Golden-winged warbler (Vermivora chrysoptera) COSEWIC assessment and status report: chapter 6

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Biology

Life cycle and reproduction

Breeding pairs raise only one brood per year with an average clutch size of 4.75 (range 2 – 6) in Ontario. Clutch sizes of subsequent broods (i.e. renests that occur after a failed nesting event) often decrease with two eggs being the minimum observed in a third nesting attempt within one breeding season. Double brooding has not been reported in this species. Birds often breed at one year of age and can continue to reproduce up to nine years of age. Average generation time for this species is two to three years (R. Fraser, unpub. data). Pairs are socially monogamous but exhibit high rates of extra-pair paternity (i.e. social fathers are not genetic fathers 33% of the time; Fraser et al., under review)

In a sample of 103 nests found in Ontario between 2001 and 2004, 55.3% (N=57) of nests successfully fledged at least one offspring, 37.9% (N=39) were depredated, and 6.8% (N=7) were abandoned. Of 435 nestlings in these nests, 256 (58.85%) fledged, while 176 (40.46%) were depredated. Average fledging success was 2.49 nestlings per breeding pair in this population (R. Fraser, unpub. data), which is similar to success rates from New York (2.3 per breeding pair; Confer et al. 2003) but lower than reported fledgling success rates from other regions (e.g. 3.7 per breeding pair in Tennessee; Klaus and Buehler 2001). Nests can be abandoned due to flooding, partial predation, Brown-headed Cowbird parasitism and death of the female (R. Fraser, unpub. data).

Birds are known to be strictly insectivorous during the breeding season. Diet consists mainly of tortricid moths and their larvae (Confer 1992). Other moths and their pupae, other winged insects, spiders and spider egg sacks are also sometimes consumed (R. Fraser, unpub. data). Similar feeding habits are also seen on the wintering grounds.

Predation

Eggs and nestlings of this species are vulnerable to a suite of predators including (in a breeding population in Ontario) the Raccoon (Procyon lotor), Red Fox (Vulpes vulpes), Coyote (Canis latrans), Short-tailed Weasel (Mustela erminea), Mink (Mustela vison), Red Squirrel (Tamiasciurus hudsonicus), Eastern Gray Squirrel (Sciurus carolinensis), Eastern Chipmunk (Tamias striatus), Fisher (Martes pennanti), Striped Skunk (Mephitis mephitis), White-footed Mouse (Peromyscus leucopus), Deer Mouse (P. maniculatus), Meadow Vole (Microtus pennsylvanicus), Woodland Jumping Mouse (Napaeozapus insignis), Common Garter Snake (Thamnophis sirtalis), Eastern Ribbon Snake (Thamnophis sauritus), Eastern Ratsnake (Elaphe o. obsoleta), Blue Jay (Cyanocitta cristata), American Crow (Corvus brachyrhynchos; Demmons, unpub. data) and American Toad (Bufo americanus; R. Fraser, unpub. data).

Adult birds likely face a less diverse group of predators, although incubating and brooding females have been taken from the nest site (R. Fraser, unpub. data).

Dispersal/migration

Spring migration in Ontario peaks around mid-May, with males typically arriving in areas of northern New York and southern Ontario in the first few days of May and females arriving one to two weeks later. Birds are assumed to have left the tropics by mid-April (Confer 1992). Fall migration peaks in late-August and early-September in areas of southern Ontario and New York (Confer 1992).

Little is known about dispersal movements in this species, although this is currently under investigation (K. Fraser, unpub. data). Mark and recapture studies indicate that some (4.3%) offspring return to their natal area (N=16/368 nestlings banded between 2001 and 2003 in Ontario) and may breed as close as 500m from the nest site where they were born (R. Fraser, unpub. data).  Of these 16 nestling returns, 11 were male while five were female.

