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2011
Chapter nine: Understanding Declines in Rusty Blackbirds
Russell Greenberg Canada Canadian Wildlife Service, [email protected]
Dean W. Demarest U.S. Fish and Wildlife Service, [email protected]
Steven M. Matsuoka U.S. Fish and Wildlife Service, [email protected]
Claudia Mettke-Hofmann Liverpool John Moores University, [email protected]
David Evers BioDiversity Research Institute, [email protected]
See next page for additional authors
Follow this and additional works at: https://digitalcommons.unl.edu/icwdm_usdanwrc
Greenberg, Russell; Demarest, Dean W.; Matsuoka, Steven M.; Mettke-Hofmann, Claudia; Evers, David; Hamel, Paul B.; Luscier, Jason; Powell, Luke L.; Shaw, David; Avery, Michael L.; Hobson, Keith A.; Blancher, Peter J.; and Niven, Daniel K., "Chapter nine: Understanding Declines in Rusty Blackbirds" (2011). USDA National Wildlife Research Center - Staff Publications. 1294. https://digitalcommons.unl.edu/icwdm_usdanwrc/1294
This Article is brought to you for free and open access by the U.S. Department of Agriculture: Animal and Plant Health Inspection Service at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USDA National Wildlife Research Center - Staff Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Russell Greenberg, Dean W. Demarest, Steven M. Matsuoka, Claudia Mettke-Hofmann, David Evers, Paul B. Hamel, Jason Luscier, Luke L. Powell, David Shaw, Michael L. Avery, Keith A. Hobson, Peter J. Blancher, and Daniel K. Niven
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/ icwdm_usdanwrc/1294 CHAPTER NINE
Understanding Declines in Rusty Blackbirds
Russell Greenberg, Dean W. Demarest, Steven M. Matsuoka, Claudia Mettke-Hofmann, David Evers, Paul B. Hamel, Jason Luscier, Luke L. Powell, David Shaw, Michael L. Avery, Keith A. Hobson, Peter J. Blancher, and Daniel K. Niven
An enormously abundant migrant. . . . The thousands of Grackles have been increased to tens of thousands. They blacken the fi elds and cloud the air. The bare trees on which they alight are foliated by them. Their incessant jingling songs drown the music of the Meadow Larks and produce, dreamy, far-away-effect, as of myriads of distant sleigh bells. E. E. THOMPSON (1891), Birds of Manitoba
Abstract. The Rusty Blackbird (Euphagus caroli- desiccation resulting from global warming. Other nus), a formerly common breeding species of likely reasons for decline include loss or degrada- boreal wetlands, has exhibited the most marked tion of wooded wetlands of the southeastern U.S decline of any North American landbird. North and mortality associated with abatement efforts American Breeding Bird Survey (BBS) trends in targeting nuisance blackbirds. In addition, the abundance are estimated to be Ϫ12.5%/yr over patchy breeding distribution of this species may the last 40 years, which is tantamount to a Ͼ95% inhibit population consolidation, causing local cumulative decline. Trends in abundance calcu- populations to crash when reduced to low levels. lated from Christmas Bird Counts (CBC) for a Progress in understanding the causes and mecha- similar period indicate a range-wide decline of nisms for observed declines has remained limited Ϫ5.6%/yr. Qualitative analyses of ornithological until recently. Here we present initial attempts accounts suggest the species has been declining to understand the habitat requirements of Rusty for over a century. Several studies document range Blackbirds and offer specific predictions associ- retraction in the southern boreal forest, whereas ated with each of the hypotheses for decline as limited data suggest that abundance may be more a way of guiding future research. stable in more northerly areas. The major hypoth- eses for the decline include degradation of boreal Key Words: contaminants, Euphagus carolinus, habitats from logging and agricultural devel- habitat use, limiting factors, population decline, opment, mercury contamination, and wetland population movements, Rusty Blackbird.
Greenberg, R., D. W. Demarest, S. M. Matsuoka, C. Mettke-Hofmann, D. Evers, P. B. Hamel, J. Luscier, L. L. Powell, D. Shaw, M. L. Avery, K. A. Hobson, P. J. Blancher, and D. K. Niven. 2011. Understanding declines in Rusty Blackbirds. Pp. 107– 126 in J. V. Wells (editor). Boreal birds of North America: a hemispheric view of their conservation links and significance. Studies in Avian Biology (no. 41), University of California Press, Berkeley, CA.
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From Carl D. Marti & Brett K. Sandercock, Boreal Birds of North America: A Hemispheric View of Their Conservation Links and Significance, Berkeley and Los Angeles: University of California Press, 2011.
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WWells_6480005_watermark.inddells_6480005_watermark.indd 1 110/20/110/20/11 1:401:40 PMPM he boreal zone provides the most exten- 1995). Breeding is closely tied to forested or tall sive forested habitat for high-latitude birds. shrubby wetlands and riparian zones (Erskine TBecause large parts of the region are inac- 1977, Avery 1995), with birds remaining largely cessible by road, even large changes in the sta- absent from adjacent upland interior forests tus of a boreal forest species may go unnoticed, and shrublands (Whitaker and Montevecchi or if detected, remain challenging to investigate 1997, 1999). Rusty Blackbirds winter almost and understand. The Rusty Blackbird (Euphagus entirely in temperate North America, where the carolinus) is a widespread boreal breeding song- core wintering area is located within the south- bird that has undergone a precipitous decline, as eastern United States (Avery 1995). The species evidenced by data collected through breeding and winters primarily in shallowly flooded wooded wintering surveys from across its North American wetlands of the Mississippi Alluvial Valley and range (Greenberg and Droege 1999, Niven et al. South Atlantic Coastal Plain. Current population 2004, Sauer et al. 2005). Unlike many migratory estimates developed using data and extrapola- species that breed in remote boreal habitats, the tions from North American Breeding Bird Sur- Rusty Blackbird winters entirely in temperate vey (BBS), the Canadian Breeding Bird Census North America, providing an opportunity to mon- Database, and other sources range from 158,000 itor the status of the entire population and evalu- to 2 million individuals (Rich et al. 2004, Savignac ate how it may be responding to threats occurring 2006), and are strongly influenced by the validity throughout the year. However, the inconspicu- of a few key assumptions that have not been rig- ous behavior of Rusty Blackbirds, coupled with orously evaluated (Rosenberg and Blancher 2005, their use of relatively inaccessible habitats dur- Thogmartin et al. 2006). ing winter (e.g., forested wetlands and swamps), complicate efforts to assess status on the less EVIDENCE FOR THE DECLINE remote wintering grounds. Thus, despite the fact that the decline of Rusty Blackbirds has spanned Analyses of long-term data sets including the several decades and has been widely recognized BBS (Sauer et al. 2005) and Christmas Bird Count for over fifteen years (Avery 1995, Link and Sauer (CBC) (Niven et al. 2004) have documented con- 1996, Greenberg and Droege 1999), only very sistent and significant declines in Rusty Blackbirds recently has research attempted to understand over the past 40 years. Additional careful review of and address the causes. In this paper, we summa- historical accounts (Greenberg and Droege 1999) rize what we currently know or strongly suspect suggests that Rusty Blackbirds had already gone about the basic ecology and conservation of Rusty from conspicuously abundant to uncommon Blackbirds, describe ongoing efforts to fill critical in many areas even before these modern survey information gaps and present a research strat- efforts began tracking them. Collectively, these egy for future work on this species. We present observations and data describe alarming and sus- this paper because of the intrinsic importance of tained population declines, range retractions, and understanding and addressing such a concerning local extirpations from across the range. decline in a formerly common and widespread bird, and to provide insights to approaches that North American Breeding Bird Survey might be applicable to other boreal species that present some of the same research challenges. BBS data currently provide the only standard- ized long-term assessment of large-scale breed- ing season abundance of the Rusty Blackbird. GENERAL DISTRIBUTION AND ECOLOGY For the period 1966–2005, these data indicate An estimated 80–90% of all Rusty Blackbirds a survey-wide population decline that averages Ϫ12 5 Ϯ 6 3 breed across the boreal forest region of North approximately . %/yr (CI95% . %/yr, America (Blancher and Wells 2005), from Alaska P Ͻ 0.01; Table 9.1, Fig. 9.1) (Sauer et al. 2005). to Newfoundland and south into the Maritime This trend corresponds to a loss of Ͼ95% of the Provinces, Adirondack Mountains, and the population since 1966, and represents one of coastal rainforest zone of southeastern Alaska the largest population declines documented by (Kessel and Gibson 1978, Godfrey 1986, Avery the BBS (Link and Sauer 1996, Sauer et al. 2005).
