Cerulean Warbler/ Paruline azurée

SPECIES ACCOUNT. Cerulean Warbler/ Paruline azurée (Setophaga cerulea)

Laura J. Yantha
Department of Biology, Queen’s University, Kingston, ON Canada K7L 3N6
email: 17ljy@queensu.ca

Taxonomy: Class: Aves. Order: Passeriformes. Family: Parulidae. Genus: SetophagaSpecies: Setophaga cerulea

The sky-blue Cerulean Warbler is a small wood-warbler that will be familiar residents of the Lake Opinicon region, although rarely spotted because it dwells high in the canopy of hardwood forests. Its former Latin name, Dendroica cerulea, was changed to Setophaga cerulea when mitochondrial and nuclear DNA phylogenetic studies of the wood-warbler family Parulidae (Lovette et al. 2010) revealed that the traditional taxonomy did not accurately reflect evolutionary history. Based on the most recent phylogenetic data, Cerulean Warblers are closely related to species that were formerly classified in the genus Parula, including Northern parulas (Setophaga americana) and tropical parulas (S. pitiayumi), as well as species observed in the Opinicon region such as American redstarts (S. ruticilla), black-throated blue warblers (S. caerulescens), and yellow warblers (S. petechia) (Lovette et al. 2010). Given the morphological and behaviorual similarities, it will not surprise avid birdwatchers that S. pitiayumi and S. americana are the closest relatives of Cerulean Warblers among extant wood-warblers (Lovette et al. 2010). Across their breeding range, one of the largest and most extensively studied breeding populations of Cerulean Warblers is found on QUBS (Queen’s University Biological Station) lands (COSEWIC 2010). These studies have focused on the species’ habitat use, behaviour, and population dynamics since 1994, and have generated abundant demographic information for QUBS and environs (e.g., Jones et al. 2004a; Buehler et al. 2008).

Description
The Cerulean Warbler is a small (8-10 g) sexually dimorphic songbird. Adult males are deep cerulean blue above and white below, with a narrow blue-black band across the breast and blue-black streaks along the flanks (sides of breast under the wings) and back (Figure 1). Male plumage colour and pattern varies substantially among individuals, and differs significantly among age class, with older males displaying brighter, more exaggerated plumage than younger males (Boves et al. 2014).

Cerulean warbler male and female
Figure 1. Cerulean warbler male (left) and female (right). Photos: Liz Purves.

Adult females are blue-green above and slightly yellow below, with a yellow-white eyebrow, lacking the neck band and streaked sides observed in males. First-fall males appear similar to adult females, with a greenish edge to their back feathers, whitish underparts, streaking that may only be present on the sides of their back, blurred ventral streaking, and a missing or restricted breast band. Both sexes display two white wing bars and white tail patches in all plumages. Cerulean Warblers usually undergo two molts each year; a complete molt on their breeding grounds prior to migration in mid-July, and a partial molt with crown, body, and rump feathers on their wintering grounds (Girvan et al. 2007).

Singing male Cerulean Warbler
Figure 2. Singing male Cerulean Warbler. Pangman Conservation Reserve. Queen’s University. Photo P-G. Bentz.

Cerulean Warblers have two distinct song categories within their song repertoire that function in different social contexts (Figure 2). In Ontario populations, their mate attraction song, used early in the breeding season, is higher in frequency than their territory defense song, which is used late in the season (McKillip and Islam 2009; Woodward 1997). The song typically begins with three buzzy notes, then four quick warbles, and ends with a single high-pitched buzz. Among males, song rate, minimum frequency, and maximum frequency are significant predictors of pairing status wherein unpaired individuals have higher song rates and frequencies than unpaired individuals (McKillip and Islam 2009). At least four distinguishable call notes have been observed in the Cerulean Warbler repertoire: (1) a metallic buzzy “zzee” note used by both sexes in alarm, while nearby a nestling or mate, when approaching or leaving an active nest, or while foraging; (2) a series of “chips” from both sexes when an intruder has invaded their territory or is close to an active nest; (3) a “tchip” produced in alarm when an intruder is close to a nest with nestlings, or in the presence of bird species that pose a threat; (4) a whisper song used by males when females are nearby (Rogers 2006). Male Cerulean Warblers prefer song posts with reduced foliage densities and predominantly select bitternut hickory (Carya cordiformis) as a song-post tree, perhaps as a behavioural adaptation to maximize song propagation and compensate for their lack of acoustic adaptation within their habitat (Aiama 2002; Barg et al. 2006a).

