Prothonotary warbler (Protonotaria citrea) COSEWIC assessment and status report: chapter 6

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Biology

Life cycle and reproduction 

The Prothonotary Warbler is highly territorial during the breeding season, but not on the wintering grounds (Lefebvre et al. 1994; Petit 1999). At breeding sites that support ample suitable habitat, males often settle in well-defended territories that are adjacent to other males (J.D. McCracken, pers. obs.).

Based on territory spot-mapping studies conducted in the core of its range in the southeastern U.S., breeding densities were reported to range from 15 to 70 territorial males per 100 ha, and average about 37 territories per 100 ha (Hamel et al. 1982, cited in NatureServe 2005). Territory size is influenced by habitat quality, habitat configuration and population density (see Petit 1999). Mean territory size was about 1.5 ha in Michigan (Walkinshaw 1953), about 0.5 ha in Tennessee (Petit 1989), and from 1-2 ha in Ontario (J.D. McCracken, pers. obs.). When feeding nestlings, adults forage in an area that is considerably larger than the male’s defended territory – averaging up to 5.4 ha for females (Reynolds 1997 cited in Petit 1999).

Polygyny is rare, but increases when density of nest sites is high (Petit 1991 in Petit 1999). At least three cases of polygyny are known in Canada, all of which involved after-second year (ASY) males (J.D. McCracken, pers. obs.).

Peck and James (1998) reported egg dates in Ontario ranging from 25 May – 3 July, but several nests with eggs have since been found as late as 21 July (Prothonotary Warbler Recovery Team, unpubl. data). 

Clutch size ranges from 2 to 8 eggs, but 4 to 6 is the most usual (Walkinshaw 1957; Blem and Blem 1992). Older females are inclined to lay larger clutches than younger females, and first clutches are larger than second clutches (Blem and Blem 1992; Blem et al. 1999a). Both clutch size and reproductive success are positively related to the abundance of insect prey (Lyons 2005). 

In the southern U.S., the Prothonotary Warbler is typically double-brooded, while northern populations are typically single-brooded (Walkinshaw 1941). Double broods in Ontario are rare, but second nesting attempts are commonplace if the first nest is destroyed early in the season (J.D. McCracken, unpubl.data).

Nests are constructed by both sexes, with the male’s involvement limited to helping place the moss foundation, while the female completes the lining (Petit 1999).  Incubation period ranges from 10-14 days and averages about 12 days (Podlesak and Blem 2002). Incubation is done entirely by the female (Petit 1999). Both parents assist with feeding the young, which remain in the nest for 9-11 days (Podlesak and Blem 2002). 

Nesting success is highly variable, and largely depends on predation and parasitism rates, which in turn are largely influenced by whether the studies are based on natural cavity nests or employ nest boxes. Well-designed nest boxes confer protection against cowbird parasitism and mammalian predators when predator guards are used. Variability is also heavily influenced by the local density of House Wrens (Troglodytes aedon), a very aggressive competing species that is known to usurp Prothonotary Warbler nests and destroy their eggs and young. For example, Walkingshaw (1941) reported that 56% of the Prothonotary Warbler eggs that were laid in nest boxes successfully fledged young in Tennessee, compared to only 26% in Michigan. He attributed the low success rate in Michigan to high densities of House Wrens. In the absence of House Wrens, 44% of Prothonotary Warbler nests in nest boxes in Mississippi were successful, fledging an average of 3.1 young (Twedt and Henne-Kerr 2001). In Ontario, annual nest success (primarily in nest boxes) over a seven-year period ranged from 44% to 67%, with most losses attributed to House Wrens (Dobbyn and McCracken 2005; McCracken and Wood 2005).

Prothonotary Warblers mature in one year and, like most small birds, generally have a short life span. Walkinshaw (1953) estimated that the average life span of males was about 2.5 years. The longevity record for the Prothonotary Warbler is about 8 years (Blem et al. 1999a). Probability estimates for annual survival of adults in Tennessee were about 53% for males and 47% for females (Petit 1999). Likewise, in one study in Virginia, about 48% of female Prothonotary Warblers that were banded as adults returned to nest at least once in subsequent years (Blem et al. 1999a). Walkinshaw (1953) reported return rates of male and females in Michigan at 50% and 20%, respectively.

Information on the sex structure of the population in Canada is limited to data collected during annual population and nesting surveys that were undertaken from 1997-2005. It is recognized that unmated males are much more likely to be located by surveyors than unmated females. Nevertheless, an average of about 37% of all territorial male Prothonotary Warblers in Canada remained unmated during the course of the breeding season (McCracken and Wood 2005). Hence, at the northern limits of its breeding range in Canada, the sex ratio appears to be skewed towards males, suggesting that males have a higher propensity to drift farther north during spring migration than females.