108 STUDIES IN AVIAN BIOLOGY NO. 41 Wells
WWells_6480005_ch09.inddells_6480005_ch09.indd 108108 88/24/11/24/11 11:16:4111:16:41 AMAM TABLE 9.1 North American Breeding Bird Survey annual trend estimates (%/yr) for Rusty Blackbird, 1966–2005 (Sauer et al. 2005)
Region Trenda Pb nc Mean Birds/Route
Alaskad Ϫ5.3 0.04 27 0.84 Yukon Territoryd Ϫ12.8 0.05 70.32 British Columbia Ϫ33.0 0.21 70.05 Ontario Ϫ14.9 0.01 11 0.24 Quebec Ϫ9.8 0.01 15 0.46 Newfoundlandd Ϫ7.7 0.20 15 2.03 New Brunswick Ϫ8.9 0.02 17 0.28 Nova Scotia Ϫ3.8 0.29 20 0.53 Maine 28.00.31 90.11 New Hampshire Ϫ0.2 0.90 60.11 New York 2.50.70 70.07 Surveywidee Ϫ12.5 <0.01 97 0.26
a Data for population trends are considered defi cient in either sample sizes (n < 14 routes) or the mean number of detected birds per route (<1.0 birds/route; Sauer et al. 2005). b Probability that the estimated trend differs from 0%/yr. c Number of routes included in the analyses. d Trends from Alaska, Yukon Territory, and Newfoundland are based on data from 1980–2005. e Surveywide trend analysis excludes data from Alaska, Yukon Territory, Newfoundland, and northern portions of some provinces.
5
4
3
2 Abundance index 1
0 1965 1975 1985 1995 2005 Year Figure 9.1. Trends in the abundance index of Rusty Blackbirds from 1966 to 2005 as estimated from the Christmas Bird Count (᭡, with 97.5% credible interval in gray) and North American Breeding Bird Survey (; Sauer et al. 2005). Christmas Bird Count data were from Bird Conservation Regions where the species was recorded on ≥4 count circles (Niven et al. 2004).
WWells_6480005_ch09.inddells_6480005_ch09.indd 109109 88/24/11/24/11 11:16:4111:16:41 AMAM Although more variable and less precise than (i.e., 150) and may contribute to the smaller con- the survey-wide trend estimate, regional BBS fidence interval on the CBC trend estimate. CBC analyses are also demonstrative of widespread data indicate a decline in all of the Bird Conser- declines (Table 9.1). vation Regions (BCR) where Rusty Blackbirds The magnitude, significance, and apparent were detected. In particular, a strong and consist- long-term consistency of the survey-wide BBS ent (3.7–5.1%/yr) decline was estimated for the trend for Rusty Blackbird provide compelling but four BCRs with the highest relative abundance somewhat limited evidence for a sharp range- (Central Hardwoods, West Gulf Coastal Plain, wide downturn in population size. First, Rusty Southeastern Coastal Plain, and Mississippi Blackbirds are detected on relatively few BBS Alluvial Valley). routes (approximately 150), many of which have Two patterns are evident in the CBC data and been surveyed sporadically and have frequent may provide insight into the Rusty Blackbird observer turnover. Combined with the low aver- decline (Fig. 9.1). First, the rate of decline has age number of detections per route, these factors diminished in recent decades. The estimated contribute to a relatively wide confidence inter- annual decline over the ten-year period from 1994 val (i.e., poor precision) for the trend estimate. to 2003 is only Ϫ2.1%/yr (Savignac 2006), whereas Second, survey coverage is limited to Ͻ30% of a period of marked decline occurred in the early the breeding range and is concentrated in the 1970s. This observation suggests that factors southern portion of the boreal forest. Thus, the contributing to declines in Rusty Blackbirds may possibility exists that survey-wide BBS data are have been particularly profound during or imme- not representative of trends in more northerly diately preceding the 1970s. Second, the relative breeding areas. For example, the survey-wide annual variation in counts was much greater trend estimates exclude data from survey routes prior to the late 1970s, and fluctuations around in Alaska, Newfoundland, Yukon Territory, and the estimated trend line have dampened as the northern portions of some provinces because population has continued to drop. Though vari- few routes in these areas encompass the long- ability might inherently diminish as abundance term period of analysis (Bystrak 1981, Sauer et al. approaches zero, this pattern is worth further 2005). Although regional trend analyses for these exploration as it may indicate that natural popula- northern areas do indicate declines of magnitude tion cycling is no longer occurring—or that the similar to the survey-wide estimate (Table 9.1), the ability of existing programs to detect significant precision of regional trend estimates is thought population changes with such low numbers is to be considerably poorer than the survey-wide rapidly approaching effort-related limits. estimate (Sauer et al. 2005). In comparing the two surveys, CBC data are less constrained by limited geographic coverage and Christmas Bird Count small sample size than are those from BBS; thus the trends estimated from CBC may have greater The CBC is an invaluable source of data on the external validity. On the other hand, the sampling status of Rusty Blackbirds because count circles effort for the BBS is far more carefully control- are distributed across the entire winter range of led, meaning that BBS data may have greater the species. Thus, we estimated winter population internal validity. Further, because the two surveys trend from CBC data for the period 1966–2005 track populations at different points in the annual following the methods of Niven et al. (2004). The cycle, valid biological explanations may account range-wide trend estimate for Rusty Blackbird for the differences in trend estimates. For exam- is Ϫ4.5%/yr (95% credible interval Ϯ 1.2%/yr; ple, the less severe decline suggested by the CBC Fig. 9.1). Though of smaller magnitude than the trend may reflect tempering of the trend estimate BBS trend estimate for the same time period by the annual production of young available for (Ϫ12.5%/yr), this decline is significant and tan- counting during early winter, whereas the BBS tamount to a total decline of approximately 85% trend is based largely on overwinter survival and for the 40-year period. This estimate is based on the recruitment of adults into the breeding popu- data from 1,611 count circles. This sample size is lation. Specifically, if fecundity or winter survivor- an order of magnitude greater than the number ship of Rusty Blackbirds increases as populations of BBS routes with Rusty Blackbird detections or densities decrease, then higher reproductive
110 STUDIES IN AVIAN BIOLOGY NO. 41 Wells
WWells_6480005_ch09.inddells_6480005_ch09.indd 110110 88/24/11/24/11 11:16:4211:16:42 AMAM 70 rare/uncommon fairly common 60 common abundant 50
40
30
20 Percent of bird accounts
10
0 <1921 1921–1950 1951–1980 >1980 Figure 9.2. Percent of regional or state ornithological accounts (n = 86) listing the Rusty Blackbird in four different abundance classes and time periods.