Distribution
Cerulean Warblers breed in the southern parts of Ontario and southwestern Quebec, and throughout the eastern United States, with a broad distribution from Minnesota south to Nebraska and east across the Appalachian Mountains (Figure 3; Buehler et al. 2020). The species is not uniformly distributed throughout its breeding range; individuals are more concentrated in the central Appalachian region but their presence more sporadic in other areas (Buehler et al. 2020). Many Cerulean Warblers breed in Ontario, primarily across the Frontenac Arch, with the population at QUBS and Frontenac Provincial Park thought to be one of the largest known of the species (COSEWIC 2010; Jones et al. 2004a).

Breeding and wintering ranges of Cerulean Warblers
Figure 3. Breeding (yellow) and wintering ranges (blue) of Cerulean Warblers. Licence. CC BY-SA 3.0. Dendroica cerulea map.svg. Cephas.

Cerulean Warblers are long-distance Nearctic-Neotropical migrants (breeds in North America and winters south) that travel across the Gulf of Mexico from their North American breeding grounds to their South American wintering grounds (Hayes 1995; Buehler et al. 2020). Their wintering range includes the Andes Mountains of northern South America, the east slopes of Columbia, Peru, Ecuador, and Venezuela, and northern Yungas forests of Bolivia in low densities (Buehler et al. 2020). Cerulean Warblers appear to adhere to Bergmann’s Rule; thus their body size positively correlates with latitude, perhaps because of migratory connectivity that has been observed in this species (Jones et al. 2005; Girvan 2003). However, recent evidence from a small geolocator-marked population of Cerulean Warblers in Indiana suggests that strong migratory connectivity of the species is unlikely; isolated breeding populations may distribute broadly and co-occur with other populations during the nonbreeding season (Delancey et al. 2020). Other data from stable-hydrogen isotopes indicates parallel migration patterns in the species, with connections between western breeding populations and southwestern wintering sites and between eastern breeding populations and northeastern wintering sites, but reveals a significant negative correlation between migratory connectivity strength and population trends (Jones et al. 2008). Weak migratory connectivity may therefore serve as a strategy that mitigates limiting factors in nonbreeding areas (e.g. deforestation, disease; Delancey et al. 2020).

Other researchers have discovered a lack of population genetic structure in Cerulean Warblers, despite sampling across their breeding range (e.g. Deane et al. 2013; Veit et al. 2005). Veit et al. (2005) believe that historical demography may be partly responsible, as their findings support a recent population expansion which might have commenced during the late Pleistocene (63,000 years ago). The lack of genetic structure may also be explained by on-going gene flow caused by naturally high dispersal rates (e.g. Haig and Oring 1988), population movement induced by habitat loss and/or fragmentation (Wade and McCauley 1988), a recent expansion of their breeding range into eastern Ontario (Oliarnyk and Robertson 1996), or the existence of a source-sink population system whereby populations are being sourced in eastern Ontario and are providing individuals to sink populations elsewhere (Veit et al. 2005).

Spring arrival of Cerulean Warblers in Ontario begins with males in the first week of May, followed by females in the second or third weeks of May (COSEWIC 2010). These patterns are consistent with those observed at QUBS in the Lake Opinicon region (Oliarnyk and Robertson 1996; Barg et al. 2006b). In Eastern Ontario, the species’ breeding season lasts for about 60-75 days from May through July (Barg et al. 2006b).

