty Blackbird embryos (e.g., Heinz et al. 2009) would help first fall can be distinguished in the field from older birds on determine whether levels of methylmercury exposure at the the basis of a distinct pale eye ring and chin. However, a pre- southeastern edge of the breeding range affect reproduction viously undocumented pre-alternate molt during winter is rates at the population level (Edmonds et al. 2010). Studies concentrated on the head feathers and obscures most of these that quantify sublethal effects of mercury exposure on nest- age-related differences by late winter. Aging birds during this lings’ growth and health and survival of adults and juveniles time remains possible by in-hand inspection. Patterns of molt remain of paramount importance. are useful in aging birds during fall and early winter and for Studies also are needed to determine whether the ecologi- identifying feathers to be collected for isotopic studies of diet cal trap proposed by Powell et al. (2010b) is geographically or migration. widespread or applies to disturbances other than logging in and near wetlands where Rusty Blackbirds breed. Studies that WHAT MORE WE NEED TO KNOW use the before–after, control–impact (BACI) design (Smith 2002) may give insight on this front, but their spatial scale will The papers in this special section have advanced our under- need to be relatively large, given the generally low densities standing of the rangewide ecology of the Rusty Blackbird and at which the species breeds. The availability of shallow water have provided both important clues into the factors limiting and its aquatic macroinvertebrates (particularly Odonata) as populations and field-tested methods that can now be applied prey appears to be central both in selection of breeding sites to more focused studies. Research to date has not delivered the (Matsuoka et al. 2010a) and in successful reproduction. Un- much-needed answer to the mysterious question: what is caus- derstanding how the composition, densities, and phenology of ing the Rusty Blackbird’s decline? The negative trend contin- macroinvertebrates influence clutch size, nestling growth, and ues, and considerable research challenges remain before we mass at fledging will be important in inferring mechanisms can reverse it. Here we provide our recommendations for what governing habitat choice and reproductive success and for as- we need to know to make meaningful progress in restoring sessing how climate change will affect populations by altering Rusty Blackbird populations to historical levels. We empha- the phenology or reducing the availability of prey by desiccat- size our urgent need to understand factors that limit popula- ing wetlands (Matsuoka et al. 2010a). tion growth as well as our still basic need to identify important Any effort to protect or restore habitats for breeding Rusty regions, habitats, and resources for targeted conservation dur- Blackbirds will need to take into account the large area they ing the nonbreeding season. require (Powell et al. 2010a), to buffer against negative edge effects on nest survival (Powell et al. 2010b), and to recognize BREEDING SEASON that alternative habitats may be equally important after fledg- Critical advances have been made in understanding breeding ing (Faaborg et al. 2010), a period that remains to be studied. populations at the northwestern and southeastern extremes of the species’ boreal range. Studies are needed in the vast NONBREEDING SEASON regions in between. Such efforts would be most useful if fo- cused on populations breeding in areas of known decline and Population dynamics and demography during the nonbreeding range retraction and were designed to evaluate various hy- season have proven to be difficult to study. The Rusty Black- potheses for the decline. For example, researchers could re- bird is notorious in the extent to which winter populations peat previous surveys or atlases of breeding Rusty Blackbirds fluctuate within a year and in successive years. Particularly (e.g., Powell 2008) and compare habitats, patterns of land use, outside of the core wintering area in the lower Mississippi Al- levels of acidification or contamination, and climatic charac- luvial Valley, it is difficult to establish sites of long-term study teristics among sites that retain breeding blackbirds and sites and to monitor the fate of individuals. Three approaches may at which the species bred formerly (Machtan et al. 2007, Mat- facilitate winter research. First, understanding movements of suoka et al. 2010a). individual birds will provide insight into how Rusty Black- Birds breeding in New England and the Maritime Prov- birds respond to fluctuations in weather, hydrological condi- inces appear to be particularly threatened by high levels of tions, and mast-crop production. Hydrogen-isotope data have mercury contamination. Research is needed to understand elucidated a major migratory divide between the western and the geographic extent of this contamination (Edmonds et al. eastern boreal forest (Hobson et al. 2010), and this pattern is 2010) and how it varies by habitat and by distance to sources of supported by a small number of band recoveries (Hamel et SPECIAL SECTION: RANGEWIDE ECOLOGY OF THE DECLINING RUSTY BLACKBIRD RUSTY DECLINING THE OF ECOLOGY RANGEWIDE SECTION: SPECIAL atmospheric pollution, natural occurring sources of mercury, al. 2009). Short-range transmitters have revealed individuals’