output and early winter survivorship resulting to abundant” to “rare or uncommon” (Fig. 9.2). from breeding population declines could poten- Much of this change in the description of the spe- tially bias early winter trend estimates upward, cies’ status occurred prior to the recent 40-year effectively dampening apparent declines. This decline documented by CBC and BBS. Analy- demonstrates an important need to understand ses of a larger sample of local checklists show post-breeding and overwinter survivorship and the same pattern in the qualitative descriptions their influence on population regulation. of Rusty Blackbird abundance (Greenberg and Droege 1999). Such a long-term decline is more Qualitative Historical Assessment consistent with trends and patterns in winter habitat loss than with environmental change in CBC and BBS trend estimates suggest an 85–95% the boreal forest, which is a more recent phenom- population decline over the past four decades, enon. Interestingly, the pattern of change in the but it is important to determine if this decline abundance descriptions for Rusty Blackbirds is represents only part of a longer historical proc- similar between the Mississippi Alluvial Valley ess. Unfortunately, no quantitative surveys are and the Southeastern Coastal Plain. available from earlier time periods. However, qualitative information on historical abundance Past and Current Breeding Distribution illustrates how population status may have been changing even before CBC and BBS (Greenberg Another line of evidence for the decline of Rusty and Droege 1999). For example, several observers Blackbirds is the contraction of the breeding in the late 19th and early 20th centuries provided range, particularly in the southern boreal forest. graphic descriptions of the high local abundance Recent surveys from Canada suggest an apparent of this species during migration in the northern range contraction from southern areas tradition- Great Plains, New England, and the Mississippi ally occupied by Rusty Blackbirds. For example, Valley (e.g., Beal 1890, Thompson 1891, J. H. recent surveys of 937 small wetlands in Alberta, Langille quoted in Beadslee and Mitchell 1965) Saskatchewan, and Manitoba have resulted in only that are not rivaled by any accounts since this 14 total detections (J. Morrisette, pers. comm.). period. Similar patterns have been observed in boreal In addition, analysis of regional and state orni- uplands; in more than 20,000 point count sur- thological accounts indicates a long-term shift in veys conducted across the western boreal forest in descriptions of Rusty Blackbirds from “common 1993–2006, only 80 locations documented Rusty
UNDERSTANDING DECLINES IN RUSTY BLACKBIRDS 111
WWells_6480005_ch09.inddells_6480005_ch09.indd 111111 88/24/11/24/11 11:16:4211:16:42 AMAM 0 25 50 100 Km
Aroostook Breeding Range N 2006 1978–1983 1949 1908 Piscataquis Figure 9.3. Range contraction of Rusty Blackbirds from the counties of eastern and northern Maine over the last century. Question marks indicate areas where knowledge of the current range remains anecdotal. Somerset Penobscot ? ? Wash.
Franklin ?
Oxford Hancock Waldo ? Kennebec
Blackbirds (S. Van Wilgenburg, J. Morrisette, recorded at fewer than 10% of surveyed wetlands and the Boreal Avian Modeling Project, http:// having apparently suitable habitat. Similar call– www.borealbirds.ca). Additionally, several wet- response surveys conducted from 2004 to 2005 at lands and lakes in eastern Saskatchewan and west- 350 wetlands in former breeding areas to the north ern Manitoba that were occupied in the 1970s are and east did not yield a single confirmed detec- apparently no longer occupied (K. Hobson and tion (Hodgman and Yates 2007). Recent anecdotal A. Smith, unpubl. data). evidence for Washington County suggests that The most complete analysis of range contrac- perhaps only a few breeding pairs remain. These tion comes from recent work in Maine, which sug- data and observations indicate that a substantial gests that the breeding range of Rusty Blackbird contraction in the breeding distribution of Rusty has contracted 65–160 km over the last century Blackbirds in Maine occurred between the 1980s (Fig. 9.3). Early records (e.g., Knight 1908) report and 2000. Rusty Blackbirds as summer residents through- Limited data from more northerly regions of out most of northern Maine and in southern parts the boreal forest suggest that range retractions of the state in Washington, Hancock, Waldo, and or general declines may not have been as pro- Kennebec counties. However, by 1950, Waldo, nounced among these more remote areas. For Kennebec, and southwestern Penobscot counties example, resurveys of 45 of the original 61 wetland were not included in the Maine breeding range sites censused in 1975 as background data for the (Palmer 1949). The Maine Breeding Bird Atlas impact of a proposed gas pipeline in the Macken- conducted from 1978–1983 confirmed the gen- zie Valley, Northwest Territories, suggested that eral distribution of breeding Rusty Blackbirds in wetland occupancy by Rusty Blackbirds had not Maine as described earlier by Palmer (Adamus changed substantially over the 30-year period 1987). Thus, although population declines may (Machtans et al. 2007). This analysis suggests that have occurred between the period of Palmer even if local declines had occurred in the North- (1949) and the Maine atlas, a substantial range west Territories, they are not of the magnitude that contraction during this time is not evident. would be expected if the population had declined Two decades later, Hodgman and Hermann at a rate similar to that detected from Christmas (2003) and L. Powell (unpubl. data) surveyed Bird Count data (Machtans et al. 2007). Data from Rusty Blackbirds in Maine using call–response the Ontario Breeding Bird Atlas suggest substan- techniques at nearly 400 wetlands in core breed- tial declines throughout the province except for ing areas in northern Aroostook, Piscataquis, the Hudson Bay Lowlands in the extreme north and Somerset counties. Rusty Blackbirds were (Cadman et al. 2008).