Habitat
Across their breeding range, Cerulean Warblers nest and raise their young high in the canopy of second-growth deciduous hardwood forests (Buehler et al. 2020). Throughout their core breeding range in the Appalachian Mountains, their habitat is characterized by steep slopes and ridgetops, in forests with openings in the canopy cover. They nest in north- to northeast-facing slopes (Nareff et al. 2019). In these southern populations, they typically avoid red oak (Quercus rubra) and red maple (Acer rubrum) trees, but will nest and forage in white oak (Quercus alba), cucumber magnolia (Magnolia acuminata), bitternut hickory (Carya cordiformis), and sugar maple (Acer saccharum; Boves et al. 2013a). Dense foliage cover at heights above 18 m characterizes their nesting habitat in the Lake Opinicon region, with sugar maple, oak (Quercus spp.), and elm (Ulmus spp.) trees dominating the canopy layer (Jones and Robertson 2001b). Nests in locations with high canopy and mid-storey cover have been the most successful at QUBS (Jones and Robertson 2001b). The January 1998 ice storm in southern Ontario and Quebec significantly altered the forest structure in the Lake Opinicon region and reduced the reproductive output of the local Cerulean Warbler population during the subsequent breeding season (Jones et al. 2001a). Fortunately, the population’s reproductive output in 1999 was not significantly different from pre-storm levels, but territory size had significantly increased and nest-site selection patterns had significantly shifted, reflecting the species’ ability to respond plastically in the face of a habitat disturbance (Jones et al. 2001a).

In the Lake Opinicon region, adult male Cerulean Warblers tend to establish clustered territories in areas with large, widely spaced trees and high canopies with dense foliage (Jones and Robertson 2001b). Fledglings can be found in mid- to upper-canopy, and, compared to parents, occupy areas closer to water bodies, with smaller, more clustered trees, fewer canopy gaps, and denser mid-storey cover (Raybuck et al. 2020). However, habitat selection depends on fledgling age, with young fledglings (0-2 days post-fledging) preferring areas with less basal area and more sapling cover, and more mature fledglings occupying areas with greater mid-story cover and fewer canopy openings (Raybuck et al. 2020).

During migration in Latin America, Cerulean Warblers use primary and secondary tropical forests, shade coffee plantations, and scrub habitat to forage and rest (Buehler et al. 2020). Across their non-breeding range in the Northern Andes, they occupy large tracts of primary or secondary tropical forests, woodlands, and coffee and cacao shade plantations on the western and eastern slopes at elevations ranging from 1,600-6,500 feet (Buehler et al. 2020; Colorado et al. 2012). Coffee shade plantations provide high-quality wintering habitats for Cerulean Warblers, as evidenced by their improved body condition, high within-season survival probabilities, and between-season return rates (Bakermans et al. 2009). Unfortunately, many farmers in Latin America are transitioning from traditional coffee plantations with a diverse canopy of trees to sun coffee plantations and pasture for cattle (Bakermans et al. 2009). This transition, along with rapid deforestation in the Andes, is contributing to extensive habitat loss for migratory birds that researchers believe is largely responsible for their declining populations (Colorado et al. 2012; González et al. 2020).

Ecology
Cerulean Warblers are aerial insectivores that feed primarily on flies (Diptera), beetles and weevils (Coleoptera), and caterpillar larvae (Lepidoptera; Buehler et al. 2020). On their wintering grounds, they are active foliage-gleaners, plucking their insect prey from substrates such as leaves, branches, and flowers, and may also feed on plant material (Muñoz and Colorado 2012). Their foraging rate increases by late winter, presumably due to improvement in body condition as the non-breeding season progresses (Muñoz and Colorado 2012). During migration, Cerulean Warblers may feed on plant material, particularly fatty masses adhered to fruits on tropical trees (Buehler et al. 2020). Foraging height may range from 2-3 m to above 45 m, with males foraging at greater heights than females on both their breeding and non-breeding grounds (MacNeil 2010; Bakermans 2008). Insect-gleaning is the primary mode of foraging for Cerulean Warblers, but sally-gleaning and lunging have also been observed, although less frequently (Muñoz and Colorado 2012; Jones et al. 2000a).

