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Rapid adoption of nestboxes by Prothonotary Warblers (Protonotaria citrea) in mesic deciduous forest

Journal: Canadian Journal of Zoology

Manuscript ID cjz-2019-0059.R2

Manuscript Type: Article

Date Submitted by the Author: 04-Aug-2019

Complete List of Authors: Mueller, Alexander; University of Memphis, Biological SciencesTwedt, Daniel; United States Government, US Geological SurveyBowers, Emerson 'Keith'; University of Memphis, Biological Sciences

Is your manuscript invited for consideration in a Special

Issue?:Not applicable (regular submission)

Keyword: ECOLOGY < Discipline, LIFE HISTORY < Discipline

https://mc06.manuscriptcentral.com/cjz-pubs

Canadian Journal of Zoology

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Rapid adoption of nestboxes by Prothonotary Warblers

(Protonotaria citrea) in mesic deciduous forest

A.J. Mueller, D.J. Twedt, and E.K. Bowers

A.J. Mueller and E.K. Bowers. Department of Biological Sciences, Edward J. Meeman

Biological Station, and Center for Biodiversity Research; University of Memphis, Memphis,

TN 38152 USA

D.J. Twedt. USGS Patuxent Wildlife Research Center, University of Memphis, Memphis TN

38152 USA

Corresponding author: E.K. Bowers (email: [email protected]; Tel.: +1 901 678 3406;

Fax: +1 901 678 4457)

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Breeding territory selection in Prothonotary Warblers (Protonotaria citrea, Boddaert 1783) is

thought to hinge on standing water, with a strong preference for low-lying areas prone to

seasonal flooding. However, we have observed this species nesting in much drier areas than

previously reported. We recently initiated a study of the Carolina Wren (Thryothorus

ludovicianus, Latham 1790) using wooden nestboxes, and nearly 60% of all nests produced in

these boxes during the initial study year were produced by Prothonotary Warblers, despite this

species being absent from our field site during the year preceding nestbox availability. Most

nests were produced in dense, closed-canopy forest with a thick shrub layer >100 m from any

water body. There was no difference in the average distance from water between nests of the

Prothonotary Warbler and those of the Carolina Wren, a habitat generalist that does not nest over

water. We then observed a 60% increase in the number of Prothonotary Warbler nests the

following year, along with significant increases in breeding productivity. Although they nested

on sites they are not thought to prefer, our observations suggest that Prothonotary Warblers may

nest in drier areas than usual if appropriate nest cavities are provided.

Key words: breeding habitat selection, Carolina Wren, nestbox, Prothonotary Warbler,

Protonotaria citrea, Thryothorus ludovicianus

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Introduction

The abundance of many species has declined rapidly over recent decades, and although the

main causes of species decline (e.g., habitat loss, invasive species, climate change) are generally

well established, exactly how these factors interact to affect changes in species abundance is

seldom clear (Didham et al. 2007). For cavity-nesting birds, the availability of natural nest sites

tends to be more limited in drier upland areas (e.g., Smith 1977; Stauffer and Best 1980),

possibly because upland forest is more likely to be logged than riparian forest (Darveau et al.

1995), thereby reducing forest age and the availability of snags in which to nest. Local

populations of cavity-using species may often be maintained through the use of artificial cavities

or nesting boxes (e.g., Spring et al. 2001). Indeed, LiBois et al. (2012) detected a significant

increase in survivorship and breeding success in Mediterranean Storm Petrels (Hydrobates

pelagicus melitensis, Boie 1822), when nesting in artificial nestboxes as compared with natural

nests. Thus, management practices that increase the availability and safety of suitable nest

cavities, either through the erection of artificial nestboxes or maintenance of snags in which

these species can nest (e.g., secondarily within old woodpecker cavities), may provide an

effective means of bolstering local densities and potentially augmenting local populations

(Slevin et al. 2018).