112 STUDIES IN AVIAN BIOLOGY NO. 41 Wells
WWells_6480005_ch09.inddells_6480005_ch09.indd 112112 88/24/11/24/11 11:16:4211:16:42 AMAM HYPOTHESES FOR THE CAUSES 2006). Subsequent declines in soybean prices, OF DECLINE however, led to afforestation efforts, principally those undertaken as part of the U.S. Department The reasons for the decline of the Rusty Blackbird of Agriculture Wetland Reserve and Conservation are currently unknown. However, multiple factors Reserve Programs (King et al. 2006). An estimated operating at different spatial and temporal scales 162,000 ha were enrolled in the Wetland Reserve are likely responsible for the chronic and acute Program during 1990–2005 (Ducks Unlimited patterns of decline observed both regionally and 2007). Considerable additional interest in affor- range-wide. Greenberg and Droege (2003) pro- estation has been spurred by utility industry pose three principal reasons, including: (1) winter investments in carbon sequestration programs habitat loss and degradation due to conversion that seek to offset carbon emissions while restor- and hydrologic alteration of bottomland hard- ing cleared forests in an ecologically compatible wood habitats; (2) breeding habitat degradation manner (Caspersen et al. 2000, Houghton 2002, due to logging, wetland drying, acidification, and Shoch et al. 2009). This pattern of habitat loss mercury contamination; and (3) direct mortality followed by stabilized habitat trends and then a associated with efforts to abate nuisance black- slow, steady gain appears to be consistent with bird problems, particularly during winter. Each the pattern seen in the CBC data, which show a of these is discussed below along with possible precipitous drop in Rusty Blackbird abundance in predictions and tests that could help in evaluat- the early 1970s. Emphasis of the Wetland Reserve ing their relative importance in explaining the and Conservation Reserve Programs on margin- observed declines. ally productive croplands will result in affores- tation activities that follow in reverse of earlier Loss or Degradation of Winter Habitat clearing patterns. This anticipated pattern sug- gests that as afforested lands mature into forests An estimated 75–80% of bottomland hardwood over the next several decades, considerable new habitats in the United States have been converted habitat may become available for wintering Rusty to agriculture since European settlement (Hefner Blackbird populations. and Brown 1988, Hefner et al. 1994, Twedt and Nonetheless, this positive scenario of a net Loesch 1999). Certainly, the gross loss of wooded future return of Rusty Blackbird winter habitat wetlands that has occurred in the southeastern needs to be kept in perspective. The great flex- United States would be consistent with a severe ibility of the agricultural community to respond long-term decline in any species dependent on to changing markets, exemplified by earlier land these habitats. The more difficult question is clearing in response to rising soybean prices, sug- whether the pattern of decline indicated by BBS gests that markets for new commodities could and CBC over the past 40 years is consistent with quickly shift the habitat balance in the south- spatial patterns and rates of forested wetland loss eastern United States back again to agricultural during the same time period (Greenberg and lands. Currently, large shifts in crop production Droege 1999). are occurring in response to new opportunities For example, some low-lying and seasonally for biofuels such as corn (Zea mays) for ethanol. flooded regions of the Mississippi Alluvial Valley Overall biofuel production is increasing faster remained uncultivated until the 1970s because of than 10% per year worldwide (Starke 2007), and low anticipated crop revenues and the high costs corn production in the United States may double associated with converting these areas into pro- in the coming decades (Ringelman 2007). Some duction farmlands. By the 1970s, high soybean of this expansion is occurring at the expense of (Glycine max) prices enticed growers to convert lands presently enrolled in wetlands conserva- and farm these areas, which may have been some tion easements. Corn production has undergone of the best remaining wintering habitats for Rusty recent expansion in the lower Mississippi Alluvial Blackbirds in the region. A distinct temporal pat- Valley in the past two years, but it is unclear how tern of land clearing from relatively drier contours this will affect currently forested areas, or areas more conducive to farming, to more flood-prone available for afforestation. areas (Rudis 2001) likely had large implications for Another consideration is the management birds dependent on forested wetlands (Twedt et al. of existing wooded wetlands. As a primarily
UNDERSTANDING DECLINES IN RUSTY BLACKBIRDS 113
WWells_6480005_ch09.inddells_6480005_ch09.indd 113113 88/24/11/24/11 11:16:4211:16:42 AMAM terrestrial, insectivorous species that forages in focused on wetland habitats. Large hydroelectric saturated or flooded soils, Rusty Blackbirds may projects and concomitant reservoir development be particularly sensitive to changes to natural for- has led to the loss of riparian habitats and wet- est flooding regimes resulting from drainage and lands in several areas (Greenberg and Droege diversions of water. In particular, the effects of 1999). For example, over 1 million ha of forest was the enormous water control projects of the Mis- flooded in central Quebec (Gauthier and Aubry sissippi River and its major tributaries (Barry 1996). Furthermore, wetland changes have also 1997) on Rusty Blackbirds is potentially quite been caused by the displacement of large volumes large but has not been estimated. In much of the of underground and surface water during oil and coastal Carolinas, wooded wetlands were histori- gas extraction (Schmiegelow et al. 1997, Whitaker cally impounded for rice production (Tompkins and Montevecchi 1999, Hobson et al. 2002, Bayne 1987). Many of these areas have regrown into sec- et al. 2005, Savignac 2006). Overall, the southern ond growth forest, but water levels are managed boreal forest has been impacted by timber har- principally for waterfowl or are not managed at vest, agriculture, mining, and oil and gas develop- all. Management of water levels appropriate for ment. Timber extraction is arguably the greatest blackbirds in both public and privately owned threat to the overall integrity of the boreal forest. impoundments represents a significant oppor- More than 60% of the commercially viable south- tunity for habitat enhancement and may also ern boreal forest has already been allocated to tim- provide a means for research to examine how ber companies. However, other forms of land use the depth, timing, duration, and spatial extent of have led to substantial habitat loss or degradation, inundation influence suitability of forested wet- particularly in the southern portion of the boreal lands for Rusty Blackbirds. Thus far, only anec- plain. Eight percent of the boreal forest biome dotal natural history of the species suggests its had been directly impacted by oil and gas extrac- sensitivity to the details of surface hydrology, and tion activities as of 2003 (Gauthier and Aubrey research is being undertaken to more rigorously 1996). Approximately 79% of the forest plain eco- test the impact of surface water conditions on the zone at the southern edge of the boreal forest in condition and survival of wintering blackbirds. Saskatchewan had been converted to agriculture since European settlement. Ongoing annual rates Loss or Degradation of Breeding Habitat of deforestation in this ecozone across southern Canada range from 0.8 to 1.7% per year (Hobson Until recently, the perception of the boreal forest et al. 2002). As a result of all this economic activ- as a vast expanse of undisturbed habitat has pre- ity, the amount of intact habitat in the southern vented many from invoking breeding habitat loss boreal forest is estimated to be no more than 75% or degradation as an important factor in boreal of the pre-European coverage (Lee et al. 2006), forest bird declines. However, several widespread and other estimates are much lower (Ricketts disturbances are noteworthy and deserve consider- et al. 1999). ation as factors contributing to recent population Boreal forests are dynamic habitats prone to nat- reductions in Rusty Blackbirds and other spe- ural disturbance regimes. More permanent habitat cies. Increasing surface air temperatures across change can result from an interaction of natural Alaska and northwestern Canada have resulted in forces and human management responses. For increases in the frequency, intensity, and extent of example, a massive outbreak of spruce budworms fire (Soja et al. 2006), as well as widespread dry- (Choristoneura fumiferana) from 1968 to 1985 ing of boreal wetlands (Klein et al. 2005, Riordan defoliated balsam fir and spruce across 55 million et al. 2006). In Alaska, the latter has resulted in a ha of forest spanning the boreal zone from Lake 19% loss of closed basin ponds, changes in water Superior east to the Atlantic coast (Blais 1983, chemistry, decreases in macroinvertebrate abun- Bolgiano 2004). This and related salvage logging dance, and invasions of woody plants (Corcoran may have caused widespread changes to Rusty 2005, Klein et al. 2005, Riordan et al. 2006). Blackbird breeding habitats. While the northern boreal forest feels the indi- The eastern boreal forest may be suffering a dis- rect effect of climate change, the southern boreal proportionate impact from the fallout of industrial plains have long been impacted by direct human pollutants in heavily populated portions of the U.S. settlement and resource exploitation, often and Canada. Industrial pollution has decreased the