As noted above, males typically arrive on their breeding grounds one week before females, and pair formation occurs shortly after female arrivals (Buehler et al. 2020). Male-female Cerulean Warbler pairs are typically monogamous, but some cases of bigamy have been observed in males (~10% in Ontario and ~7% in the Cumberland Mountains of Tennessee; Barg et al. 2006b; Boves and Buehler 2012). Pairs often engage in reciprocal vocalizations (male call followed immediately by female call), especially during the incubation period (Barg et al. 2006b). During the hour before sunrise throughout the breeding season, males engage in collective bouts of dawn singing, a behavior which may have many social functions, including mate attraction and intrasexual communication (Barg et al. 2006b; Amrhein and Erne 2006; Woodward 1997). Early in the breeding season (May-June), males are more likely to sing dawn song and sing in longer bouts than later in the season, a behaviour which is influenced more by weather than social factors (Macdonald and Islam 2019). While females build their nests, males direct whisper songs at them, possibly to ensure pair-bond maintenance (Barg et al. 2006b). Males also demonstrate guarding behaviour and feed their partners during the nest building stage; when departing their nest for an off-bout, females often chip, perhaps to warn males that the nest is unprotected, and males usually exhibit nest vigilance until the female returns (Barg et al. 2006b). Males often bring females food while they are incubating, with lepidopteran larvae found to comprise 100% of the presented food items (Barg et al. 2006b).

The relationship between male Cerulean Warbler plumage traits and mate choice has been an focus for some researchers, although not extensively studied due to the difficulties of capturing individuals within this species (e.g., Boves et al. 2014; Purves et al. 2016). Boves et al. (2014) examined four plumage traits in males: crown coloration, rump coloration, breast band width, and tail patch size; although white tail patches are common to both sexes of Cerulean Warblers, males tend to have more exaggerated patches than females (Pyle 1997). They found rump colouration was associated positively with provisioning rate and negatively with body mass, and tail patch size to indicate quality and condition from the previous breeding season; thus, females should rely on plumage traits to choose their mates (Boves et al. 2014). However, plumage and reproductive performance within age classes were not significantly related, perhaps due to the high productivity of the breeding seasons during which this study took place that reduced the impact of individual differences on reproductive output, or other reasons listed by Boves et al. (2014). At QUBS, white tail patches may function as an honest signal of quality, as patches are significantly larger on males two years or older compared to 1-year-olds, and males with longer wing length (which provides an estimate for structural size) compared to those with shorter wings, but how patch size influences mate choice is unknown (Purves et al. 2016).

Both male and female Cerulean Warblers choose their nest site together, and often select the lateral limb of a deciduous tree with concealment above and below by clustered leaves or vines (Boves and Buehler 2012; Bent 1953). Their primary nest materials are grapevine bark and tent caterpillar or spider silk, but they may also incorporate plant fibers, lichen, or horsehair, and embellish the exterior with bracken fungus or sodden paper (Buehler et al. 2020). The bulk of the nest-building is done by the females, but males may assist in the early stages, particularly by gathering silk (Boves and Buehler 2012). In the Cumberland Mountains of Tennessee, May 10 is the average nest-initiation day, with nest construction lasting for an average of ~5 days (Boves and Buehler 2012).  In the Opinicon region, females allocate 4-7 days for nest building (Barg et al. 2006b).

Cerulean Warblers raise one brood per breeding season, and females lay 2-5 eggs per clutch (COSEWIC 2010; Buehler et al. 2020). Double-brooding is rare and has only been observed in Ontario (Barg et al. 2006b). Studies conducted at QUBS have found that females spend 7 days for egg laying and 10-12 days for incubation, with an average of 2.7 young fledging successfully from each nest (Barg et al. 2006b; Buehler et al. 2008). During the last days of the nestling period (days 10-11), both parents share nestling-feeding duties, and feed younger nestlings less frequently than older ones (Barg et al. 2006b). Females have been observed to either stand at the edge of the nest while the male feeds the nestlings or take the prey item from the male and feed it to the nestlings herself (Auer et al. 2016). Females are responsible for maintaining active nests by supplying new material, but both parents contribute to nest sanitation by removing fecal sacs (Boves and Buehler 2012; Buehler et al. 2020). Upon nest failure, females will reuse parts of the old nest to build a new nest, and will re-nest up to two times regardless of the stage their failed nest had reached (Buehler et al. 2020).

Blood parasites commonly infect avian species, and Cerulean Warblers are no exception (e.g. Gibb et al. 2005). One study found the endoparasite Haemoproteus paruli in 36 out of the 112 Cerulean Warblers examined, with no significant associations between prevalence and age class or body condition (Gibb et al. 2005). However, southern populations had higher parasite prevalence than their northern counterparts, perhaps due to regional differences in arthropod abundance and thus different activity levels of vectors harbouring the endoparasites (Gibb et al. 2005).