Adults show strong site-fidelity and exhibit high return rates (64-88% of adult males and 35-53% of adult females) to the same breeding location in subsequent years (R. Fraser, unpub. data). Both males and females have been documented as inhabiting the same breeding territory for seven years (R. Fraser, unpub. data). Return rates in some areas of the United States are much lower (e.g. 16% in West Virginia; R. Canterbury, pers. comm.) suggesting that patterns of high return rates are not to be interpreted as being a range wide phenomenon and are likely attributable a variety of factors such as changes to the breeding habitat and/or to the surrounding landscape.

Interspecific interactions

Hybridization with Blue-winged Warblers - Overview

One of the key factors implicated in Golden-winged Warbler decline is hybridization with the Blue-winged Warbler. While traditionally breeding in allopatry after glaciers receded, these sister species were brought into geographic contact in the 1800s when clearing of the land facilitated the movement of Blue-winged Warblers into the Golden-winged Warbler breeding range in the northeastern United States (Gill 1980). Therefore, hybridization between these two superspecies is a relatively recent phenomenon exacerbated by anthropogenic disturbances to the landscape (Mayr and Short 1970; Gill 1980). The current hybrid zone extends from the eastern reaches of the species’ range through to Minnesota, and is rapidly moving northward (Figure 4).

Figure 4.  Hybrid index indicating remaining areas of allopatry and areas of contact.  Golden-winged Warbler in yellow, Blue-winged Warbler in blue, hybrid zone in green (Data courtesy of K. Rosenberg, GOWAP, Cornell Lab of Ornithology).

In areas of sympatry, hybridization occurs with great frequency and has been seen in all areas of contact (e.g. Gill 1980; Confer 1992). Hybridization tends to negatively affect the Golden-winged Warbler with local extirpation occurring within 50 years of Blue-winged Warbler arrival being the norm (Gill 1997), although replacement can occur within as few as four or five years (Gill 2004).  Advancing Blue-winged Warbler females apparently lead the introgression causing an asymmetric and apparently rapid introgression of Blue-winged Warbler DNA into Golden-winged Warbler populations (Gill 1997). These results suggest that Blue-winged Warblers have a genetic advantage over Golden-winged Warblers and this may explain the replacement of Golden-winged Warblers by Blue-winged Warblers in areas of contact. 

Recent genetic analyses, however, indicate that there is bidirectional gene flow in at least five currently mixed populations (Michigan, Ohio, West Virginia, New York and Ontario) suggesting that Blue-winged Warblers do not genetically swamp Golden-winged Warblers in all areas of contact (Shapiro et al., 2004; Dabrowski et al., in press). These results are important because they suggest that patterns of mitochondrial introgression that vary by site may be related to differences in geographic variables, in local population sizes, as well as subtle between-species differences in mate-choice and habitat preferences.

Further work examining habitat use and genetic analyses are needed from sympatric and allopatric populations of these species throughout the entire breeding range to elucidate the true impact of hybridization on populations of Golden-winged Warblers.

Hybrid phenotypes

When Blue-winged and Golden-winged Warblers interbreed they tend to produce one of two phenotypes assigned the names “Brewster’s Warbler” (“V. leuchobronchialis”) and “Lawrence’s Warbler” (“V. lawrencei”), although exceptions are sometimes observed. The “Brewster’s Warbler” exhibits plumage characteristic of a Golden-winged Warbler with a yellow wing patch and white underparts (although some have speculated that a first generation “Brewster’s Warbler” may exhibit yellow underparts; Parkes 1951), but they lack the striking black throat and eye patch (Figure 5). The “Lawrence’s Warbler” looks mostly like a Blue-winged Warbler with yellow underparts and white wing bars. However, the “Lawrence’s Warbler” exhibits the throat and eye patch characteristic of the Golden-winged Warbler (Figure 5), likely representing a combination of recessive characters (Parkes 1951).

Figure 5.  Male “Lawrence’s Warbler” (left) and a male “Brewster’s Warbler” (right). (Photos by Rachel Fraser).

Later generations of hybrids may show only subtle signs of introgression and may be classified as one of the parental species without close (i.e. in-the-hand) examination (Short 1963, 1969). Furthermore, both hybrid phenotypes sing one of the two parental song types, and very rarely a combination of them both (Ficken and Ficken 1968; Gill and Murray 1972). The latter two factors could have pronounced implications for the use of surveys to identify/locate Golden-winged Warblers.