The complexity of these potential interactions makes it difficult to ascertain best practices for

ameliorating population declines. For example, the Prothonotary Warbler (Protonotaria citrea,

Boddaert 1783), a Neotropical-Nearctic migratory bird (Tonra et al. 2019) whose endemic

breeding range lies predominantly in the Southeastern United States, was once much more

abundant across their range than they are currently (Sauer et al. 2017). Habitat loss, both on the

breeding and wintering grounds and migratory route, has likely been the biggest contributor to

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reductions in their abundance (Petit 1999; Tonra et al. 2019); however, additional factors, such

as competition with heterospecifics for a limited number of nesting sites on the breeding grounds

(sensu Slagsvold 1978; Newton 1994; Petit 1999), may have also contributed to their decline.

Breeding habitat selection in Prothonotary Warblers has long been known to be determined

largely by the presence of standing water and seasonally flooded bottomland forests (Petit and

Petit 1996; Petit 1999). Reportedly, nesting pairs of this cavity-nesting species will almost

universally build nests directly over or within 5 m of standing water (Petit 1999, Tirpak et al.

2009). Low-lying forest areas prone to seasonal flooding are regarded as the preferred breeding

sites for these birds (Kahl et al. 1985; Blem and Blem 1991; Petit and Petit 1996; Tirpak et al.

2009). However, competition for territories and nest cavities may shift nest locations from wet

areas to drier sites that may be up to 100 m from water (Petit and Petit 1996). Indeed, these

upland areas are usually depauperate of nesting Prothonotary Warblers. However, our

observations suggest that Prothonotary Warblers will readily nest on much drier sites than

previously thought if nest boxes are provided, thereby potentially relaxing nest site limitations.

We recently initiated a study of Carolina Wrens (Thryothorus ludovicianus, Latham 1790)

using nesting boxes erected ca. 1.5 m above ground in upland forest. To our surprise, most of the

nests in these boxes during the initial year of study were produced by Prothonotary Warblers,

even though no Prothonotary Warblers were observed at this site in the summer preceding the

deployment of our nestboxes, despite intensive sampling efforts that would not have left this

species undetected if it had, in fact, been present. Prothonotary Warblers and Carolina Wrens

breed in sympatry and occupy relatively similar ecological niches, as both are insectivorous and

nest in preformed cavities, which can be a critically limiting resource for reproduction (Haggerty

and Morton 2014). They are also relatively similar in size and, thus, experience overlap in the

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size of preferred nesting cavities. We therefore suspect that they might experience interspecific

competition over limiting nesting sites (sensu Slagsvold 1978; Newton 1994; Petit 1999).

In this study, we assess demographic parameters (e.g., reproductive success, return rate) of a

study population of Prothonotary Warblers breeding in sympatry with Carolina Wrens, both

using wooden nestboxes, to assess whether the provision of artificial nestboxes might bolster the

abundance of declining Prothonotary Warblers (even in the presence of a potential competitor).

We first describe evidence of the absence of Prothonotary Warblers from our study site prior to

the availability of nestboxes. Then, we assessed the distance over which both Prothonotary

Warblers and Carolina Wrens nested away from standing water. The Carolina Wren nests almost

universally over dry land and is more of a habitat generalist (Haggerty and Morton 2014) than

the Prothonotary Warbler, making it useful as a natural control. Thus, if water is a more

important determinant of the presence or absence of nesting Prothonotary Warblers than Carolina

Wrens, a representative woodland passerine, we predicted that Prothonotary Warblers would nest

closer to water bodies, on average, than the Carolina Wrens nesting at or study site. We then

assessed the breeding success of warblers using our nestboxes. Because breeding success often

has a positive effect on the return of adults to breeding sites in subsequent years (Harvey et al.

1979; Greenwood and Harvey 1982; Hoover 2003), we expected that a high rate of nest success

in 2017 would be accompanied by increases in abundance and breeding success during the

second year of nestbox availability. We also tested whether breeding success was associated with

the subsequent return of these individuals during the following breeding season, predicting a

positive effect of breeding success on return rates.