Brood parasitism by the Brown-headed Cowbird (Molothrus ater) in Cerulean Warbler nests has been reported throughout their breeding grounds, with variation across their geographic range (Friedmann 1963; Friedmann et al. 1977; Nicholson 2004; Buehler et al. 2008; Boves et al. 2013b). In the Cumberland Mountains of Tennessee, Nicholson (2004) reported Cowbird parasitism in 1 out of 52 Cerulean Warbler nests monitored. However, rates of parasitism have exceeded 10% in Ontario, Michigan, Indiana, and the Mississippi Alluvial Valley (Buehler et al. 2008; Rogers 2006), which researchers suggest have contributed to increased nest abandonment, predation, and loss of reproductive output in the Cerulean Warbler (Buehler et al. 2008; Hamel and Woodson 2000). The impact that cowbirds have on this species is not wholly understood because the reported rates of parasitism are likely an underestimate due to the difficulty for humans to locate Cerulean Warbler nests and identify brood parasitism (Buehler et al. 2020).

Conservation Status
The North American Breeding Bird Survey has reported significant declines in the Cerulean Warbler population, with -2.63%/yr from 1966-2015 (Figure 4; Sauer et al. 2017), which are largely attributed to habitat destruction on both the breeding and wintering grounds (Robbins et al. 1992; Bakermans et al. 2009). Partners in Flight predicts a remaining global population of 285,000 by 2041 if the current rates of decline continue (Buehler et al. 2020). The International Union for Conservation of Nature (IUCN) reported the Cerulean Warbler as vulnerable across its breeding and non-breeding ranges from 2004-2018, but changed the species  status to ‘near threatened’ in 2019 due to slower rates of decline that have been observed in populations (BirdLife International 2019). In Canada, the Cerulean Warbler is listed as Endangered (BirdLife International 2019), and is Endangered, Threatened, or under protective watch across US states (Buehler et al. 2020). A mark-recapture study, conducted from 1995-2004 at QUBS, indicated a stable Cerulean Warbler population; however, low average fecundity observed in the study suggests that immigration of warblers from other areas may be sustaining the population (Jones et al. 2004a).

Map of Cerulean Warbler declines.
Figure 4: Changes in breeding Cerulean Warbler abundance from 1966-2015. From the North American Breeding Bird Survey (Sauer et al. 2017).

Extracting accurate density estimates of Cerulean Warbler populations is important for implementing appropriate conservation efforts. Among various point count censusing techniques, Jones et al. (2000b) recommends the variable circular-plot (VCP) methodology for broad-scale surveys, where stations are established in a manner that avoids more than one observation of the same bird and observers count the number of birds seen or heard within a fixed time period at each station (Reynolds et al. 1980). However, the particulars of a population (e.g. habitat, population trends) should be considered before a censusing technique is chosen.

Management Guidelines for Enhancing Breeding Habitat in Appalachian Hardwood Forests by Wood et al. (2013) recommends silviculture as a tool to increase Cerulean Warbler populations wherein partial harvesting promotes oak regeneration, understory vegetation, and maintenance of a structurally diverse forest that is favourable for the species. A recent test of the Guidelines in the central Appalachian region reported a 100% increase in Cerulean Warbler territory density from pre-harvest to two years post-harvest, and a positive correlation between Cerulean abundance and percentage of basal area of their preferred tree species (e.g. oak, sugar maple, hickories) or of trees with diameters 40.6 cm (Nareff et al. 2019). Other studies have also demonstrated success from following recommendations in the Guidelines (e.g. Sheehan et al. 2014), but the long-term effects of this management system on Cerulean Warbler populations is unknown. The management plan in Canada includes acquiring and conserving land that serves as the species’ habitat and conducting research to determine the optimal silviculture practices for Cerulean Warblers in Ontario and Quebec (Environment Canada 2011).