16_MS100153.indd 774 11/25/10 3:31:21 PM RUSTY BLACKBIRD DECLIne 775 SPECIAL SECTION: RANGEWIDE ECOLOGY OF THE DECLINING RUSTY BLACKBIRD space use and roost locations over a few weeks (C. Mettke- weather. The relative importance of wetland conditions and Hofmann, unpubl. data), but more advanced tracking tech- the distribution and abundance of mast need to be assessed nology is necessary to clarify how blackbirds move during through correlational and experimental studies. For example, migration and in winter. Second, we need to locate and de- experiments in which water in impoundments is drawn down termine the spatial consistency of the remaining concentra- may be useful for testing how habitats can be managed to ben- tions of the species. Establishing locations of nonbreeding efit the species. “hot spots” and comparing landscape and local attributes of these spots to areas with only small numbers of blackbirds CROSS-SEASONAL PERSPECTIVE may disclose key factors for roosting and feeding. In an at- tempt to harness the largely untapped power of myriad bird- As for all declining migratory species, ultimately what is ers who comb the countryside daily, the International Rusty needed is a model of annual population growth that connects Blackbird Working Group and eBird have established a Rusty demographic processes throughout the year so that the relative Blackbird Winter Hotspots Blitz to locate and monitor areas importance of various factors on fecundity, survival, and re- of high concentrations. Once an array of sites for comparison cruitment can be integrated into models of population dynam- is established, standardized protocols for assessing hydrologi- ics. The cross-seasonal approach may be particularly needed cal conditions, weather, and mast-crop abundance need to be to understand when and where deficits in juveniles’ or adults’ instituted. survivorship limit population growth because nesting success A few studies focusing on the winter distribution of the is not clearly limited (Matsuoka et al. 2010b). Rusty Blackbird in various habitats are underway. Research- Studies might focus particularly on the Rusty Blackbird’s ers are using counts (Luscier et al. 2010), a variety of mea- Atlantic flyway population, which likely is smaller than the sures of condition (C. Mettke-Hofmann and P. Newell, unpubl. Mississippi flyway population and which appears to be at data), and age and sex ratios to compare sites. Refining these higher risk of extirpation from a combination of higher mer- measures should continue. In particular, wintering Rusty cury contamination during the breeding season and more hab- Blackbirds are often difficult to capture and have proven al- itat loss and alteration throughout the annual cycle. Analysis most impossible to recapture. The seemingly simple task of of potential genetic differentiation between Mississippi and developing reliable techniques for trapping continues to be a Atlantic flyway populations is underway and may provide im- high priority for winter work. Ultimately, our ability to esti- portant fuel to future debates as to whether this population mate apparent survival within a winter and in successive win- deserves protective status under the U.S. or Canadian endan- ters will be critical if we hope to develop year-round models gered species acts. of population dynamics, models that could be a primary tool for evaluating the causes of decline. To date, small-scale at- CONCLUSIONS tempts to obtain survivorship data have met with failure be- cause of the mobility of the birds and the low rate of resighting The attention now paid to a “common” species with a plung- of color-banded individuals. Progress will come when efforts ing population underscores a sea change in the approach to bird at banding and resighting become more regional or if appropri- conservation over the past two decades. The mantra, first ut- ate tracking technology is developed. tered by Rosalie Edge in 1932 (Furmansky 2009), that “the time Research on the population dynamics of songbirds has to save a species is while it is common; the only way to save a focused increasingly on the role of habitats in which the birds species is to never let it become rare” is now widely appreci- stop over during migration. Stopover habitat may be particu- ated among conservation biologists. Such a proactive approach larly critical in spring, when migration is more contracted and has been a philosophical underpinning of the Partners in Flight selective pressure to arrive on the breeding grounds early and program to conserve migratory birds (Rich et al. 2004) and the in good physical condition is high (Rotenberry and Chandler motivation of International Rusty Blackbird Working Group to 1999). With more information on habitat use and movement, it undertake the research