Distribution

Breeding Range

The Connecticut Warbler is an uncommon and localized breeder from northeastern British Columbia (38 Davidson, P. J. A., R. J. Cannings, A. R. Couturier, D. Lepage, and C. M. Di Corrado (Editors) (2015). The Atlas of the Breeding Birds of British Columbia, 2008–2012. Bird Studies Canada, Delta, BC, Canada. Close ) and southwestern Northwest Territories (39 Machtans, C. S. (2000). Extra-limital observations of Broad-winged Hawk, Buteo platypterus, Connecticut Warbler, Oporornis agilis, and other bird observations from the Liard Valley, Northwest Territories. Canadian Field-Naturalist 114(4):671–679. Close ), east through north-central and central Alberta (40 Federation of Alberta Naturalists (2007). Atlas of Breeding Birds of Alberta: A Second Look. Federation of Alberta Naturalists, Edmonton, AB, Canada. Close ), central and southern Saskatchewan (Saskatchewan Breeding Bird Atlas), central and southern Manitoba (41 Artuso, C., A. R. Couturier, K. D. De Smet, R. F. Koes, D. Lepage, J. McCracken, R. D. Mooi, and P. Taylor (Editors) (2018). The Atlas of the Breeding Birds of Manitoba, 2010–2014. Bird Studies Canada, Winnipeg, MB, Canada. Close ), north-central Ontario south to the northern shore of Lake Superior and Sault St. Marie (42 Cadman, M. D., D. A. Sutherland, G. G. Beck, D. Lepage, and A. R. Couturier (Editors) (2007). Atlas of the Breeding Birds of Ontario 2001–2005. Bird Studies Canada, Environment Canada, Ontario Field Ornithologists, Ontario Ministry of Natural Resources, and Ontario Nature, Toronto, Ontario, Canada. Close ), and west-central Quebec east to Lac St. Jean and Chicoutimi (43 Robert, M., M. Hachey, D. Lepage, and A. G. Couturier (Editors) (2019). The Second Atlas of the Breeding Birds of Southern Québec. Regroupement QuébecOiseaux, the Canadian Wildlife Service (Environment and Climate Change Canada) and Bird Studies Canada, Quebec, QC, Canada. Close ). Breeding range includes the northern Great Lakes region of the United States in northeastern Minnesota (44 Pfannmuller, L., G. Niemi, J. Green, B. Sample, N. Walton, E. Zlonis, T. Brown, A. Bracey, G. Host, J. Reed, K. Rewinkel, and N. Will (2017). The First Minnesota Breeding Bird Atlas (2009–2013). Close ), northern Wisconsin (45 Cutright, N. J., B. R. Harriman, and R. W. Howe (2006). Atlas of the Breeding Birds of Wisconsin. Wisconsin Society of Ornithology, Waukesha, WI, USA. Close ), and northern Michigan (Upper Peninsula and northern Lower Peninsula) (46 Chartier, A. T., J. J. Baldy, and J. M. Brenneman (Editors) (2011). The Second Michigan Breeding Bird Atlas. Kalamazoo Nature Center, Kalamazoo, MI, USA. Close ).