Research Needs
Considering status of conservation concern for Cerulean Warblers, determining forest management plans across its geographic range is a top research priority, as failing to do so could lead to the species’ extinction or at least extirpation in many areas. Although some silviculture practices have produced positive results (Sauer et al. 2017; Nareff et al. 2019), another study found negative associations with preferred habitat features and nest success (Boves et al. 2013a), indicating the need for site-specific management strategies and additional research concerning how fitness is affected under a variety of habitat conditions. Implementing management plans that can simultaneously benefit other at-risk species must be analyzed (Sheehan et al. 2014) as well as long-term studies of the effects of forest management on Cerulean Warbler viability (Buehler et al. 2020). Further, little is known about their stopover and winter habitat needs, but this information is required for forest management on their non-breeding grounds. The importance of small forest patches (Amaya-Espinal and Hostetler 2019) and shade coffee plantations (Bakermans et al. 2009) has been evidenced, but more information is needed to devise a specific management plan for migratory birds on their wintering grounds—in particular, the relationship between landscape features in coffee plantations and bird species richness, their spatial distribution and dietary needs, the effect of insecticides and pesticides on bird viability, and how to balance both economic and conservation needs of migratory birds (Colorado et al. 2016; Narango et al. 2019).

Literature & Further Reading

  1. Aiama, D.M. 2002. The influence of leaf-out on song post selection by male Cerulean Warblers (Dendroica cerulea). B.Sc. Honours thesis, Department of Biology, Queen’s University, Kingston.
  2. Amaya-Espinal, J.D. and M.E. Hostetler. 2019. The value of small forest fragments and urban tree canopy for Neotropical migrant birds during winter and migration seasons in Latin American countries: A systematic review. Landscape and Urban Planning 190: 103592.
  3. Amrhein, V. and N. Erne. 2006. Dawn singing reflects past territorial challenges in the winter wren. Animal Behaviour 71: 1075–1080.
  4. Auer, S.A., K. Islam, J.R. Wagner, K.S. Summerville, and K.W. Barnes. 2016. The diet of Cerulean Warbler (Setophaga cerulea) nestlings and adult nest provisioning behaviors in southern Indiana. The Wilson Journal of Ornithology 128: 573-583.
  5. Bakermans, M.H. 2008. Demography and habitat use of Cerulean Warblers on breeding and wintering grounds. Ph.D. dissertation, Ohio State University, Columbus.
  6. Bakermans, M.H., A.C. Vitz, A.D. Rodewald, and C.G. Rengifo. 2009. Migratory songbird use of shade coffee in the Venezuelan Andes with implications for conservation of Cerulean Warbler. Biological Conservation 142: 2476-2483.
  7. Barg, J.J., D.M. Aiama, J. Jones and R.J. Robertson. 2006a. Within the territory: habitat use and microhabitat selection by male Cerulean Warblers. The Auk 123: 795-806.
  8. Barg, J.J., J. Jones, M.K. Girvan, and R.J. Robertson. 2006b. Within-pair interactions and parental behavior of Cerulean Warblers breeding in eastern Ontario. The Wilson Journal of Ornithology 118: 316-325.
  9. Bent, A. C. 1953. Life histories of North American wood warblers. United States National Museum Bulletin 203.
  10. BirdLife International. 2019. Setophaga cerulea. The IUCN Red List of Threatened Species 2019: e.T22721740A153691320.
  11. Boves, T. J. and D. A. Buehler. 2012. Breeding biology, behavior, and ecology of Setophaga cerulea in the Cumberland Mountains, Tennessee. Southeastern Naturalist 11: 319-330.
  12. Boves, T.J., D.A. Buehler, J. Sheehan, P.B. Wood, A.D. Rodewald, J.L. Larkin, P.D. Keyser, F.L. Newell, A. Evans, G.A. George, and T. B. Wigley. 2013a. Spatial variation in breeding habitat selection by Cerulean Warblers (Setophaga cerulea) throughout the Appalachian Mountains. Auk 130: 46–59.