During the period 1999-2005, the national recovery team captured and banded a total of 35 adult Prothonotary Warblers during the nesting season (Table 1). Ward (2005) noted that age structure should be skewed in favour of older birds in isolated populations where immigration is low. Indeed, this appears to be the case for Prothonotary Warblers in Canada, where after-second year (ASY) birds accounted for about 71% of the captured population (Table 1). The proportion of known yearling males (indicative of immigration) accounted for only about 18% of the breeding population, which appears consistent with Ward’s (2005) findings for isolated populations.

The Prothonotary Warbler is an insectivore, gleaning insects from leaves, twigs and branches, mostly from the subcanopy layer up to about 7 m in height (Petit 1999). During the breeding season, favoured foods include caterpillars, flies, midges, and spiders. Breeding males and females that are attending nests may segregate according to foraging height and prey items, with males foraging higher than females and more apt to deliver caterpillars to nestlings than females (Petit 1999; J.D. McCracken, pers. obs.). If fledged young move into the upper canopy, adults will adjust their foraging height (and presumably diet) accordingly.

Predation/parasitism

Some protection from some types of potential nest predators is probably conferred because Prothonotary Warbler nests are situated in cavities over open water (e.g.Nice 1957; Hoover 2006). Moreover, it is generally believed that nest boxes offer protection against many forms of predation (e.g. Nilsson 1986; Moller 1989; Blem et al. 1998; 1999b; Mitrus 2003; McCracken and Wood 2005), compared to nests in natural cavities. 

Including studies involving nest boxes, nest predation rates are highly variable, ranging from: 2.6% to 53.3% in Tennessee (Petit et al. 1987; Petit 1989; Petit 1991; Petit and Petit 1996); 15.5% in Virginia (Blem and Blem 1992); 27.6% in Wisconsin (Flaspohler 1996); 44% in Michigan (Walkinshaw 1941); 42% in Illinois (Hoover 2006) and about 40% in Ontario (J.D. McCracken, unpubl.data).

Excluding avian predators, losses of Prothonotary Warbler young and eggs are attributed to fox snakes (Elaphe vulpina), raccoons (Procyon lotor), mice (Peromyscus spp.), weasels (Mustela spp.), and squirrels (Walkinshaw 1938; Bent 1953; Guillory 1987; Petit 1989; Blem and Blem 1992; Flaspohler 1996; Petit and Petit 1996; Petit 1999; Hoover 2006). In Canada, the raccoon is probably the predominant mammalian predator at cavity nests and in unprotected nest boxes affixed to trees, whereas the southern flying squirrel (Glaucomys volans) is the predominant mammalian predator on nests placed in protected nest boxes affixed to metal poles (J.D. McCracken, unpubl. data).

In the northern part of the Prothonotary Warbler’s breeding range the destruction of eggs and young by House Wrens is apt to be the single most serious cause of Prothonotary Warbler nest failure (Walkinshaw 1941; Bent 1953; Walkinshaw 1953; Best and Fondrk 1995; Flaspohler 1996; Knutson and Klaas 1997; McCracken and Wood 2005). Moreover, unlike the protection that nest boxes afford against many other kinds of predators, they confer no protection against House Wrens (Doherty and Grubb 2002; McCracken 2004; Dobbyn and McCracken 2005). In Canada, House Wrens figure most prominently in the destruction of Prothonotary Warbler nests at sites that do not have large, unbroken areas of forest interior (Dobbyn and McCracken 2005; McCracken et al.2006). As such, some habitat changes can lead to increased interspecific interactions with wrens (see Limiting Factors and Threats). 

Brood parasitism from Brown-headed Cowbirds (Molothrus ater) may also limit population size and contribute to population declines by reducing productivity of Prothonotary Warblers (McCracken 1984; Flaspohler 1996). For a cavity-nesting species like the Prothonotary Warbler, cowbird parasitism rates in natural cavities can be surprisingly high: 21% in Tennessee (Petit 1989; 1991); 25.7% in Iowa (based upon data in Bent 1953); 26.9% in Wisconsin (Flaspohler 1996); and 27.1% in Ontario (Peck and James 1998). Many Prothonotary Warbler breeding studies are based on artificial nest structures, which usually confer protection against parasitism (Walkinshaw 1991; Best and Fondrk 1995; Flaspohler 1996; McCracken and Wood 2005), because nest hole diameter is typically smaller than in many natural situations, preventing access to cowbirds. In addition to the amount of edge in local sites, it is likely that land-use patterns and regional forest fragmentation determine the regional abundance of cowbirds (Flaspohler 1996). Distance from the historical heartland of the cowbird’s range may also be a factor (Hoover and Brittingham 1993).

Physiology

Most information on physiology relates to migration and wintering energetics. There does not appear to be anything unusual about the Prothonotary Warbler’s energetics, though it may be able to replenish spring fat reserves faster than some other warbler species (Moore and Kerlinger 1987, cited in Petit 1999). As a trans-Gulf migrant (see Petit 1999), it relies upon an ability to lay down extensive fat deposits before carrying out long, non-stop flights over open water. 