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Materials and Methods

Study site

This study was conducted within predominately deciduous hardwood forest of the 252 ha

Edward J. Meeman Biological Station (35.363° N, 90.017° W) located on the Bluff Hills

(Omernik level IV ecoregion) of the Mississippi Valley Loess Plains (U.S. Environmental

Protection Agency 2013). Although a relatively small study site, the local flora and fauna is

similar to that found in typical Protonotary Warbler habitat, so patterns described here are likely

representative of those that would be observed over larger geographic areas. Approximately 5

km east of the Mississippi River in southwestern Tennessee, this mixed mesophytic forest is

characterized by American Beech (Fagus grandifolia, Ehrh.), Sweetgum (Liquidambar

styraciflua, L.), Tulip Poplar (Liriodendron tulipifera, L.), and White (Lepidobalanus) and Red

(Erythrobalanus) Oak subgenera (Quercus spp., L.) with a distinctive understory of Red Buckeye

(Aesculus pavia, L.), as well as Spicebush (Lindera benzoin, L.), Pawpaw (Asimina triloba,

Adanson), Blue Beech (Carpinus caroliniana, L.), and Dogwood (Cornus florida, L.).

Although average yearly precipitation is ca. 135 cm, elevations that are >50 m above the

nearby (ca. 3 km) Mississippi Alluvial Plain and the marked topographic relief of the study area

restrict the area and duration of flooding. Even so, several small ponds and ephemeral creeks are

distributed throughout the site (Fig. 1). Creek beds were dry for a majority of the season (late

April through July), although water was present during and briefly (1-2 d) following significant

rainfall events.

Field procedures

Fieldwork began in May 2016 as the initial year of an ongoing Monitoring Avian Productivity

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and Survivorship project (MAPS Station 16764, MEEM, UOFM; Institute for Bird Populations;

https://www.birdpop.org/pages/maps.php). As a bird-banding and data-gathering activity in

support of bird conservation through demographic modeling, organized by The Institute for Bird

Populations, this station was located in the south-central portion of the study area (Fig. 1).

Comprised of a network of 12 fine-mesh “mist” nets (12 x 3 m), these nets were operated for ~6

h during each of 8 days per year, once every ~10 days from mid-May through early August. We

banded captured birds with leg bands and recorded their sex and age. For many songbirds,

changes in feather condition due to molt afford the ability to determine age of adult “after-

hatching-year” birds as either: “second-year” (birds hatched during last year’s breeding season),

or “after-second-year” (birds older than 2 years). In addition, for all species encountered on the

study site, we recorded the presence and observed breeding activity as individuals were

encountered during each day of operation.

Prior to the 2017 breeding season, we distributed 220 nestboxes over ca. 100 ha of secondary

deciduous forest (Fig. 1), including the same area as the MAPS banding station. Nestboxes,

made of 1.9 cm thick pine boards, had interior dimensions of 10.1 cm wide, 14 cm deep, with

rear height of 13.3 cm tapering to 12.1 cm at the front (see Fig. 1 inset). These are shorter than

many commonly used wooden nestboxes, and, thus, have smaller internal volumes that

Prothonotary Warblers may favor (Petit et al. 1987). Nestboxes were placed ca. 1.5 m above

ground on metal poles with a 51-cm aluminum predator baffle (i.e., pizza dish) immediately

beneath each nestbox (Fig. 1 inset) and located on a roughly 80-90 m interval grid within a

forested area on the Loess Bluffs atop the Mississippi Alluvial Plain.

Beginning in 2017, we checked nestboxes at least twice weekly for the formation of new

nests from March through August, detecting every nest produced in the nestboxes. We captured

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https://www.birdpop.org/pages/maps.php

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females of both species at the nest during the second half of incubation by either capturing them

inside nestboxes or using mist nets placed outside the box. We also captured all male

Prothonotary Warblers at this time, whereas in many instances male Carolina Wrens could not be

captured until after eggs hatched. Upon capture, we weighed (0.1 g) all adults using a digital

scale, measured their tarsus length (0.1 mm) with dial calipers, and the length (0.5 mm) of their

flattened wing cord and tail feathers using a stopped rule to determine whether morphological

size predicted return rates to the study site in future years. All adults were banded with a unique

U.S. Geological Survey (USGS) aluminum leg band. Both female and male wrens, and male

warblers, were also banded with 3 additional colored leg bands arranged in unique combinations

so they could be visually identified and observed subsequently without capture (we did not

colorband female Prothonotary Warblers because their sexual dichromatism allowed us to

delineate the male from a female at any given nest, which is not possible in the monochromatic

Carolina Wren). Once eggs hatched, we subsequently monitored nests until fledging.