Current descriptions of the breeding range may underestimate the true distribution across North America. The Boreal Avian Modelling Project (47 Boreal Avian Modelling Project (2012). Annual Report: April 2011–March 2012. University of Alberta, Edmonton, AB, Canada. Close ) used Maxent software to model species distributions based on relative habitat suitability, climate variables, and land cover in the boreal forest of Canada and the northern United States. Their models found more suitable breeding habitat and occurrence data for the Connecticut Warbler than previously described limits of the breeding range. The actual breeding range may extend farther north in both eastern and western Canada. For example, Hennigar et al. (48 Hennigar B., J. P. Ethier, and D. R. Wilson (2019). Experimental traffic noise attracts birds during the breeding season. Behavioral Ecology 30:1591–1601. Close ) reported an extralimital record near Happy Valley-Goose Bay, Labrador based on call notes recorded during the breeding period (16 May–10 July). Wells et al. (49 Wells, J., D. Childs, F. Reid, K. Smith, M. Darveau, and V. Courtois (2014). Boreal birds need half: maintaining North America’s bird nursery and why it matters. Boreal Songbird Initiative, Seattle, Washington, Ducks Unlimited, Memphis, Tennessee, and Ducks Unlimited Canada, Stonewall, Manitoba. Close ) estimated that over 80% of the breeding range is located in the boreal forest of Canada, similar to earlier estimates of 90% by Sodhi and Paszkowski (50 Sodhi, N. S., and C. A. Paszkowski (1995). Research on wood warblers in Canada. Journal of Field Ornithology 66(2):260–266. Close ) and 92% by Cheskey et al. (51 Cheskey, E., J. Wells, and S. Casey-Lefkowitz (2011). Birds at risk, the importance of Canada’s boreal wetlands and waterways. Natural Resource Defense Council Technical Report. NRDC Reports Department, New York, NY, USA. Close ). Winger and Pegan (52 Winger, B. M., and T. M. Pegan (2021). Migration distance is a fundamental axis of the slow-fast continuum of life history in boreal birds. Ornithology 138:1–18. Close ) estimated that the migration distance between the centroids of the breeding and overwintering ranges was 6,111 km.

Overwintering Range

The Connecticut Warbler overwinters in South America, although the exact limits of the overwintering range are poorly defined. Earlier authors (e.g., 53 Meyer de Schauensee, R. (1970). A Guide to the Birds of South America. Livingston Publishing Company, Wynnewood, PA, USA. Close , 54 Ridgely, R. S., and G. Tudor (1989). The Birds of South America. Volume 1: The Oscine Passerines. University of Texas Press, Austin, TX, USA. Close , 55 Restall, R., C. Rodner, and M. Lentino (2006). Birds of Northern South America: An Identification Guide. Christopher Helm, London, United Kingdom. Close ) typically defined the overwintering range to include both northern South America and Amazonia, but these and works often included records of transient individuals (or potential transients) recorded during October, November, April, and May (outside mid-winter). In an analysis based largely on sightings, Paynter (56 Paynter, R. A., Jr. (1995). Nearctic Passerine Migrants in South America. Publications of the Nuttall Ornithological Club 25, Cambridge, MA, USA. Close ) commented that based on limited records, the Connecticut Warbler seems to overwinter mainly on the edges of the Amazon basin. Subsequently, Hilty (57 Hilty, S. L. (2003). Birds of Venezuela. Princeton University Press, Princeton, NJ, USA. Close ) considered the Connecticut Warbler as a transient in Venezuela, with specimen records from 23 September‒31 October and late April‒8 May, and Ridgely and Tudor (58 Ridgely, R. S., and G. Tudor (2009). Field Guide to the Songbirds of South America: the Passerines. University of Texas Press, Austin, Texas, USA. Close ) reported the species as mainly or entirely a transient north of southern Amazonia. In southeastern Peru, reported to be a rare passage migrant (59 Schulenberg, T. S., D. F. Stotz, D. F. Lane, J. P. O'Neill, and T. A. Parker III (2007). Birds of Peru. Princeton University Press, Princeton, New Jersey, USA. Close ); e.g., four individuals mist-netted at Tambopata Reserve from 3–17 November were believed to be transients (29 Parker, T. A., III (1982). Observations of some unusual rainforest and marsh birds in southeastern Peru. Wilson Bulletin 94(4):477–493. Close , 60 Sorrie, B. A. (2016). Early records of Connecticut Warbler Oporornis agilis in Amazonian Peru. Cotinga 38:46. Close ).