  13. Boves, T.J., D.A. Buehler, J. Sheehan, P.B. Wood, A.D. Rodewald, J.L. Larkin, P.D. Keyser, F.L. Newell, G.A. George, M.H. Bakermans, A. Evans, T.A. Beachy, M.E. McDermott, K.A. Perkins, M. White and T.B. Wigley. 2013b. Emulating natural disturbances for declining late-successional species: A case study of the consequences for Cerulean Warblers (Setophaga cerulea). PLOS One 8 (1):e52107.
  14. Boves, T.J., D.A. Buehler, P.B. Wood, A.D. Rodewald, J.L. Larkin, P.D. Keyser, and T.B. Wigley. 2014. Multiple plumage traits convey information about age and within-age-class qualities of a canopy-dwelling songbird, the Cerulean Warbler. Auk 131: 20-31.
  15. Buehler, D.A., J.J. Giocomo, J. Jones, P.B. Hamel, C.M. Rogers, T.A. Beachy, D.W. Varble, C.P. Nicholson, K.L. Roth, J.J. Barg, R.J. Robertson, J.R. Robb, and K. Islam. 2008. Cerulean Warbler reproduction, survival, and models of population decline. Journal of Wildlife Management 72: 646-653.
  16. Buehler, D.A., P.B. Hamel, and T. Boves. 2020. Cerulean Warbler (Setophaga cerulea), version 1.0. In Birds of the World (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.
  17. Colorado Z., G.J., P.B. Hamel, A.D. Rodewald, and D. Mehlman. 2012. Advancing our understanding of the non-breeding distribution of Cerulean Warbler (Setophaga cerulea) in the Andes. Ornitologia Neotropical 23: 307-315.
  18. Colorado Z., G.J., D. Mehlman, and G. Valencia-C. 2016. Effects of floristic and structural features of shade agroforestry plantations on the migratory bird community in Colombia. Agroforestry Systems 92: 677–691.
  19. COSEWIC. 2010. COSEWIC assessment and update status report on the Cerulean Warbler Dendroica cerulea in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. x + 40 pp.
  20. Deane, P.E., K.D. McCoy, R.J. Robertson, T.P. Birt and V.L. Friesen. 2013. Minimal genetic structure in the Cerulean Warbler despite evidence for ecological differentiation among populations. The Condor 115: 178-185.
  21. Delancey, C.D., K. Islam, G.R. Kramer, G.J. Macdonald, A.R. Sharp, and B.M. Connare. 2020. Geolocators reveal migration routes, stopover sites, and nonbreeding dispersion in a population of Cerulean Warblers. Animal Migration 7: 19-26.
  22. Environment Canada. 2011. Management Plan for the Cerulean Warbler (Dendroica cerulea) in Canada. Species at Risk Act Management Plan Series. Environment Canada, Ottawa. iii + 19 pp.
  23. Friedmann, H. 1963. Host relations of the parasitic cowbirds. U.S. National Museum Bulletin, Volume 233. Washington, DC, USA.
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  25. Gibb, C.E., Jones, J., Girvan, M.K., Barg, J.J., and R.J. Robertson. 2005. Geographic variation in prevalence and parasitemia of Haemoproteus paruli in the cerulean warbler (Dendroica cerulea). Canadian Journal of Zoology 83: 626-629.
  26. Girvan, M.K. 2003. Examining dispersal and migratory connectivity in Cerulean Warblers (Dendroica cerulea) using stable isotope analysis. M.Sc. thesis, Queen’s University, Kingston.
  27. Girvan, M.K., J. Jones, D.R. Norris, J.J. Barg, T.K. Kyser, and R.J. Robertson. 2007. Long-distance dispersal patterns of male Cerulean Warblers (Dendroica cerulea) measured by stable-hydrogen isotopes. Avian Conservation and Ecology 2: 3.
  28. González, A.M., S. Wilson, N.J. Bayly, and K.A. Hobson. 2020. Contrasting the suitability of shade coffee agriculture and native forest as overwinter habitat for Canada Warbler (Cardellina canadensis) in the Colombian Andes. The Condor 122: duaa011.
  29. Haig, S.M. and L.W. Oring. 1988. Genetic differentiation of piping plovers across North America. The Auk 105: 260-267.
  30. Hamel, P. B. and C. A. Woodson. 2000. Brown-headed cowbird removes egg from Cerulean Warbler nest. The Migrant 71: 1-3.
  31. Hayes, F.E. 1995. Definitions for migrant birds: What is a Neotropical migrant? The Auk 112: 521-523.
  32. Jones, J., P.R. Perazzi, E.H. Carruthers, and R.J. Robertson. 2000a. Sociality and foraging behavior of the Cerulean Warbler in Venezuelan shade-coffee plantations. The Condor 102: 958–962.
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Reviewers: Raleigh J. Robertson (Queen’s Univ.) and Stephen C. Lougheed (Queen’s Univ.)

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