114 STUDIES IN AVIAN BIOLOGY NO. 41 Wells
WWells_6480005_ch09.inddells_6480005_ch09.indd 114114 88/24/11/24/11 11:16:4211:16:42 AMAM quality of wetlands in the northeastern United States During 1974–1992, 83 roosts were sprayed with a and eastern Canada by lowering pH, depleting envi- surfactant (PA-14) which killed approximately 38 ronmental calcium (Greenberg and Droege 1999), million blackbirds, principally Common Grackles and increasing concentrations of methylmercury (Quiscalus quiscula), European Starlings, Red-winged (MeHg) (Lovett et al. 2009). Although aquatic eco- Blackbirds, and Brown-headed Cowbirds (Molothrus systems sensitive to environmental mercury load- ater) (Dolbeer et al. 1997). Rusty Blackbirds have ing are well established as having adverse impacts been estimated as comprising less than 1% of the on piscivorous birds such as the Common Loon birds in these mixed-species winter roosts (Meanley (Gavia immer) (Burgess and Meyer 2008; Evers et al. and Royall 1976). Thus, it was estimated that only 2008), only recently has the availability of MeHg 120,000 Rusty Blackbirds were affected by the PA-14 in wetland birds been identified as a major threat applications at the 83 winter roosts (Dolbeer et al. (Schwarzbach et al. 2006). Concentrations deemed 1997). The use of the surfactant was discontinued to have adverse effects on egg hatchability, based on when the EPA registration lapsed in 1992 and was Heinz et al. (2008), have been documented for wild not renewed. The period covered by this program breeding populations of Icterids, including the Red- coincides with that of the steepest decline as docu- winged Blackbird (Agelaius phoeniceus) (Evers et al. mented by the CBC data. The mortality estimates 2005) and Rusty Blackbird (Evers, pers. comm.). from control efforts do not suggest a magnitude that Current sampling efforts of Rusty Blackbird tis- is equivalent to that of the global population declines sues have documented significantly higher blood (which would be on the order of millions of birds mercury concentrations in breeding versus winter- based on available trend and population estimates). ing individuals, with highest levels recorded in the Nonetheless, it would be instructive to examine the northeastern United States (BioDiversity Research regional patterns of decline and how they relate to Institute, unpubl. data). Reasons for elevated levels specific control events. Currently, the Rusty Black- may be related to Rusty Blackbirds foraging on high bird–specific mortality estimates and the population trophic level invertebrates (e.g., arachnids; Cristol estimates based on CBC appear to be insufficiently et al. 2008) from low-pH wetlands with frequent precise to support such an analysis. water level changes that are conducive to high meth- The toxicant DCR-1339 (Starlicide®) is cur- ylation rates (Driscoll et al. 2007). rently applied in rice-growing areas of Texas and It should be noted that a number of other spe- Louisiana to reduce blackbird depredations to cies that co-occur with Rusty Blackbirds in boreal early-sprouting rice (Cummings and Avery 2003). wetlands during the breeding season are also suf- Laced bait is applied on staging areas to affect birds fering steep declines over the past few decades. entering and leaving large winter blackbird roosts. These include Lesser Scaup (Aythya affinis); Black In parts of the Rusty Blackbird winter range, this (Melanitta nigra), Scoters; Horned Grebe (Podiceps toxicant is also used at feedlots and dairies for auritus), White-winged (M. fusca), and Surf (M. per- starling control (Homan et al. 2005). There is spicillata); and Lesser Yellowlegs (Tringa flavipes) no estimate of the take of Rusty Blackbirds from (Austin et al. 2000, Hannah 2004, North America DRC-1339 through either of these applications. Waterfowl Management Plan Committee 2004, Similarly, there is no information on how Rusty Sauer et al. 2005, U.S. Fish and Wildlife Service Blackbirds might be affected by legal removal of 2006). Thus, Rusty Blackbirds may be responding blackbirds as authorized in U.S. regulations under to factors causing degradation in boreal wetlands a blackbird depredation order (50 CFR 21.43) or by that are having much broader impacts. illegal shooting and trapping activities.
Blackbird Control Efforts Migratory Allee Effect
Rusty Blackbirds are not considered crop pests, but Regardless of the external factors driving the they do join other blackbirds and European Star- decline of Rusty Blackbirds, two aspects of its lings (Sturnus vulgaris) in large communal winter life history may contribute to strong, negative roosts. Because of nuisance, property damage, and density dependence at low population size (Allee health concerns, winter roosts have been subjected effect). First, the species is often patchily distrib- to extensive control programs in the southeastern uted, following the local occurrence of appropri- United States (Garner 1978, Heisterberg et al. 1987). ate wetland habitat across the landscape. Second,
UNDERSTANDING DECLINES IN RUSTY BLACKBIRDS 115
WWells_6480005_ch09.inddells_6480005_ch09.indd 115115 88/24/11/24/11 11:16:4211:16:42 AMAM the migratory nature of the species means that Act, largely because of the flexibility this affords individual birds settling to breed across the boreal the Canadian government in addressing crop landscape may not coalesce into viable breeding depredation and other nuisance situations caused populations because of difficulty in locating mates by blackbirds. or forming the loose nesting colonies observed Until January 2011, the Rusty Blackbird could in Alaska and Newfoundland (see below under be legally taken in the United States without a per- Assessing Habitat Use on the Breeding Ground). mit under an existing depredation order for black- A similar argument was made to account for the birds, cowbirds, grackles, crows, and magpies (50 last stages of extinction of the Bachman’s Warbler CFR 21.43). This order facilitates the lethal con- (Vermivora bachmani; Wilcove and Terborgh trol of these birds when “committing or about to 1984). In particular, birds nesting in groups commit depredations . . . or when concentrated may be much more successful on a per capita in such numbers and manner as to constitute a basis than individual pairs due to group defense health hazard or other nuisance.” In conjunction against nest predators, as has been found in Red- with all affected stakeholders, the U.S. Fish and winged Blackbirds (Picman et al. 1988, Yasukawa Wildlife Service has excluded Rusty Blackbirds et al. 1992). Taken together, the regional popula- from this depredation order in acknowledgment tion size at which an extinction vortex (Gilpen of the increasing vulnerability of Rusty Blackbird and Soule 1986) is reached may be relatively high populations to extirpation, the minimal threat for migratory species, such as Rusty Blackbirds, they have on crops or other commodities, and compared to local resident species. This may their limited potential to congregate in numbers explain rapid range contractions in regions, such constituting a health or safety concern. as the southern boreal forest, where today seem- Non-regulatory designations add weight for the ingly appropriate breeding habitat is available but prioritization of funding and other resources ded- unoccupied. icated to conservation, research, and monitoring of this species. Probably the most significant list- LEGAL AND CONSERVATION STATUS ing for the Rusty Blackbird is its recent inclusion on the IUCN Red List as a “vulnerable” species OF THE RUSTY BLACKBIRD (IUCN 2007). NatureServe (2006) ranks Rusty Despite the evidence for a profound population Blackbirds as being secure at the range-wide decline, Rusty Blackbirds have only very recently scale, but notes vulnerabilities at state and provin- received heightened conservation attention from cial levels. Partners in Flight’s North American governmental and private conservation entities. Landbird Conservation Plan (Rich et al. 2004) lists In the United States, Rusty Blackbirds receive the Rusty Blackbird as a species of Continental Con- same legal protection afforded most migratory cern that is moderately abundant and widespread, birds under the Migratory Bird Treaty Act (16 USC but experiencing declines and high threats. The 703–711). The act establishes a federal prohibition U.S. Fish and Wildlife Service has designated against unauthorized pursuit, hunting, and kill- Rusty Blackbird as a Bird of Conservation Concern ing of birds identified in various bilateral treaties (U.S. Fish and Wildlife Service 2002), which earns between the United States and Great Britain (for the species additional attention and consideration Canada), Japan, and Russia. However, the act does in various activities of the agency. Similarly, the not provide for the protection of habitats, nor does Committee on the Status of Endangered Wildlife it mandate proactive conservation and manage- in Canada recently identified Rusty Blackbird as a ment to sustain populations of protected species. species of Special Concern (Savignac 2006). Further, the act does allow for the permitted take Also outside of the regulatory arena was the of protected species when they threaten agricul- formation of the International Rusty Blackbird tural crops or human health or safety. In Canada, Working Group in April 2005. This ad hoc group the Migratory Birds Convention Act (1994, c. 22, has consisted of approximately over 60 scientists, s. 19) implements the bilateral treaty between biologists, and program managers focused on (1) Canada and the United States for the protection developing an overarching research and monitor- of migratory birds. However, this act does not ing strategy to understand and reverse the spe- confer the same protections afforded blackbirds cies’ decline; (2) providing information about the in the United States by the Migratory Bird Treaty species and its decline to the greater scientific,