During the 2018 breeding season, we captured and identified females and their social mates

at all nests, with the exception of nests that failed prior to our being able to capture the tending

adults (i.e., nests that failed during the first half of the incubation period or before). Thus, we

were able to assess the probability that any Prothonotary Warbler breeding in 2017 would return

to breed in the local population in 2018. All nests in 2017 were assigned a treatment

corresponding to an experimental manipulation of nest-cavity temperature as part of another

study (Mueller et al. 2019), but there were no overall effects of this temperature treatment on

return rates. All adults and nestlings in 2018 were handled in a similar manner as in 2017.

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Data and analyses

We first assessed variation among nests in their distance from standing water, obtaining

approximate distances from each nestbox occupied by either Prothonotary Warblers or Carolina

Wrens to the nearest known water source using Google Earth (https://earth.google.com/web/).

We used this information to compare the approximate distance either species nested away from

one of these water sources (Fig. 1) using a one-way ANOVA to test for a difference between

species in the approximate distance from water they built their nests (water is not thought of as a

determinant of Carolina Wren breeding territory selection). We then investigated changes in

breeding success between years using both a generalized linear mixed model (GLMM) and

mixed-model ANOVA (including individual identity as a random effect). The GLMM included a

binary response distribution that assessed whether a given bird would be multi-brooded (i.e., rear

multiple broods of young within a single season or just a single brood) in 2017 vs. 2018, and we

used a mixed-model ANOVA to analyze differences between years in total reproductive success

(i.e., number of young fledged). We then used a similar GLMM to analyze whether individuals

breeding in 2017 returned to breed in the study population in 2018 using a binary response (1 =

returned, 0 = did not return) including morphological variables and reproductive success (i.e.,

total number of young fledged) in 2017 as predictors.

Ethical note

This research was conducted in accordance with the ABS/ASAB Guidelines for the Treatment of

Animals in Behavioural Research and Teaching, and with the Guide for the Care and Use of

Laboratory Animals. All activities complied with current laws of the United States, the

University of Memphis Institutional Animal Care and Use Committee, U.S. Geological Survey

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bird banding permits #24052 and #22215, and the Tennessee Wildlife Resources Agency permit

#3950.

Results

In 2016, the year prior to the availability of nestboxes, none of the 222 birds captured during

operation of the MAPS station were Prothonotary Warblers. Moreover, this species was not

among the 37 species whose presence was recorded during any days of operation. However, in

2017, the initial year in which nesting boxes were available, 12 of 235 captured birds were

Prothonotary Warblers and their presence, typically singing males, was recorded during all but

one day of operation. During the next year, 2018, the number of captured Prothonotary Warblers

fell to 3 of 223, but we again recorded the presence of this species during all but one day of

operation – the species being undetected during the late July day of operation in both years.

In 2017, the first year in which nestboxes were available, over 60% of all nests in our

nestboxes (33 of 52) were produced by Prothonotary Warblers. None of these nestboxes were

positioned over standing water, and many were placed greater than 100 m away from any body

of water (Fig. 2). In fact, there was no difference between Prothonotary Warblers (average

approximate distance from water = 107 ± 12 m, mean ± SE; min = 2 m, max = 285 m) and

Carolina Wrens (mean ± SE distance to water = 121 ± 15 m; min = 2 m, max = 285 m) in the

proximity of their nests to water (F1, 50 = 0.55, P = 0.4613).

Twenty-seven of the 33 Prothonotary Warbler nests (ca. 82%) successfully fledged young.

Of the six nests that failed prior to fledging, five failed as a direct result of abandonment of eggs

or nestlings, and one failure was attributable to an infertile clutch in which the female continued

to incubate eggs for over three weeks prior to abandoning the nest. In other words, no nests

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failed because of depredation.

