rsos.royalsocietypublishing.org
Research
Cite this article: Fournier AMV, Mengel DC,
Krementz DG. 2018 Sora (Porzana carolina)
autumn migration habitat use. R. Soc. open sci.
5: 171664.
http://dx.doi.org/10.1098/rsos.171664
Sora (Porzana carolina)
autumn migration habitat
use
Auriel M. V. Fournier1,† , Doreen C. Mengel3 and
David G. Krementz2
1 Arkansas Cooperative Fish and Wildlife Research Unit, Department of Biological
Sciences, and 2 US Geological Survey, Arkansas Cooperative Fish and Wildlife Research
Unit, Department of Biological Sciences, University of Arkansas, Fayetteville, AR, USA
3 Resource Science Division, Missouri Department of Conservation, Columbia, MO, USA
AMVF, 0000-0002-8530-9968
Received: 18 October 2017
Accepted: 17 April 2018
Subject Category:
Biology (whole organism)
Subject Areas:
ecology
Keywords:
resource use, sora, Porzana carolina,
autumn migration, wetland
Author for correspondence:
Auriel M. V. Fournier
e-mail: aurielfournier@gmail.com
†
Present address: Mississippi State University,
1815 Popps Ferry Road, Biloxi, MS 39532, USA.
Electronic supplementary material is available
online at https://dx.doi.org/10.6084/m9.
figshare.c.4088609.
Palustrine wetland management across the USA is often
conducted under a moist soil management framework aimed
at providing energetic resources for non-breeding waterfowl.
Moist soil management techniques typically include seasonal
water-level manipulations and mechanical soil disturbance
to create conditions conducive to germination and growth
of early successional, seed-producing wetland plants. The
assumption is that providing stopover and wintering habitat
for non-breeding waterfowl will also accommodate life-history
needs of a broader suite of migratory waterbirds including
shorebirds, wading birds and marsh birds. Although studies
of wetlands provide some evidence to support this assumption
for shorebirds and wading birds, there is less information on
how other marshbirds respond. Sora (Porzana carolina) are a
species of migratory rail that depend on wetlands year round
as they migrate across North America. It is a species for which
the consequences of wetland management decisions directed
towards non-breeding waterfowl are unknown. We conducted
nocturnal surveys on 10 public properties in Missouri, USA
during autumn migration during 2012–2016 to examine sora
habitat use in wetland impoundments managed to enhance
the production of moist soil vegetation. We found a positive
relationship with sora presence and mean water depth and
annual moist soil vegetation; sora used, on average, deeper
water than was available across surveyed impoundments and
used locations with a higher percentage of annual moist soil
vegetation than was available. We found a negative relationship
with sora use and upland vegetation, woody vegetation and
open water. We found sora using deeper water than have
previously been reported for autumn migration, and that moist
soil management techniques used on Missouri’s intensively
managed public wetland areas may be compatible with sora
autumn migration stopover habitat requirements.
2018 The Authors. Published by the Royal Society under the terms of the Creative Commons
Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted
use, provided the original author and source are credited.
1. Introduction
................................................
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Wetlands are among the nation’s most imperiled ecosystems with the conterminous USA experiencing an
estimated 53% wetland loss since pre-European settlement [1]. Freshwater emergent wetlands (wetlands
dominated by erect, rooted, herbaceous hydrophytes, [2]) have suffered the highest per cent loss
among all freshwater vegetated wetland types (i.e. forest, shrub and emergent) with approximately
17% of emergent wetlands remaining in 1954 lost in the 55-year period 1954–2009 [1]. The central US
landscape is highly altered owing to agricultural conversion and associated infrastructure (i.e. levees
and ditches), navigation considerations associated with major riverways and other human developments
such as roads and urban expansion. The degree of altered processes (i.e. disrupted flood pulses, stream
or river channels constrained by levees, ditching to decrease flood duration that once created and
maintained riverine wetlands) requires that public rehabilitated wetlands be intensively managed to
provide a variety of habitats and resources for wetland-dependent wildlife as well as to accommodate
wetland recreational opportunities for the agency’s stakeholders. A common management approach
taken by wetland managers is to maximize production of early successional wetland plants; this
approach is known as moist soil management [3]. Moist soil management is typically associated with
providing energetic resources for autumn migrating waterfowl by creating conditions favourable to
germination and growth of seed-producing plants adapted to moist or saturated growing conditions. The
energetic resources (i.e. seeds) are made available by incrementally inundating wetland impoundments
throughout late summer, autumn and early winter to provide habitat for non-breeding waterfowl,
beginning with early migrating dabbling ducks (blue-winged teal, Anas discors) and continuing through
to late-arriving dabbling ducks (i.e. mallards, Anas platyrhynchos).
A key assumption of moist soil wetland management is that providing habitat for the full suite of
waterfowl species (Anatidae—ducks, geese and swans) will also accommodate life-history needs of a
broader suite of waterbirds including shorebirds, wading birds and marshbirds. Although studies of
moist soil wetlands provide some evidence to support this assumption regarding shorebirds and wading
birds [4], there is less information on how marshbirds respond to the vegetative and habitat conditions
resulting when wetlands are managed with a focus on non-breeding waterfowl. Marshbirds encompass
a group of wetland-dependent birds including rails, bitterns and moorhens [4] that are generally
inconspicuous and tend to inhabit wetlands with robust, perennial vegetation, making them difficult
to detect [5]. Overall, limited distributions combined with low detection probability have resulted in
marshbirds being among the least studied avian groups [6] and, for most species of marshbirds, there
is limited information on population levels, breeding ecology, migration patterns and overall habitat
requirements [7].