116 STUDIES IN AVIAN BIOLOGY NO. 41 Wells
WWells_6480005_ch09.inddells_6480005_ch09.indd 116116 88/24/11/24/11 11:16:4311:16:43 AMAM conservation, and resource management com- nests noted near active nests (Kennard 1920, munity as well as to bird enthusiasts and the Shaw 2006, R. Corcoran, pers. comm.). Recent general public; and (3) serving as a forum for the studies in Alaska found nests as close as 75 m to real-time exchange of information among part- one another but more typically Ͼ250 m apart. At ners having a stake in Rusty Blackbird conserva- sites with multiple pairs in Alaska, adults joined tion. As of 2008, members of the working group into groups of 3–7 individuals to mob potential had initiated research programs in both northern predators near nests, particularly during the boreal habitats (particularly Alaska and Maine) late nestling stage (Corcoran 2006, Shaw 2006, and southeastern bottomland hardwood ecosys- P. Meyers, pers. obs.). tems (Mississippi, Arkansas, and South Caro- The species breeds in bogs, wet meadows, or lina), supported by U.S. Fish and Wildlife Service, along ponds, lakes, and streams (Kennard 1920, Department of Defense Legacy Program, U.S. Gabrielson and Lincoln 1959, Ellison 1990, Avery Geological Survey, and the U.S. Forest Service. In 1995, Sinclair et al. 2003). To many observers, the 2008, efforts were initiated to collaborate with the gestalt of a Rusty Blackbird breeding area includes eBird program (Cornell Laboratory of Ornithol- a mixture of open water, flooded meadow or float- ogy and the National Audubon Society) to locate ing emergent vegetation, and conifers or tall “hotspots” for the species during the winter and shrubs. The dominant breeding habitat, however, migration periods. All of these efforts are helping varies among the principal regions where it has better describe the spatial and temporal distribu- been studied. For example, Rusty Blackbirds are tion of Rusty Blackbirds as a basis for ongoing almost completely restricted to beaver ponds in and planned research, monitoring, and conserva- New England, whereas in western Alaska they can tion activities. be locally common in shrub and meadow vegeta- tion along rivers, and in southern Alaska they are EFFORTS TO FILL CRITICAL found along sloughs in early successional forests INFORMATION GAPS dominated by Sitka spruce (Picea sitchensis). Many observers have commented that Rusty Blackbirds Habitat loss and degradation figure prominently can be seen flying great distances within or out- in hypotheses for why the Rusty Blackbird is side of the wetlands where their nests are located. declining. Therefore, a complete understanding Telemetry studies are needed to determine how of habitat requirements and what features con- extensive an area they require to forage during tribute to habitat quality is necessary to evaluate the breeding season and the nature of the overall habitat-based hypotheses. habitat mosaic on which they depend. Based on anecdotal accounts, it seems unlikely Habitat Use on the Breeding Ground that the nest site itself is limiting the distribution of the species. This species builds a large nest Understanding of the life history of the Rusty low (Ͻ6 m) in small live or dead conifers (Picea Blackbird has advanced surprisingly little since or Abies spp.) or tall shrubs (Salix or Alnus spp.) Bent (1958). In particular, information on habi- (Kennard 1920, Gabrielson and Lincoln 1959, tat use on the boreal breeding grounds is scant Ellison 1990, Sinclair et al. 2003, Shaw 2006). and largely restricted to observations of limited Although dependence on small conifers for nest numbers of birds in New England (Kennard 1920, sites is particularly marked in New England (Ken- Ellison 1990) and recent studies in Alaska (Corc- nard 1920, Ellison 1990), the preferred tree or oran 2006, Shaw 2006, P. Meyers, unpubl. data). shrub species used for nesting appears to vary Rusty Blackbirds are generally described as breed- regionally. For example, along the drainage of the ing solitarily and at low densities, but may also be Yukon River and its tributaries in interior Alaska, found in small groups of a few to several pairs in over 75% of 37 nests were found in live or dead Alaska and Newfoundland (Peters and Burleigh willows and often over water—fewer nests were 1951, Gabrielson and Lincoln 1959, Ellison 1990), found in live or dead spruce (Picea glauca and P. where the species appears to be most abundant mariana; Corcoran 2006, Shaw 2006, K. Martin, (Sauer et al. 2005; Table 9.1). Rusty Blackbirds are unpubl. data, K. Sowl, unpubl. data). In contrast, patchily distributed and have often been reported in the coastal rainforest zone on the upper Cop- occupying the same locations annually, with old per River Delta, Alaska, 87% of 17 nests were
UNDERSTANDING DECLINES IN RUSTY BLACKBIRDS 117
WWells_6480005_ch09.inddells_6480005_ch09.indd 117117 88/24/11/24/11 11:16:4311:16:43 AMAM found in Sitka spruce and the remainder in In contrast to feeding areas, night roosts were alder (P. Meyer, unpubl. data). It is likely that more likely in fields than in scrub or forests. Pre- these regional patterns reflect differences in the ferred night roosts (20 to 400 birds) were often in availability of small trees rather than geographic afforestation areas with dense vegetation near the variation in preference. ground, but were also in fields with short vegeta- tion, or trees or shrubs in residential yards—in Winter Habitat Characteristics the latter of which birds joined large roosts with other blackbird species. The preference for these As in other temperate zone blackbirds, Rusty relatively open, treeless habitats for night roosts is Blackbirds have two distinct habitat needs dur- surprising given the avoidance of these habitats ing the non-breeding season: foraging areas and during feeding. Most of the trees in foraging areas roosting sites. In terms of foraging areas, Rusty are leafless in winter; birds may be quite conspic- Blackbirds winter primarily in wet bottomland uous when using them as roosts. Thus, the ample hardwood forests (Avery 1995). They are com- low cover in new afforestation areas and vegetated mon in areas of continuous semi-flooded forest, fields and the dense foliage of evergreen trees in but in drier areas they can be seen in association residential yards may give roosting blackbirds with smaller wooded wetlands, such as beaver important protection against nocturnal predators ponds. They are also regularly found in more and cold temperatures. Most Rusty Blackbirds are open habitats such as pecan (Carya illinoiensis) found either in single-species roosts or mixed with orchards and in forest fragments along creeks some Red-winged Blackbirds. However, as winter (Mettke- Hofmann unpubl. data). The extent to progresses, Rusty Blackbirds are more frequently which Rusty Blackbirds use each of these habitats found in large mixed-species blackbird roosts. is poorly quantified, as is the effect that the use of these different habitats has on fitness. Within FUTURE RESEARCH these habitats, birds show a preference for forag- ing near the edge of shallow water without regard Now that it is understood that the Rusty Black- for understory vegetation density or distance to bird has suffered both a long-term and precipi- forest edge. Habitat preference, in part, seemed tous range-wide decline, future research needs to reflect the availability of preferred food. Rusty to address three separate, but related needs: Blackbirds appeared to depend on two distinct (1) a testing of predictions associated with the dietary items: (1) small acorns and pecans, which proposed causes of decline to begin to evalu- are often eaten while associating with Common ate which factors are of paramount importance; Grackles, whose large, strong bills are able to (2) a plan consisting of concrete, proactive man- crack nutshells; and (2) invertebrates picked from agement recommendations to reverse the decline; water or soil, or captured after flipping leaf lit- and (3) continued monitoring of the population to ter and floating vegetation. Consequently, Rusty provide both regional and global data to facilitate Blackbirds appeared to select areas with the the first two objectives. proper species of mast-producing oaks and hicko- ries, or areas having a surface mosaic of water and Testing Hypotheses for the Decline moist soil that supported the appropriate inverte- brate fauna. The qualitative information so far Because resources are limited and the distribu- suggests that Rusty Blackbirds prefer forests with tion of the species is vast, our ability to determine mature oaks (particularly willow oaks, Quercus the causes of the decline will depend on the for- phellos), areas with small creeks, and patchily mation of specific hypothesis that can be tested inundated areas of shallow water and exposed by gathering information from well-defined, substrate. Thus, overstory composition, the avail- focal studies. The most important hypotheses are ability of mast-producing individuals of key oaks listed in Table 9.2, along with an attempt to define and hickories, the depth and extent of inundation, key data that will allow us to test key predictions and general soil moisture regimes all appear to from these hypotheses. Developing specific be important characteristics for future studies of testable predictions for these various breeding- winter habitat quality. The relative importance of ground factors is difficult because of the lack of these factors needs further evaluation. access to study this sparsely distributed species.