We observed a 64% increase in the absolute number of Prothonotary Warbler clutches during

the 2018 breeding season, with 54 clutches initiated. The number of nests produced by Carolina

Wrens also increased substantially (N = 19 clutches in 2017, 117 clutches in 2018). Of the 87

Prothonotary Warbler nests produced over the two years, 71 nests successfully fledged young,

and the percentage of nests initiated that successfully fledged young was no different between

years (2017: 81.8%, 2018: 81.5%; = 0.35, P = 0.5524). Out of 87 nests over the two-year 𝜒21

span, only two of them failed because of depredation.

Reproductive performance also increased significantly from 2017 to 2018. For example, the

proportion of adults that produced multiple broods of young within a season increased over this

time ( = 5.95, P = 0.0148; Fig. 3A), and this was associated with an increase in the total 𝜒21

number of young fledged by any given individual, on average, from 2017 to 2018 (F1, 100 =

16.62, P < 0.0001; Fig. 3B).

We then investigated predictors of whether individuals breeding in 2017 returned to breed in

the study population in 2018. Greater than 50% of all adults breeding in 2017 returned to breed

in our nestboxes in 2018 (23 of 45 marked adults returned), typically in close proximity to their

previous years’ nestbox (6 returners used the same nestbox in successive years). Our analysis of

return rates revealed a set of sex-specific effects of breeding success and morphological size in

influencing whether a given bird returned to breed in 2018 (Table 1). Specifically, breeding

success positively predicted return rate, as expected, but this effect was only present within

females, whereas for males there was, if anything, a negative relationship between reproductive

success and return rate (Table 1; Fig. 4A). Wing length and body mass were also associated with

sex-specific return rates. For male warblers, there was no effect of wing length (Fig. 4B) but a

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positive effect of body mass (Fig. 4C) on return rate, whereas females with longer wings were

actually less likely to breed in the subsequent year (Fig. 4B). In following up the interactive

effect of sex and mass, visual inspection of the data revealed a clearly non-linear relationship

between body mass on female return rates, but a linear effect for males (Fig. 4C). This is

evidenced by a statistical interaction between sex and the quadratic term for body mass (i.e., sex

× mass2: = 3.86, P = 0.0495), indicating that the effect of mass depended on the sex of the 𝜒21

individual (Fig. 4C).

Discussion

Despite intensive sampling efforts, Prothonotary Warblers were not observed at our study site

during the year prior to the availability of nestboxes, likely due to a paucity of permanent water

or seasonal flooding. Indeed, mist-netting and observations of the avian community at this site

began in the year preceding the availability of nestboxes, and no Prothonotary Warblers were

captured nor were any heard singing during that breeding season. Thus, we speculate that the

nesting of this species in this previously unoccupied site was attributable to the increased

availability of nesting cavities provided by our nestboxes. With an increase in available nesting

sites, territory quality may be less important in determining its suitability for breeding. Indeed, in

a multi-year study using 560 nestboxes, Petit and Petit (1996) found that a large majority of

Prothonotary Warbler nests (83 of 86 nests) were produced in artificial nestboxes rather than

natural cavities.

Notably, Twedt et al. (2001) increased both species richness and density of cavity-nesting

species in young (<10 year-old), Eastern Cottonwood (Populus deltoides, Bartram ex Marshall)

agroforests, that lacked natural cavities, through provision of artificial cavities: Prothonotary

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Warbler nests accounted for 29 of 77 nests constructed over 2 years in 173 available nestboxes.

In addition, the small cavity size (1800 cc) of the artificial nesting boxes used in this study may

have been particularly attractive to Prothonotary Warblers. When offered a choice of wooden

nestboxes with 3500 cc cavities or smaller cavity (1800 cc) paper nest boxes, Prothonotary

Warblers produced 28 of 29 nests within the smaller-cavity boxes (Twedt et al. 2001).