Some studies have suggested that hybrids are at a mating disadvantage and therefore may have overall lower fitness than do the parental species (Confer and Tupper 2000; Confer and Barker 2002). Recent quantitative analyses of reproductive success, however, show that hybrids are not at a disadvantage and in fact hybrid fitness and extra-pair fertilizations are likely playing a major role in the ongoing hybridization between Blue-winged and Golden-winged Warblers (Fraser et al. under review; Gill 2004).

Genetic distinctiveness

Prior to the publication of results from genetic analyses it was suggested that Blue-winged and Golden-winged Warblers had a common ancestry that occurred during the recent glacial maxima at approximately 20,000 years before present (e.g., Short 1963). As such, and given the existence of extensive hybridization in many areas, some researchers suggested that Blue-winged and Golden-winged Warblers should be considered conspecific (Mayr and Short 1970).

However, Gill (1997) reported that the nucleotide divergence between these 2 species was estimated to be 3.2% based on RFLP analyses. This suggests these lineages are substantially older and more differentiated than was previously suggested (Short 1963; Shapiro et al. 2004). Later work by Dabrowski et al. (in press) and Shapiro et al. (2004) determined the level of mtDNA sequence divergence to be approximately 4.5%. This level of divergence between ancestral Blue-winged and ancestral Golden-winged Warbler haplotypes is several orders of magnitude greater than expected if these lineages split near the Pleistocene-Holocene boundary. In comparison with other passerine genera the level of nucleotide divergence between the ancestral Blue-winged and ancestral Golden-winged haplotype groups is equivalent to the separation of many other pairs of taxa that comprise clear biological species (reviewed in Johnson and Cicero 2004).

Regions of allopatry

Currently allopatric populations of Golden-winged Warblers only occur in the most extreme northern reaches of the breeding range, as well as at its highest nesting elevations in the Appalachian Mountains (Confer and Knapp 1981; Figure 4).  Genetic purity of these populations has not yet been established, but is currently under investigation (R. Fraser, unpub. data). Even populations that were thought to be “safe havens” are now showing evidence of hybrid or Blue-winged Warbler arrival. For example, a previously allopatric population of Golden-winged Warblers in Ontario saw the first arrival of hybrids and Blue-winged Warblers in the late 1980s. By 2004 17% of the breeding population was made up of hybrid phenotypes (Blue-winged Warbler occurrence remains low at <1%; R. Fraser, unpub, data). In addition, there have already been five reports of Blue-winged Warblers and one hybrid from Saskatchewan (Saskatchewan Bird Atlas Project; J. Keith, pers. comm.), while Manitoba reports only one hybrid sighting to date (a female “Brewster’s Warbler” in 1932; Manitoba Museum - MARC record #2963). One male Blue-winged Warbler was sighted in Manitoba (Manitoba Avian Research Committee; Manitoba Museum); however, this observation took place during the fall and therefore likely represents a disoriented hatch year bird and is not indicative of Blue-winged Warblers expanding their range into Manitoba (K. Hobson, pers. comm.).

If Blue-winged Warbler breeding range advances continue, currently allopatric populations of Golden-winged Warblers can be expected to come into contact with Blue-winged Warblers in the near future making extirpation via hybridization and competition more likely, but not a certainty, as the Sterling Forest site in New York demonstrates.

Adaptability

Currently there are no studies on the adaptability of this species directly. Because of the ephemeral nature of early successional scrub environments this species uses on the breeding ground, it presumably deals well with reestablishment of breeding populations in suitable habitat (NatureServe 2004) when others have grown to a stage where they are no longer of use. Indeed, the species has been known to move into areas that have been recently logged, or burned, and they readily move into areas that are intermittently farmed (Klaus and Buehler 2001). Edge creation experiments carried out since 1997 near Elgin, Ontario have documented that the species will move into an area of suitable habitat within three years of its creation (R. Fraser, unpub. data).