Prothonotary warbler (Protonotaria citrea) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

In Canada, the breeding range of the Prothonotary Warbler appears to be limited primarily by suitable climate and habitat availability (Tischendorf 2003). Judging from abundance mapping derived from the Breeding Bird Survey (Sauer et al. 2005), the Prothonotary Warbler’s northern breeding range may be influenced further by densities of its principle nest-site competitor – the House Wren. The core breeding ranges of the two species demonstrate little overlap.

Loss/degradation of breeding habitat

The decline of Prothonotary Warbler populations in the U.S. is attributable to loss and degradation of its wetland habitat (Petit 1999). A similar pattern of habitat loss has occurred in the species' breeding range in Canada, where nearly all deciduous swamp forests have been drained to varying degrees and/or have been cut-over (see Habitat Trends). Drainage of swamp forests, whether through ditching, agricultural tiling, municipal drains or irrigation, depletes the water table and removes standing water. Degradation of breeding habitat in this way poses one of the most significant, widespread and ongoing threats facing the species in Canada.

Adverse effects of both flooding and drought have been reported. In riparian habitats, short-term floods can be a major cause of Prothonotary Warbler nest failures (Walkinshaw 1953; Flaspohler 1996; Knutson and Klaas 1997). At the same time, however, Knutson and Klaas (1997) suggest that periodic major flooding can benefit Prothonotary Warblers by maintaining suitable floodplain habitat in the face of competition with House Wrens for nest sites. Meanwhile, long-term high water events in swamp forest can result in massive dieback of trees, rendering habitat unsuitable (Page 1996). Likewise, the species does not persist very long at sites where successive droughts dry up swamp forest habitat (McCracken et al. 2006).

The Prothonotary Warbler will likely be sensitive to climate change, especially at the northern periphery of its range. Periodic high water levels on the Great Lakes appear to benefit the species (McCracken et al. 2006; Wormington 2006). Most climate change scenarios in the Great Lakes basin predict lessening amounts of precipitation, increased evaporation of surface water, and reduced ground water supplies (e.g. Smith et al. 1998; Environment Canada 2001). Because the species is so highly dependent on the availability of water, it seems likely that the negative effects stemming from lower water supplies would offset any northward range expansion that might otherwise be expected to occur as a result of warming temperatures (see Matthews et al. 2004).

Development activities can also contribute to the loss of habitat. One regularly occupied nesting site (Turkey Point) was destroyed when it was developed into a marina in the late 1970s (McCracken 1984). Because some jurisdictions in southern Ontario still do not have tree-cutting bylaws, some forms of development are also likely to involve removal of large swaths of forest and drainage of swamps. For example, a proposal to develop a large swamp forest, which was believed to support one or more Prothonotary Warblers, into a golf course was eventually stopped, but not before the site had been severely fragmented by ditching and the clearing of fairway lanes (McCracken and Mackenzie 2003). In addition, residential/estate development adjacent to swamp forests is apt to artificially increase local populations of nest predators (e.g. raccoons) and/or competitors (e.g. House Wrens).

Likewise, logging disturbances that take place in occupied habitat create forest openings and edge habitat that can result in increases in numbers of nest competitors, nest predators, and nest parasites. Overharvest by logging also reduces the amount of open water cover in swamp forests through heightened evaporation. The increased light penetration can also result in rapid encroachment of invasive plants (e.g. Phragmites). In addition, removal of standing dead timber results in loss of nesting cavities.

Loss of wintering habitat

Coastal mangrove forest in Latin America is highly threatened by deforestation for building supplies, charcoal production and resort development (Terborgh 1989; Petit em>et al. 1995; and reviewed by McCracken 1998a). Mangrove habitat is also under increasingly intense pressure from commercial shrimp farmers (e.g. Arendt 1992). Loss and degradation of wintering habitat is believed to have a strong effect on wintering Prothonotary Warblers (Lefebvre et al. 1994; Warkentin and Hernández 1996; McCracken 1998a), and is likely contributing to the species’ decline continentally. A relatively high level of site fidelity on the wintering grounds is believed to increase the species' sensitivity to habitat loss and disturbance (e.g. Holmes and Sherry 1992; Warkentin and Hernández 1996). 

Invasive forest insects

Forest insect infestations have the potential to kill large numbers of trees. While this could benefit Prothonotary Warblers in the short term through the creation of more dead stubs, the long-term impact is expected to be severe if the affected trees make up a large proportion of the canopy. Anything that significantly opens the tree canopy is apt to result in significant degradation in habitat quality, whether it is through encroachment of invasive plants or increased numbers of wrens and cowbirds.

The emerald ash borer (Agrilus planipennis) is of increasing concern in southern Ontario, since ash is a frequent subdominant tree in swamp forests here (McCracken et al. 2006). The Asian long-horn beetle (Anoplophora glabripennis) presents another important emerging concern, depending upon its ability to spread beyond its present area of containment in the Toronto region and its affinity for silver and red maple, which are typically dominant trees in Prothonotary Warbler swamp habitats.

Invasive plants

Two invasive species of plants – Phragmites and European black alder (Alnus glutinosa) – can significantly degrade Prothonotary Warbler breeding habitats, particularly when water levels are low and/or when canopy cover is reduced.

Within the last decade at Rondeau Provincial Park, Phragmites has expanded dramatically through many of the slough forests, especially in the larger and more open sloughs and in areas where canopy closure has been reduced due to catastrophic tree windthrow (see below; McCracken et al. 2006). Because the Prothonotary Warbler requires expanses of open water, this invasive emergent effectively fills in the open pools of water, rendering nesting habitat unsuitable. European black alder is a highly invasive shrub that can also significantly degrade Prothonotary Warbler nesting habitat in open swamp forest conditions, in much the same way as Phragmites. It is becoming increasingly abundant in one important nesting area (McCracken et al. 2006).

Catastrophic weather events

The intensity and frequency of storms (including hurricanes) on both the wintering and breeding grounds is anticipated to increase as a result of climate change. Owing to the Prothonotary Warbler’s clumped and restricted distribution in Canada, catastrophic weather events along the north shore of Lake Erie pose a serious threat to the species. Such an event occurred at Rondeau Provincial Park in July 1998, resulting in a dramatic change in forest structure (Larson and Waldron 2000). The number of Prothonotary Warblers nesting at Rondeau has declined markedly since then, likely because the more open canopy has favoured House Wren populations (Dobbyn and McCracken 2005; McCracken et al. 2006). A similar severe weather event at Point Pelee in the late 1970s resulted in significant windfall and may have reduced habitat suitability for Prothonotary Warblers there (D.A. Sutherland fide V. McKay and A. Wormington). 

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