presented in this special section. may be possible to look for cross-seasonal effects, where con- The Rusty Blackbird, a formerly common species, has ditions during winter and spring migration affect reproduc- suffered both a chronic long-term decline and an acute recent tive performance (Norris et al. 2006). decline. Most losses attracted no notice or comment. Like that The emerging picture is that two resources appear to of many birds breeding in North America’s boreal forest, the be critical to the success of nonbreeding Rusty Blackbirds: basic natural history of the Rusty Blackbird is poorly known. semi-flooded wetlands where blackbirds forage for aquatic Research over the past five years has been an exercise in catch- arthropods and fish and the mast of small-seeded oaks and ing up, so we may systematically evaluate causes of the de- domesticated pecans. Aquatic arthropods and fish provide a cline. We have accumulated a number of plausible hypotheses protein-rich diet, but they are available only from unfrozen for the decline, so the phase of testing hypotheses has begun in water. Mast provides energy easily procured regardless of earnest. The Rusty Blackbird has a number of attributes that
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make even the simplest research difficult: most of the breeding Fa a b o rg , J., R. T. Ho l m e s , A. D. An d e r s , K. L. Bi l d s t e i n , K. M. range is inaccessible, breeding populations are often sparse and Du gg e r , S. A. Ga u t hr e a u x Jr., P. He g l u n d , K. A. Ho b s o n , patchy, migratory and wintering populations are unpredictable A. E. Ja h n , D. H. Jo h n s o n , S. C. La t t a , D. J. Le v e y, P. P. Ma rr a , C. L. Me r k o r d , E. No l , S. I. Ro t h s t e i n , T. W. Sh e rry , T. S. and often hard to detect, and during the winter the birds are Si l l e t t , F. R. Th o mp s o n III, a n d N. Wa r n o c k . 2010. Recent difficult to catch and often almost impossible to recapture. But advances in understanding migration systems of New World land in many ways the Rusty Blackbird is typical of other common birds. Ecological Monographs 81:3–48. yet declining species. Although there has been a philosophical Fu rm a n s k y , D. Z. 2009. Rosalie Edge, hawk of mercy: the activist shift in conservation to focus on protecting species while they who saved nature from the conservationists. University of Geor- gia Press, Athens, GA. are still common, the effort and resources required to deter- Gr e e n b e rg , R. 2008. Bye-bye blackbird. Zoogoer, July–August, mine the factors underlying the decline of any particular spe- p. 9–15. cies are often daunting. However, the research is critical to any Gr e e n b e rg , R., a n d S. Dr o e g e . 1999. On the decline of the Rusty attempt at population recovery. The most important looming Blackbird and the use of ornithological literature to document question is do we have the will and the resources to address the long-term population trends. Conservation Biology 13:553–559. Gr e e n b e rg , R., D. W. De m a r e s t , S. M. Ma t s u o k a , C. Me t t k e - mystery of the Rusty Blackbird’s decline. Ho f m a n n , M. L. Av e ry , P. J. Bl a n ch e r , D. Ev e r s , P. B. Ha m e l , K. A. Ho b s o n , J. Lu s ci e r , D. K. Ni v e n , L. L. Po w e l l , a n d D. ACKNOWLEDGMENTS Sh a w . 2011. Understanding declines in Rusty Blackbirds. Studies in Avian Biology, in press. We thank each of the authors of the papers included in this spe- Ha m e l , P. B., D. De St e v e n s , T. Leininger, a n d R. Wi l s o n . 2009. cial section as well as the other members of the International Historical trends in Rusty Blackbird nonbreeding habitat in for- Rusty Blackbird Working Group (http://nationalzoo.si.edu/SCBI/ ested wetlands, p. 341–353. In T. Rich, C. Arizmendi, C. Thomp- MigratoryBirds/Research/Rusty_Blackbird/twg.cfm) for sharing son, and D. Demarest [e d s .], Proceedings of the 4th International with us their insights into this declining bird’s ecology and conserva- Partners in Flight Conference, McAllen, TX, 13–16 February tion needs. The free exchange of information in the workshops, sym- 2008. posia, and many, many conference calls organized by the working Ha m e l , P. B., a n d E. Oz d e n e r o l . 2009. Using the spatial filter- group has helped us move rapidly toward addressing the Rusty Black- ing process to evaluate the nonbreeding range of Rusty Black- bird’s decline—continued cooperation in this spirit will be needed if bird Euphagus carolinus, p. 334–340. In T. Rich, C. Arizmendi, populations of this once abundant bird are ultimately to be recovered. C. Thompson, and D. Demarest [e d s .], Proceedings of the 4th International Partners in Flight Conference, McAllen, TX, LITERATURE CITED 13–16 February 2008. Ha n n a h , K. C. 2004. Status review and conservation plan for the Au s t i n , J. E., A. D. Af t o n , M. G. An d e r s o n , R. G. Cl a r k , C. M. Rusty Blackbird (Euphagus carolinus) in Alaska. Alaska Bird Cu s t e r , J. S. La w r e n c e , J. B. Po l l a r d , a n d J. K. Ri n g e l m a n . 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