There are very few mid-winter records in Brazil, where Sick (61 Sick, H. (1993). Birds in Brazil: A Natural History. Princeton University Press, Princeton, NJ, USA. Close ) reported the species on the Rio São Lourenço (Mato Grosso) in southwestern Brazil in December and January (e.g., AMNH specimen 128089). More recently, additional records suggest that the Connecticut Warbler is a regular overwintering resident in Bolivia. At least 16 individuals were documented in late December 2012 and late January 2019 in the upper Machariapo Valley (1,200‒1,500 m) of northwestern Bolivia, suggesting that other inter-Andean valleys in the region could harbor overwintering populations (62 Aponte M. A., D. Ric, O. Maillard, D. F. Lane, R. S. Terrill, A. G. Calle, R. Ramirez, M. A. Montenegro, R. Arispe, L. H. Acosta, M. M. Salvatierra, W. S. Pantoja, G. Sánchez, and D. Aliaga-Pantoja (2022). New and noteworthy observations on the distribution of birds in Bolivia. Cotinga 44:9–18. Close ). Herzog et al. (63 Herzog, S. K., V. H. Garcia-Soliz, and S. E. Davis (2009). Status of the Cerulean Warbler (Dendroica cerulea) at the southern terminus of its non-breeding range, with a review of other Nearctic-neotropical migrant Parulidae in Bolivia. Ornitología Neotropical 20(1):121–130. Close ) reported two birds observed in mid-February at Tucavaca in the Chaco of southeastern Bolivia, and two individuals mist-netted in early April at Santa Cruz de la Sierra in central Bolivia; they summarized additional unpublished reports from nearby locations in the Bolivia Chaco (Estancia El Cañon, Fortín Ravelo, and Palmar de las Islas). A study that placed geolocators on Connecticut Warblers from across the breeding range found that recaptured individuals (n = 9) were estimated to have overwintered in an area that encompassed the northern Gran Chaco ecoregion, including southwestern Brazil, eastern Bolivia, and northern Paraguay (64 Hallworth M. T., E. Bayne, E. McKinnon, O. Love, J. A. Tremblay, B. Drolet, J. Ibarzabal, S. Van Wilgenburg, and P. P. Marra (2021). Habitat loss on the breeding grounds is a major contributor to population declines in a long-distance migratory songbird. Proceedings of the Royal Society B 288(1949). Close ), though, at present, confirmed records for Paraguay are needed. The species could overwinter elsewhere in South America, but most well-documented records (e.g., photo, specimen) fall within migratory periods. Two interesting exceptions include a 9 December record at Laguna Pedropalo in central Colombia (65 Freeman, B. G., S. L. Hilty, D. Calderón-F., T. Ellery, and L. E. Urueña (2012). New and noteworthy bird records from central and northern Colombia. Cotinga 34:33–42. Close ) and a 19 January record from Puerto Rico (see eBird).

Because the Connecticut Warbler is secretive and breeds and overwinters in remote areas of boreal and tropical forest, respectively, it remains difficult to asses changes in its distribution. Indeed, breeding and overwintering ranges have yet to be fully described. Recent additions to the breeding range (e.g., eastern Quebec; 66 Gauthier, J., and Y. Aubry (1996). Les Oiseaux Nicheurs du Québec: Atlas des Oiseaux Nicheurs du Québec Meridional. Environnment Canada, Région du Québec: Association Québécoise des groupes d'ornithologues, Sociétié Québécoise de protection des oiseaux, Service Canadien de la Faune, Quebec, Canada. Close , 43 Robert, M., M. Hachey, D. Lepage, and A. G. Couturier (Editors) (2019). The Second Atlas of the Breeding Birds of Southern Québec. Regroupement QuébecOiseaux, the Canadian Wildlife Service (Environment and Climate Change Canada) and Bird Studies Canada, Quebec, QC, Canada. Close ) were likely due to an increase in survey effort lack of earlier survey effort and probably are not evidence of range expansion. In Minnesota, other than the loss of the small nesting population in Isanti County, the distribution changed remarkably little over the past 100 years (67 Janssen, R. B. (1987). Birds in Minnesota. University of Minnesota Press, Minneapolis, Minnesota, USA. Close , 44 Pfannmuller, L., G. Niemi, J. Green, B. Sample, N. Walton, E. Zlonis, T. Brown, A. Bracey, G. Host, J. Reed, K. Rewinkel, and N. Will (2017). The First Minnesota Breeding Bird Atlas (2009–2013). Close ).

