Prothonotary warbler (Protonotaria citrea) COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

During the breeding season in Canada, Prothonotary Warblers occupy mature and semi-mature, deciduous swamp forest and riparian floodplains (McCracken 1984; Petit 1999). Tree cover is typically dominated by silver maple (Acer saccharinum), ash (Fraxinus spp.), and yellow birch (Betula alleghaniensis), often with a mature buttonbush (Cephalanthus occidentalis) component in open flooded areas. Canopy closure ranges from incomplete to complete, but is usually extensive enough to limit the development of an extensive herbaceous and shrubby plant understory. Nest sites are usually shaded for at least part of the day (Blem and Blem 1991; 1992; Best and Fondrk 1995).

Permanent and semi-permanent pools of open water are characteristic, and nests are typically situated over standing or slow-moving water. At Ontario sites, water depth typically ranges from 0.5 to 1.5 m, and the surface area of water typically represents between 70% and 100% of the territory (J.D. McCracken, pers. obs.). Pools in these territories may be 1 ha or more in size, though sites containing smaller pools will also be readily occupied, especially if several pools are in close proximity (J.D. McCracken, pers. obs.).

The Prothonotary Warbler was described as an area-sensitive species by Keller et al. (1993), Petit (1999), and Thompson et al. (1993), but not by Robbins et al. (1989) or Hodges and Krementz (1996). In riparian forests in the southeast U.S., Prothonotary Warbler populations can probably be conserved if a 100 m wide corridor of suitable habitat is protected (Hodges and Krementz 1996), while Kilgo et al. (1998) found that the species’ probability of occurrence was greatest in forests that were at least 500 m wide. In Canada, over 95% of known breeding sites occur within forest patches that are at least 100 ha in size (Flaxman and Lindsay 2004).

The Prothonotary Warbler is a secondary-cavity nester, often using naturally formed tree cavities in dead or dying trees, but more commonly occupying cavities that have been excavated by chickadees (Parus sp.) and Downy Woodpeckers (Picoides pubescens; Petit 1999). Properly designed nest boxes are readily accepted, and perhaps even preferred (e.g. Best and Fondrk 1995; McCracken et al. 2006). Indeed, Twedt and Henne-Kerr (2001) demonstrated that local breeding densities of Prothonotary Warblers were enhanced by nest box provisioning, effectively showing that cavity availability is a limiting factor (see also Petit 1999). Prothonotary Warblers have also been known to nest in a variety of unusual situations, including a coffee can, tin pail, glass jar, an old hornet’s nest, a mailbox (Bent 1953), and an empty Red-winged Blackbird (Agelaius phoeniceus) nest (Petit and Petit 1988).

As noted above, nest sites are almost always directly above water. Cavities are invariably shallow and usually situated at low heights (typically about 2 metres from the ground or water; Petit 1999). Males often build one or more incomplete “dummy” nests (Bent 1953; Petit 1999; Blem and Blem 1992). The female often selects one of these to complete, but may also build an entirely new nest on her own. In any case, several suitable cavities appear to be required in each territory, in order to accommodate both the functional nest plus one or more “dummy” nests (Petit 1999).

Nests are constructed primarily of green mosses, often with some liverworts and dead leaves (Petit 1999). They are lined with fine rootlets, lichens and grasses. Moist green moss is an essential material in both incomplete (“dummy”) and functional nests (e.g. Blem and Blem 1994). There is no information on whether particular species of mosses are favoured, but Anomodon attenuatus and Bryohaplocladium microphyllum were the dominant species used in Virginia (Blem and Blem 1994). Both species are common in Ontario (D.A. Sutherland, pers. comm.2006). 

No information has been published on habitat use during the post-fledgling period (Petit 1999). Fledged young are apt to range widely (Petit 1999), often occupying the upper tree canopy within 250 m or more of the nest site, regardless of the availability of standing water (J.D. McCracken, pers. obs.).

The Prothonotary Warbler's key wintering habitat is coastal mangrove forest in Central America and northern South America (Lefebvre et al. 1992; Lefebvre et al. 1994; Petit et al. 1995; Lefebvre and Poulin 1996; Warkentin and Hernández 1996). It also winters in swamps and wet woodlands and occasionally in drier woodlands (including pine forest), mainly below 1300 m (Bent 1953; Arendt 1992; Curson 1994). The habitat preferences (e.g. structure, species composition, spatial characteristics, stand age, moisture regimes) of wintering Prothonotary Warblers have not yet been quantitatively described, though black mangrove (Avicennia germinans) forest is a primary habitat type in Venezuela, Panama and Costa Rica (Lefebvre et al. 1994; Lefebvre and Poulin 1996; Warkentin and Hernández 1996; Woodcock et al. 2004).

Habitat trends

In the contiguous U.S., only 10% of the original bottomland forest habitat remains (Dickson et al. 1995). In the southeastern U.S., forested wetlands are being lost at a very high rate (Winger 1986; Hefner et al. 1994). Losses have been particularly high in coastal Louisiana and the Carolinas (U.S. Dept. Interior 1994) -- two of the species’ core breeding areas.

Likewise, much of southwestern Ontario’s historical wetlands and forests have disappeared, been heavily fragmented, and/or have been drained for agricultural purposes (see Snell 1987; Page 1996). In southern Ontario, Snell (1987) estimated that wetlands had been reduced by about 1.5 million ha (61%) from the time of European settlement to 1982. The great majority of this loss occurred before the 1960s. Nevertheless, between 1967 and 1982, wetlands in southern Ontario had been reduced by about 39,000 ha, mostly due to agricultural activities (Snell 1987). Since 86% of the wetlands that were then remaining in southern Ontario were forested swamps (Snell 1987), most of the recent loss is assumed to have involved this type of habitat. There are now few, large intact blocks of deciduous swamp forest remaining in this region. In recent decades, habitat loss has slowed considerably, with the introduction of provincial policies designed to protect significant wetlands in southern Ontario. Nevertheless, swamp forests continue to be drained through the large system of municipal drains and tile drainage.

On the wintering grounds, loss of coastal mangrove forest is high and likely increasing (Petit et al. 1995; McCracken 1998a; see Limiting Factors and Threats).

Habitat protection/ownership

In Canada, the majority of habitat that has been occupied by Prothonotary Warblers since 1981 occurs on public lands that are typically afforded strong to very strong levels of habitat protection (see also Appendix A):

Federal lands

The species nests regularly at Big Creek National Wildlife Area and occasionally at Long Point National Wildlife Area (Norfolk Co.). It also occasionally nests at Point Pelee National Park (Essex Co.) when water levels are high.

Provincial Crown lands

Prothonotary Warblers nest regularly at Rondeau Provincial Park (Chatham-Kent), and sporadically at Wheatley Provincial Park (Essex Co.) and Pinery Provincial Park (Lambton Co.).

Conservation Authority lands

Nesting is fairly regular at Holiday Beach Conservation Area (Essex Co.) and Backus Woods (Norfolk Co.), and sporadic at Hillman Marsh Conservation Area (Essex Co.).

Other Conservation lands

Small numbers nest fairly regularly at Coote’s Paradise (Hamilton-Wentworth).

There are no known nesting occurrences on First Nations lands. On private lands, several known nesting occurrences occur within "Environmentally Sensitive Areas" that are afforded various levels of protection through municipal plans and/or the Provincial Policy Statement on significant wetlands. Several privately owned sites are managed by hunt clubs, which typically have a strong interest in conserving wetland habitat.

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