118 STUDIES IN AVIAN BIOLOGY NO. 41 Wells
WWells_6480005_ch09.inddells_6480005_ch09.indd 118118 88/24/11/24/11 11:16:4311:16:43 AMAM However, while a complete picture of the status of exploration of individual CBC circles could shed a widely distributed boreal population is impos- light on the temporal and geographic patterns sible, evidence to evaluate the cause of decline of decline in relation to available data on habitat could be developed with strategically placed change. In the future, focused surveying, using studies. The site occupancy approach is being site occupancy approaches in different regions of used for surveys on breeding areas in Maine the wintering ground, can be compared to habi- (L. Powell, unpubl. data) and Alaska (S. Matsuoka tat change as well. These abundance data, in con- and D. Shaw, unpubl. data) and, along with more junction with information on condition and other detailed life history information, can help test correlates of blackbird fitness, could be used to specific breeding ground–related hypotheses. assess habitat quality, which can be added to pre- For example, because each of the listed hypoth- dictive models for future decline. Other issues for eses for decline has a distinct geographic pattern ongoing and future research that will illuminate for the likelihood of impact, the local population the nature of the Rusty Blackbird decline include: trend and critical aspects of life history (particu- larly mating success, breeding success, nestling • a more quantitative analysis of the loss of growth, or post-fledging survival) could be meas- Rusty Blackbirds due to control efforts; ured at key sites in the boreal region. If simple • sampling of Rusty Blackbirds for mercury habitat loss or degradation was the driving force, and other contaminants in different portions then the populations should decline or contract of their breeding and winter range; away from areas where habitat has been destroyed • comparisons of demographic variables, or degraded. But if further environmental fac- such as nest success and survivorship, tors affect the quality of habitat, such as MeHg for different regions of the breeding and contamination or changes in food supply due wintering season; to global warming and boreal wetland drying, then the reproductive output and particularly the • telemetry studies that reveal the details of growth, condition, and survival of young should how Rusty Blackbirds use the habitat mosaic be affected even in areas of seemingly appropri- for different aspects of their life history; ate habitat. For example, support for the effect • further study of winter roosting behavior of mercury contamination would be provided if and how this affects access to preferred (1) Rusty Blackbirds have declined disproportion- habitat and vulnerability to blackbird control; ately in areas that are most affected by mercury • an examination of the possible role of accumulation; (2) Rusty Blackbird blood and disease and parasites in the decline of Rusty tissue show relatively high levels of mercury Blackbird populations; and in birds from a region of greatest decline; and (3) reproductive anomalies associated with chronic • more detailed studies of how forest manage- mercury toxicity are documented. Similarly, ment and hydrological interventions affect acidification of wetlands should cause a distinct habitat quality, particularly on the wintering geographic pattern of decline associated with grounds. specific effects of calcium stress, such as eggshell thinning and poorly developed skeletons in young Management Recommendations birds. Finally, if declines are disproportional in regions experiencing greater wetland drying or An overall management strategy will have to await on a local level, smaller bodies of water might be more definitive information on the causes of the more immediately affected, and the growth rate Rusty Blackbird decline. However, many of the and fledging success of birds in these areas may hypothesized causes for decline lend themselves be significantly lower than in less-affected areas to general conservation and environmental meas- (Table 9.2). ures that are not exclusive to the Rusty Blackbird. Similarly, the case for loss or degradation of For example, to the degree the development of winter habitat can be evaluated by exploring the boreal wetlands has contributed to the decline, correlation between population declines and land efforts to protect and restore these ecosystems use changes at a finer scale than has been accom- will contribute to the recovery of populations. plished thus far. For a retrospective analysis, Similarly, control of emissions that contribute to
UNDERSTANDING DECLINES IN RUSTY BLACKBIRDS 119
WWells_6480005_ch09.inddells_6480005_ch09.indd 119119 88/24/11/24/11 11:16:4311:16:43 AMAM TABLE 9.2 Possible causes, predictions, and necessary data to test predictions for declines in the Rusty Blackbird
Hypothesis Prediction Data Needed
Breeding Season
Breeding Habitat Loss Steeper declines in areas/regions Temporal and spatial data on suffering greatest loss or habitat loss, preferably that conversion of habitat. collected in association with monitoring data (e.g., habitat loss along survey routes). Boreal Habitat Degradation Wetland drying Steeper declines, lower fi tness, Detailed studies of site and lower food availability occupancy and reproductive in regions with the strongest performance across boreal apparent effect of drying wetland systems of differing (e.g., decreased surface water size and varying vulnerability availability). to drying. Fitness correlates: low chick growth rate and condition (e.g., mass). Methyl mercury (MeHg) and Steeper declines and lower fi tness MeHg levels from areas with wetland acidifi cation (also in regions with higher levels Rusty Blackbird surveys. includes wintering habitat of MeHg contamination or Tissue samples from different conditions) acidifi cation. boreal regions. High levels of MeHg in tissues Tissue samples from different from these regions. winter regions if strong Fitness correlates: low adult connection with breeding survival and chick growth rates, region can be established. high nest failure, and other Focused studies of reproductive reproductive anomalies. For success from wetlands with acidifi cation, egg shell thinning different levels of MeHg and skeletal deformities in contamination. chicks. Allee Effect Local breeding populations Detailed information on site extirpated as regional occupancy, mating success, populations decline in the and reproductive success absence of any identifi able coupled with regional data on deterioration of habitat. population trends. As populations decline, more sites will be populated by unmated birds.