One potential explanation for the changes in Prothonotary Warbler abundance at our site

could be that our nestboxes provided the only nest cavities within a forest largely depauperate of

natural cavities, which we think unlikely. Although we did not quantify the availability of natural

nest cavities at the study site, there are many dead and hollow tree trunks within this mature

forest, and we have observed other cavity-nesting bird species [e.g., Carolina Chickadee (Poecile

carolinensis, Audubon 1834), White-breasted Nuthatch (Sitta carolinensis, Latham 1790),

Tufted Titmouse (Baeolophus bicolor, L. 1766)] using natural cavities. Moreover, woodpecker

species, including the Pileated Woodpecker (Dryocopus pileatus, L. 1758), Red-headed

Woodpecker (Melanerpes erythrocephalus, L. 1758), Downy Woodpecker (Picoides pubescens,

L. 1766), and Red-bellied Woodpecker (Melanerpes carolinus, L. 1758), are common within the

study area. These species contribute significantly to the creation of natural nest cavities in North

American forests (Cockle et al. 2011). Thus, it is unlikely that this forest harbored an unusually

low abundance of natural cavities for nesting.

We note, however, that we think it unlikely that Prothonotary Warblers colonized this area

solely in response to the installation of nestboxes. Contiguous forest extends westward from our

study site with >1500 ha of upland Chickasaw Bluff forest (same potential habitat as our field

site) and >3000 ha of bottomland Mississippi Alluvial Plain forest at a distance of 3 – 5 km.

These bottomland forests have only slight topographic relief, but contain a variety of forest types

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that range from cypress-dominated sloughs and bayous, containing an abundance of Bald

Cypress (Taxodium distichum, L.), through less flood-prone riverfront forest, which may contain

Eastern Cottonwood, Sweetgum, American sycamore (Platanus occidentalis, L.), and

Cherrybark Oak (Quercus pagoda, L.)(Hodges 1997). Typically associated with Prothonotary

Warblers, these bottomland forests are a likely origination source for birds breeding at our study

site.

It is possible that the birds nesting in our boxes in 2017 were poor-quality or first-year

breeders and were unable to obtain territories in their more strongly preferred floodplain sites. In

a previous study, Petit and Petit (1996) found a non-significant tendency for older adults to

obtain preferred nesting sites over water, although a large proportion of nests in this preferred

territory type are also produced by first-year breeders (Petit and Petit 1996). Because of the

proximity of our study site to suitable bottomland forest (ca. 3-10 km) and the availability of

safe, dry nesting locations (our nestboxes), the area within our field site may have suited these

individuals as a close second choice to their preferred territories on the floodplain. If this is the

case, then the success of these birds over the two field seasons may be due to the relatively low

abundance of conspecifics competing for resources in the immediate vicinity (Petit and Petit

1996). Notably, all female Prothonotary Warblers captured during operation of the MAPS station

during 2017 that were successfully aged beyond “after-hatching-year,” were aged as “second-

year” birds (3 of 8). However, all females captured during 2018 were recaptures of birds banded

the previous year and were “after-second-year” birds. Moreover, the high nesting success, as

well as high return rate (>50%) and site fidelity of adults we observed for Prothonotary Warblers

is inconsistent with the hypothesis of poor-quality or first-year breeders and indicates that

experienced, “after-second-year” birds will also nest in this, now-established, drier upland site.

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It is also worth noting that nearly none of the nests produced in these nestboxes experienced

depredation. Rates of nest depredation for all nests within natural populations can range from ca.

40-60% (Martin and Li 1992; Martin 1993). Cavity-nesting birds generally experience lower

rates of nest depredation than cup-nesting birds (Martin and Li 1992; Martin 1995); however,

even with nestboxes and predator baffles, nest destruction can still reach up to ca. 50% of nests

in some cavity-nesting study populations (Bowers et al. 2016). It is worth noting that the

population studied by Bowers et al. (2016) was the House Wren (Troglodytes aedon, Vieillot

1809), a species in which infanticide and conspecific nest destruction is widespread (Johnson

2014). Indeed, in northern portions of the Prothonotary Warbler range that overlap with that of

the House Wren, the latter species represents a major source of mortality and nest failure (Petit

1999). It is also possible that the reduced depredation we observed could be explained by the

novelty of our nestboxes within the area. In some subpopulations, nests in natural cavities can be

depredated to a greater extent than those in nestboxes even without predator baffles (Slevin

2018). However, the low rates of depredation continuing through the second breeding season

(rate of nest failure did not change from 2017 to 2018), despite a significant increase in nestbox

usage by Prothonotary Warblers and Carolina Wrens, does suggest a level of efficacy for our

predator baffles. Thus, we suspect that a combination of (i) the deterrence of ground-dwelling

predators (e.g., snakes, raccoons, mice) by our predator baffles and (ii) a lack of sympatric House

Wrens may account for the near-complete lack of depredation we observed, although further

research on this issue is needed.