During nesting, there is evidence that the use of moist green moss in nests has a favourable damping effect on extremes of temperature and humidity (Blem and Blem 1994). 

Dispersal/migration

The Prothonotary Warbler is a long-distance migrant that typically begins to arrive in southern Ontario in the first week of May and departs by September (James 1991). In spring, males precede females to the breeding grounds (Blem and Blem 1992; Petit 1999), and older birds typically precede younger birds (J.D. McCracken, pers. obs.).

Because of the species’ rarity, no important areas that concentrate migrating Prothonotary Warblers are recognized in Canada. Nevertheless, during spring migration, the species occurs most regularly in small numbers along the north shore of Lake Erie (e.g. Holiday Beach, Pelee Island, Point Pelee, Rondeau, Long Point). There are very few records of the species at these locations in the fall, probably because these sites are not used as fall stop-over destinations. The species appears to simply depart the province in one migration jump.

The Prothonotary Warbler is a nocturnal migrant. In Canada, it is believed to migrate solitarily, but it frequently occurs in small flocks when undertaking trans-Gulf migrations and on the wintering grounds (see Petit 1999).

Little information is available on post-fledging dispersal, but most young likely remain within about 250 m of the natal area for the first few weeks (Petit 1999; J.D. McCracken, pers. obs.). Petit (1999) reported an instance of a fledgling that was about 6 weeks old wandering several kilometres.

First-year Prothonotary Warblers are not very site-faithful to natal areas (Walkinshaw 1941, 1953; McCracken et al. 2006; Wood 2006), and likely disperse widely. However, after-second-year (ASY) individuals, particularly males, exhibit strong fidelity to breeding sites, often using the same nest sites in successive years (Walkinshaw 1941, 1953; McCracken and Wood 2005; Wood 2006). In one study in Virginia, up to 48% of females that were banded as adults returned to the same forest to nest at least once in subsequent years (Blem et al. 1999a). Many of the breeding locations in Canada have a long record of occupancy, though the species does not necessarily recur at every site in every year (McCracken 1984, 1987).

To date, the only recorded long-distance dispersal event in Canada involves an adult female that was originally colour-banded at a nest north of Long Point, ON and relocated three years later nesting at Rondeau, ON – a dispersal distance of about 120 km (McCracken and Wood 2005).

There is incomplete information on the degree of year-to-year site fidelity to wintering sites, but there is every indication that the Prothonotary Warbler is quite site faithful, with annual rates of returns ranging from about 13% to 29% (McNeil 1982; Faaborg and Arendt 1984; Lefebvre et al. 1994; Warkentin and Hernández 1996; Woodcock et al. 2005). 

Interspecific interactions

Petit (1999) reported that Prothonotary Warblers were intolerant of many species during the breeding season. Where they occupy the same breeding territory, males interact particularly aggressively towards Yellow Warblers (Dendroica petechia), presumably because of similar colouration and chip notes (J.D. McCracken, pers. obs.). There is one known instance of hybridization between a male Prothonotary and a female Yellow Warbler (Speirs 1956).

As noted earlier, interspecific interactions with House Wrens are often intense. Not only do wrens directly destroy Prothonotary Warbler eggs and young, but they also build many “dummy” nests, often filling every available cavity in their territory with sticks. This directly displaces nesting Prothonotary Warblers, and indirectly reduces cavity availability, thereby further increasing competition for nest sites. Moreover, the sticks are liable to persist in the cavities for many years, effectively rendering them unsuitable for other species. House Wrens produce at least two broods per year, which means that their impacts extend throughout the duration of the Prothonotary Warbler’s nesting season.

In very open areas, Tree Swallows (Tachycineta bicolor) can also be serious competitors for nest sites (Best and Fondrk 1995; McCracken et al.2006). However, because Tree Swallows nest relatively early and are typically single-brooded, competition for nest sites begins to decline at the end of June, and they are not considered as serious a competitor as wrens. Moreover, unlike wrens, swallows do not aggressively destroy the eggs of competitors, nor do they usurp other cavities by building “dummy” nests.

Adaptability 

Although the Prothonotary Warbler has rather exacting habitat requirements at all times of the year, it readily makes use of artificial structures for nesting, and readily occupies various types of nest boxes, including wax-board milk cartons (e.g. Fleming 1986; Hoover 2006) and plastic bottles (e.g. Fondrk 1996). This attribute has resulted in the proliferation of numerous nest box programs, many of which have apparently been effective at bolstering local populations of Prothonotary Warblers (see Petit 1999). However, there is also evidence that nest box provisioning could be ineffectual, and potentially even detrimental, at sites where House Wren densities are high (Walkinshaw 1991; Dobbyn and McCracken 2005).

Prothonotary Warblers appear to be very “tame,” and tolerate human presence at all times of the year, even in the vicinity of the nest site (Walkinshaw 1957; Petit 1999).