Non-breeding Season
Non-breeding Habitat Loss Abundance correlates with Land use trend data to analyze changes in area of de- and with long-term data on afforestation. blackbird abundance (e.g., land Predictions can be refi ned as use within CBC circles). details of habitat quality are Bird responses to different habitat developed from studies. types in terms of condition and survival. TABLE 9.2 (continued)
WWells_6480005_ch09.inddells_6480005_ch09.indd 120120 88/24/11/24/11 11:16:4311:16:43 AMAM TABLE 9.2 (CONTINUED)
Hypothesis Prediction Data Needed
Non-breeding Season
Non-breeding Habitat Degradation: Within intact habitat, declines Fine-scale data on the patterns of Changes in Forest Composi- occurred in areas with greatest decline and the condition of tion (mast-producing trees) and changes in the hydrology and bottomland hardwood forest. Hydrology composition of bottomland Atlas-level data on the distribution forests. of winter concentrations of Rusty Remaining concentrations are Blackbirds with appropriate found in areas managed for habitat data. ephemeral shallow water and high abundance of small acorn- producing trees. Blackbird Control Mortality from targeted and Number of Rusty Blackbirds taken diffuse control efforts is by control efforts at large roosts. suffi cient to have an impact on Systematic data on the geographic population processes. scope of blackbird control Geographic and temporal pattern and the composition of roosts in declines correlates with coupled with long-term, site- major blackbird control efforts. specifi c population trend data.
global warming, acid rain, and MeHg contamina- Ensuring that appropriate habitat is maintained tion may help restore Rusty Blackbirds to existing in proximity to preferred nesting and roosting wetlands where they have disappeared. Similarly, areas will require detailed understanding of how policies and actions that protect and restore bot- Rusty Blackbirds use the complex habitat mosaic tomland hardwood forests and other wooded wet- associated with both boreal and southern wooded lands would provide more habitat for wintering wetlands. populations. Certain more specific management recommen- Monitoring dations may also be made proactively, based on the ongoing research focused on the habitat use Because the Rusty Blackbird has a large and largely and roosting behavior of the species. For example, inaccessible breeding distribution throughout the Rusty Blackbirds appear to suffer indirectly from boreal forest, population monitoring is likely to control efforts focused on blackbirds in general. be based on the limited number of boreal BBS Rusty Blackbirds often feed and roost in areas routes and the more geographically comprehen- where relatively few individuals of other black- sive, but less rigorously gathered, CBC data. It is birds occur. Control efforts that avoid prime habi- unlikely in the near future that a more extensive tat for Rusty Blackbird flocks and roosts would monitoring program focused on Rusty Blackbirds reduce the incidental mortality that might be will be implemented at either end of the annual occurring. Wooded wetlands are often managed cycle. It may be most strategic to incorporate more for waterfowl and other wildlife. Managing the focused monitoring into studies aimed at testing flooding regime of these areas to create appropri- the specific hypotheses for declines (see Testing ate foraging habitat may be another approach to Hypotheses for Declines). For example, region- increase the quality of protected habitats. Finally, ally based citizen-science atlases or surveys using afforestation efforts that include management for site occupancy approaches could be implemented preferred food plants, such as oaks and hickories, in each of the BCRs that are most important in for Rusty Blackbirds may also enhance habitat supporting wintering populations of Rusty Black- quality for recently reclaimed agricultural lands. birds. Such a survey effort could gather relatively
UNDERSTANDING DECLINES IN RUSTY BLACKBIRDS 121
WWells_6480005_ch09.inddells_6480005_ch09.indd 121121 88/24/11/24/11 11:16:4311:16:43 AMAM unbiased information on the use of different Austin, J. E., A. D. Afton, M. G. Anderson, R. G. Clark, habitats, which could later be correlated with land C. M. Custer, J. S. Lawrence, J. B. Pollard, and use change information. This effort could be made J. K. Ringelman. 2000. Declining scaup popula- more efficient by concentrating on the late winter, tions: issues, hypotheses, and research needs. 28 254 263 when the populations are more stable and sing- Wildlife Society Bulletin : – . 1995 ing greatly increases the detectability of blackbird Avery, M. L. . Rusty Blackbird (Euphagus caro- linus). A. Poole and F. Gill (editors), The birds of flocks. Increasing the breeding season coverage North America, No. 200. Academy of Natural Sci- is even more problematic. However, since appro- ences, Philadelphia, and American Ornithologists’ priate wetlands are often patchy and discrete, a Union, Washington, D.C. program of conducting site occupancy surveys in Barry, J. M. 1997. Rising tide: the great Mississippi a small number of representative regions seems flood of 1927 and how it changed America. Touch- within the realm of feasibility. Increasing the stone Books, New York, NY. coverage of Rusty Blackbird surveys across the Bayne, E. M., S. Boutin, B. Tracz, and K. Charest. 2005. boreal forest region would be most feasible if Functional and numerical responses of Ovenbirds incorporated with existing surveys for other types (Seiurus aurocapilla) to changing seismic explora- of birds, as is being done with waterfowl surveys tion practices in Alberta’s boreal forest. Ecoscience in the Yukon Territory (P. Sinclair, unpubl. data) 12:216–222. or could be done with surveys of boreal-nesting Beadslee, C. S., and H. D. Mitchell. 1965. Birds of shorebirds (V. Johnston, unpubl. data). the Niagara frontier. Buffalo Society of Natural Sciences, Buffalo, NY. Beal, F. E. 1890. Food of the bobolink, blackbirds, and ACKNOWLDGMENTS grackles. USDA Biological Survey Bulletin 13. The authors wish to thank our colleagues on the Bent, A. C. 1958. Life histories of North American International Rusty Blackbird Working Group for blackbirds, orioles, tanagers, and their allies. continued discussions of the scientific and conserva- U.S. National National Museum Bulletin 211. tion issues involved in Rusty Blackbird conservation. Smithsonian Institution, Washington, DC. The USDA Forest Service provided housing, cars, and Blais, J. R. 1983. Trends in the frequency, extent, and logistical support. Furthermore, we thank Kathryn severity of spruce budworm outbreaks in east- Heyden, Gerhard Hofmann, Catherine Ricketts, and ern Canada. Canadian Journal of Forest Research Carl Smith for valuable field assistance during telem- 13:539–547. etry. The Yazoo National Wildlife Refuge (NWR) pro- Blancher, P. and J. V. Wells. 2005. The boreal forest vided housing for our helpers. We thank the Theodore region: North America’s bird nursery. Canadian Roosevelt NWR Complex, the Northern Mississippi Boreal Initiative, Ottawa, ON, and Boreal Songbird NWR Complex, Leroy Percy State Park, the Delta Initiative, Seattle, WA. National Forest, the Delta Research and Extension Bolgiano, N. C. 2004. Cause and effect: changes in Center, and Frederick Ballard and other individu- boreal bird irruptions in eastern North America als who granted permission to work on their refuges relative to the 1970s spruce budworm infestation. and lands. We are grateful to Robin Corcoran, Kate American Birds 58:26–33. Martin, Paul Meyers, and Christine Sowl for providing Burgess, N. M., and M. W. Meyer. 2008. Methylmer- unpublished data on nest sites and breeding behav- cury exposure associated with reduced productivity ior in Alaska. Jeff Wells supplied important informa- in Common Loons. Ecotoxicology 17:83–91. tion on the status of boreal forests. Financial support Bystrak, D. 1981. The North American Breeding Bird was provided by the U.S. Fish and Wildlife Service, Survey. Studies in Avian Biology 6:34–41. the Canadian Wildlife Service, the Society for Tropical Cadman, M. D., D. A. Sutherland, G. G. Beck, D. Lepage, Ornithology, the Arthur-von-Gwinner Foundation, and A. R. Couturier (editors). 2008. Atlas of the breed- the Max Planck Institute for Ornithology, Friends ing birds of Ontario. Bird Studies Canada, Environ- of the National Zoo, and the Legacy Program of the ment Canada, Ontario Field Ornithologists, Ontario Department of Defense. Ministry of Natural Resources, and Ontario Nature. Caspersen, J. P., S. W. Pacala, J. C. Jenkins, G. C. Hurtt, P. R. Moorcroft, and R. A. 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