In conclusion, our observations suggest that areas that are unused by Prothonotary Warblers,

due to the lack of standing water, may be used if appropriate nesting cavities are provided and

the areas are located near typical habitat. We also suggest that predator baffles protecting the

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small-cavity nest boxes (~1800 cc) may increase their usage and reduce depredation on nests

within artificial cavities. Future work will assess relationships between nesting success, return

rates, and their effects on occupancy in this newly established study population.

Acknowledgements

We thank the staff of the Edward J. Meeman Biological Station for use of the field site and

logistical support. We also thank Than Boves, Blaine Elliott, Daniel McAuley, and two

anonymous reviewers for helpful comments that improved the manuscript. Funding was

provided by the Department of Biological Sciences at the University of Memphis and a student-

research grant from the Tennessee Ornithological Society.

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Fig. 1. Schematic representation of nestbox arrangement. Open circles represent unoccupied

nestboxes, filled black stars represent Prothonotary Warbler (Protonotaria citrea) nests, and

open diamonds represent Carolina Wren (Thryothorus ludovicianus) nests produced in 2017.

Nestboxes (inset) had a predator baffle beneath the nestbox, a slot entrance, and a removable

lid held in place by nails at each side of the box. Creekbeds with ephemeral streams (i.e.,

with periodically flowing water) are indicated by solid, black lines, permanent ponds appear

with black fill, and the dashed outline represents the field-station boundary. Topographic

layer is from the National Map (United States Geological Survey).

Fig. 2. Among-nest variation in distance to water (including ephemeral streams that may or may

not have contained water).

Fig. 3. Differences between years in (A) the probability that Prothonotary Warbler (Protonotaria

citrea) adults would produce at least two broods within a breeding season, and (B) the

average number of fledglings produced by these individuals over the course of the breeding

season (summed across nesting cycles when individuals produced multiple broods). Plotted

are least-squares means ± SE.

Fig. 4. Sex-differences in the probability of adults breeding in 2017 returning to breed in the

study population in 2018 in relation to their (A) total reproductive success in 2017 (i.e., total

number of fledglings produced), (B) wing length, and (C) body mass. Curves represent

predicted values obtained from a GLMM (see Methods).

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N

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0

1

2

3

4

5

Num

ber

of nests

Approximate distance to water (m)

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Draft0.0

0.2

0.4

0.6

0.8

1.0

2017 20180

1

2

3

4

5

6

7

2017 2018

Pro

babili

ty o

f

multip

le b

roods

Tota

l fledglin

gs

A B

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Draft0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10

FemalesMales

0.0

0.2

0.4

0.6

0.8

1.0

11 12 13 14 15 16 17

FemalesMales

Pro

babili

ty o

f re

turn

ing

Total fledglings

Pro

babili

ty o

f re

turn

ing

Body mass (g)

0.0

0.2

0.4

0.6

0.8

1.0

64 66 68 70 72 74 76

FemalesMales

Pro

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f re

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Wing length (mm)

A B C

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Table 1. Effects on interannual return rate of

adult Prothonotary Warblers.

χ2 P

Body mass 0.56 0.4536

Tarsus length 0.56 0.4554

Wing length 4.19 0.0406

Total fledglings produced 0.08 0.7764

Sex 1.66 0.1972

Mass × Sex 3.04 0.0814

Tarsus length × Sex 2.27 0.1322

Wing length × Sex 4.26 0.0391

Total fledglings × Sex 4.58 0.0324

Note: degrees of freedom = 1 for all predictors

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