Toews (68 Toews, D. P. L. (2017). Habitat suitability and the constraints of migration in New World warblers. Journal of Avian Biology 48:1614–1623. Close ) studied habitat suitability in the western boreal forest for several species of warblers with breeding ranges largely or exclusively east of the Rocky Mountains. His modeling found a lower amount of suitable (but presently unoccupied) breeding habitat in the western boreal forest for the Connecticut Warbler as compared to Magnolia Warbler (Setophaga magnolia), Palm Warbler (Setophaga palmarum), and Wilson's Warbler (Cardellina pusilla). As a result, these eastern species have greater opportunity for future range expansion in Alaska and the Yukon compared to the Connecticut Warbler.

Influence of Glacial Cycles on Current Distribution

Zink and Gardner (69 Zink, R. M., and A. S. Gardner (2017). Glaciation as a migratory switch. Science Advances 3:e1603133. Close ) showed that glacial cycles could have affected the historical overwintering and breeding ranges of North American migrants. During the last glacial maximum (LGM) about 21,000 years ago, their model suggests a contraction of the current overwintering range of the Connecticut Warbler. During the LGM, the overwintering range was restricted to northern South America and west to include parts of Ecuador and Peru. It also extended northward into parts of Central America as far north as the west coast of Mexico and Guatemala. There were not enough data to model changes in the breeding range. Until the entire overwintering range is described, it will be impossible to interpret new overwinter records as evidence of changes in distribution.

Climate Change and Future Distribution

Ornithologists have also begun to model long-term changes to the ranges of North American breeding birds in response to climate change. Alterations and weakening of current barriers to range expansion by climate change may provide opportunities for colonization and range expansion. One barrier is the northwestern cordillera in western Canada that contains the uppermost regions of the Rocky Mountains, Mackenzie Mountains, and Selwyn Mountains. Based on a comprehensive analysis of current climate, paleoclimate, migratory strategy, and life-history characteristics, Stralberg et al. (70 Stralberg, D., S. M. Matsuoka, C. M. Handel, F. K. Schmiegelow, A. Hamann, and E. Bayne (2017). Biogeography of boreal passerine range dynamics in western North America: past, present, and future. Ecography 40:1050–1066. Close ) predicted that this barrier will be reduced by climate change. Boreal species such as the Connecticut Warbler would then extend their breeding ranges into this boreal region of Alaska. The expected probabilities of occurrence for this species in the Alaska boreal region were 14.6% from 2011–2040, 77.3% from 2041–2070 and 96.3% from 2071–2100.

Winker and Gibson (71 Winker, K., and D. D. Gibson (2018). Some broad-scale effects of recent and future climate change among migratory birds in Beringia. in Trends and traditions: Avifaunal change in western North America (W. D. Shuford, R. E. Gill Jr., and C. M. Handel, Editors). Studies of Western Birds 3. Western Field Ornithologists, Camarillo, CA, USA. pp. 432–440. Close ) also predicted that future populations of the Connecticut Warbler at the northern region of its breeding range may expand north and west into western Canada in response to climate change and global warming. They hypothesized that species such as the Connecticut Warbler, with breeding ranges that project north and west in Canada, are likely candidates to continue in those directions as the climate warms, making suitable habitat more available. They base their prediction, in part, on examples such as the current breeding range of the Swainson's Thrush (Catharus ustulatus), which now occupies areas of the Northwest Territories and British Columbia. These areas may have been unsuitable during glacial times.

Although range expansion to the north and west may occur in response to climate change, other parts of the breeding range are expected to decrease in size or disappear. Stralberg and Bayne (72 Stralberg, D., and E. Bayne (2013). Modeling avifaunal responses to climate change across Alberta’s natural regions. Alberta Biodiversity Monitoring Institute, Edmonton, AB, Canada. Close ) constructed bioclimatic niche models that predict changes over the next century to the breeding distribution of the Connecticut Warbler in Alberta. Their model shows that future refugia for this species in Alberta would be reduced to 47% of its current core area from 2011–2040, 10% of the core area from 2041–2070 and 0% of the core area from 2071–2100. The extent to which similar losses would occur in other parts of the breeding range needs further study.