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COSEWIC Assessment and Update Status Report on the Cerulean Warbler in Canada

Biology

General

Despite broad public and scientific interest in the Cerulean Warbler, the basic biology of this species remains poorly documented and understood. Although considerable advances have been made in the last 10 years (e.g. Oliarnyk and Robertson 1996; Hamel 2000a; Jones and Robertson 2001; Jones et al. 2001; Barg 2002), there remain considerable gaps in our knowledge. Rather that go into an exhaustive review of research to date, this discussion of Cerulean Warbler biology is restricted to those aspects directly relevant to the status update and survival of the species.


Reproduction

Cerulean Warblers generally raise only a single brood per year, although double-brooding has been recorded (Barg et al. unpublished data). Pairs will re-nest up to 4 times in a season following nest failure (Hamel 2000a; Barg et al. unpublished data). Female Cerulean Warblers typically lay 3 or 4 eggs per clutch. Incubation generally lasts 11-12 days and the nestlings typically fledge after 8-10 days (Oliarnyk 1996; Hamel 2000a; Barg et al. unpublished data). Over an 8-year period in eastern Ontario, average fecundity was 1.9 fledglings per breeding pair (Jones et al. in review). Most successful pairs fledged 3 or 4 fledglings but, in any given season, only two-thirds of pairs have successful nests. There can be large fluctuations in annual fecundity, ranging from 0.4 to 2.2 fledglings per breeding pair (Jones et al. 2001, unpublished data).

Brood parasitism by Brown-headed Cowbirds (Molothrus ater) has been suggested as an important factor in the decline of the Cerulean Warbler in the core of its breeding range (Robbins et al. 1992). However, there is considerable variation in its prevalence (Hamel 2000a). At the Queen’s University Biological Station, no cowbird fledglings have been produced by Cerulean Warblers in the last 8 years (Oliarnyk 1996; Jones et al. unpublished data). Parasitism rates are much higher elsewhere (e.g. Mississippi Alluvial Valley, Hamel 2000a).


Survival

Jones et al. (in review) used capture-mark-recapture models to estimate survival for adult male Cerulean Warblers in an eastern Ontario population that has been studied since 1994. Adult male survival probability (49%) was constant over time in the best-supported model. No estimates exist for females or juveniles. The current longevity record is an 8-year-old male that was banded in eastern Ontario in 1997 as an adult (i.e., at least 3 years old), and has been present every year since then, including in 2002 (Jones and Barg unpublished data). Typical life span for males is likely 3-4 years.


Movements/Dispersal

As of 1999, 1399 Cerulean Warblers had been banded in North America (Hamel 2000a). Only 1 of these birds has been recovered away from the banding location (Leberman and Clench 1975 in Hamel 2000a) suggesting strong site-fidelity in adults (Hamel 2000a, Jones et al. in review, Barg et al. unpublished data) and limited gene flow among locations. In a genetic study of five Cerulean Warbler geographic clusters throughout the breeding range (including 2 in Ontario), estimates from microsatellite data revealed sufficient levels of gene flow to prevent genetic differentiation through drift (Veit 1999). Thus, dispersal between populations (presumably by young birds) undoubtedly plays an important role in Cerulean Warbler population dynamics. Some evidence exists for limited natal philopatry in this species (Barg et al. unpublished data).

Migration appears to occur over approximately 2 months in spring and 4 months in fall (Hamel 2000a). Fall migration tends to start early; there are records of individuals arriving in South America in August (Dunn and Garrett 1997).


Nutrition and Interspecific Interactions

Cerulean Warblers appear to be strictly insectivorous during the breeding season, consuming homopterans, larval lepidopterans, dipterans and coleopterans (Sample et al. 1993, Hamel 2000a, Barg et al. unpublished data). Cerulean Warblers will avail themselves of nectar resources during the non-breeding season in South America (Jones et al. 2000b). Nestlings and fledglings are fed larval lepidopterans almost exclusively. No empirical evidence exists to support the conjecture that food availability is limiting in Cerulean Warbler populations; however, it likely plays an important role in population regulation given the well-documented importance of food abundance to breeding migrant songbirds (Newton 1998).

Aggressive interactions between Cerulean Warblers and Least Flycatchers (Empidonax minimus), Red-eyed Vireos (Vireo olivaceus), and American Redstarts (Setophaga ruticilla) are common in Ontario but their consequences are unknown (Varey 1998; Barg pers. obs.).


Behaviour/Adaptability

There has been conjecture in the literature that the Cerulean Warbler exhibits a degree of “coloniality” during the breeding season (Robbins et al. 1992), likely due to conspecific attraction. This phenomenon has not been studied in any detail but could have significant bearing on conservation or management plans for this species if confirmed.  At QUBS, territory size ranges from 0.1 to 2.4 ha (Oliarnyk 1996; Barg 2002). However, territory size does not appear to be directly related to pair density (Jones pers. obs.); large portions of apparently suitable habitat go unoccupied each breeding season.

Cerulean Warblers appear to be relatively resilient in the face of habitat disturbance, both anthropogenic and natural (Jones 2000, Jones et al. 2000b, 2001, 2002). In eastern Ontario, they breed successfully in forests managed for the production of maple syrup and for shelterwood silviculture (Oliarnyk 1996, Jones 2000). On the wintering grounds, they can be found in good numbers in shade coffee plantations (Jones et al. 2000b, 2002). Similarly, this species is frequently found in second-growth habitat during migration (Land 1970, Ridgely and Gwynne 1989, Stiles and Skutch 1989).

Jones et al. (2001) examined the effects on forest structure and Cerulean Warbler reproductive success of the January 1998 ice storm (the worst in documented Canadian history, Kerry et al. 1999). This storm resulted in a significant reduction in the amount of foliage in the forest canopy of the study area in eastern Ontario the following spring. This was followed by a significant decline in Cerulean Warbler reproductive output in the 1998 breeding season. In 1999, Cerulean Warblers demonstrated a significant increase in territory size and a significant shift in nest-site location patterns; these shifts were accompanied by a significant increase in reproductive success. The 1999 shifts in territory size and nest-site location patterns were effected by the same individuals that were failed breeders in 1998. This suggests that Cerulean Warblers possess a degree of plasticity in their habitat preferences and that this plasticity rendered the population somewhat resilient to certain disturbances. One of the more reliable locations for Cerulean Warblers in Quebec (Mont Saint-Hilaire) was damaged by an ice storm in the early 1980s; very few sightings have been recorded there since the storm (Bannon and Robert 1996).

Cerulean Warblers exhibit two behaviours that may render individuals particularly vulnerable. The first is the high site fidelity exhibited by adults. Despite their apparent resilience to certain disturbances, the fact that adults seem unable to “recognize” habitat degradation may keep birds in unsuitable habitats. The second is their protracted migration periods in both spring (2 months) and fall (4 months). It is unclear whether the protracted appearance of Cerulean Warbler migration is due to individuals taking a long time to migrate or due to the species taking a long time to migrate (i.e. some individuals migrate early and some late). If the former scenario is true, this long traveling period could not only be subjecting individuals to high physiological stresses but may also be exposing them to high probability of predation (more travel days = more days at risk) as well as habitat disturbances along the length of their migration routes. Migration has been implicated as a major time of mortality for Cerulean Warblers (Jones et al. in review) and other wood-warblers (Sillett and Holmes 2002).