Our work focuses on one marshbird, the sora (Porzana carolina), a species of migratory rail that is a
common autumn migrant and is often detected in wetland impoundments managed under a moist soil
management scenario [8]. Sora are habitat generalists that forage primarily on seeds throughout autumn
migration, which may explain why sora are found in high densities in thick moist soil vegetation in the
autumn [9]. However, wetland management decisions, specifically timing of water-level manipulations,
directed towards non-breeding waterfowl have unknown consequences on the suitability of stopover
habitat for this species.
Much of the previous sora autumn migration research has been conducted in Missouri [8], although
this previous work was completed on single sites and was based on opportunistic observations.
Findings from these studies tend to indicate that sora use habitat based more on water conditions
and plant structure rather than specific plant species [9–12]. Sora were detected during autumn
migration in shallowly inundated (2–45 cm) habitats dominated by annual, seed-producing wetland
vegetation, including Panicum, Echinochloa and Bidens spp. [10–12]. Sora have been found in high
densities around wetlands that were dewatered (less than 15 cm) in accordance with local irrigation
practices during late summer in Colorado, thereby reducing available habitat and concentrating
food resources [13]. High densities of sora were also recorded on wetlands inundated (5–15 cm)
during the early part of autumn migration (early September) in Missouri [11]. Sora migration in
Missouri begins in early August and continues through to late October, with the peak of migration
occurring in late September [14]. Our objective was to examine sora habitat use on public wetlands
in Missouri during the entire span of their autumn migration under a standardized protocol to
help inform wetland managers of the habitat use of sora during autumn migration, and to form
a foundation for future work examining wetland management for the full suite of wetland bird
species.
2
2. Material and methods
2.2. Surveys
We conducted surveys by driving systematic nocturnal transects spaced 30 m apart on ATVs for 3 h
each night, starting a half hour after sunset [16]. We spaced these transects based on work by Fournier
& Krementz [16] who equipped sora with very high frequency transmitters to examine the behaviour
of sora in response to the survey method [16]. They determined that transects spaced 30 m apart were
sufficient to avoid double counting [16]. Transects were completed in a serpentine fashion across an entire
impoundment, so the length of transect varied with impoundment size [16]. We used a spotlight to scan
for flushing, walking or swimming sora, while the observer was slowly moving along the transect on
the ATV and took a global positioning system (GPS) location at the point where an individual was first
detected. We surveyed in each of 5 years from August through to October and visited each National
Wildlife Refuge or Conservation Area four times a year (roughly every two weeks; figure 1 and table 1;
electronic supplementary material, table S1).
2.3. Vegetation data
We collected habitat data during daylight hours following a night of surveys under an available versus
used framework. This approach is commonly carried out in the habitat use literature [17–20] and involves
collecting the same kind of habitat data at available points and used points. Available points are defined
as random points (randomly distributed within the boundary of the impoundment using the spsample()
function in R, [21,22]) within the wetland impoundments surveyed which, together with the GPS points
where sora were observed on surveys, allowed us to examine sora habitat use. Sora-used points (i.e. used
points) are points where sora were detected, and collectively, these used points represent the habitat
used by sora at the time of the survey. We sampled 20 random available points within each wetland
impoundment each time the impoundment was surveyed for sora. We sampled vegetation associated
with up to 20 sora points within each wetland impoundment each time the impoundment was surveyed;
however, if habitat data were collected from less than 20 used points, it was because less than 20 sora
were detected the previous night.
We placed 25 m diameter plots around each available and sora-used point and, because we were
interested in habitat use by individual sora, we considered the plot (available and used) to be the
................................................
Missouri is one of six mid-latitude states in the USA (Iowa, Illinois, Indiana, Kentucky, Missouri and
Ohio) that suffered greater than 80% loss of their historical wetland acres [1]. To compensate for this
degree of wetland loss, the Missouri Department of Conservation (MDC) and the US Fish and Wildlife
Service (USFWS) invested in acquiring and restoring wetland areas distributed geographically across
Missouri. These Conservation Areas (public land parcels owned by MDC) and National Wildlife Refuges
(public land parcels owned by USFWS) are located within riverine floodplains; seven are associated with
mid-size rivers (stream order 6–8) and eight are associated with either the Missouri or Mississippi Rivers.
These state and federal properties have served as core areas for the development of regional wetland
complexes as wetland development on private land has occurred in close juxtaposition to the public
properties.
We selected public (i.e. MDC Conservation Areas and USFWS National Wildlife Refuges, figure 1),
intensively managed wetland areas from across Missouri, from all wetlands within the state, because of
their active moist soil management and historic importance for migrating waterfowl [15]. Areas were
chosen (10 total) from wetland complexes in four regions (figure 1; electronic supplementary material,
table S1). Regions are areas of two to four National Wildlife Refuges or Conservation Areas that are close
to each other and share similar hydrology, surrounding landscape characteristics and weather. Wetland
managers assigned to areas selected for surveys were consulted regarding the wetland impoundments
under a moist soil management regime, and survey sites were selected from these impoundments. We
surveyed moist soil wetland impoundments (a wetland surrounded by a levee, with manual water-level
manipulation; 4.5–300 ha in size; mean = 26.5 ha; table 1) using selection criteria based on whether or not
we were able to access the impoundment with an all-terrain vehicle (ATV) because some have interior
ditches that cannot be crossed, and whether or not we could complete a survey within an hour and a half
survey window.
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2.1. Study site
3
4
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Clarence Cannon NWR
Ted Shanks CA
BK Leach CA
Loess Bluff NWR
Nodaway Valley CA
Swan Lake NWR
Fountain Grove CA
50
Duck Creek CA
Otter Slough CA
Ten Mile Pond CA
40
30
–120
–100
–80
Figure 1. Study sites in Missouri, USA surveyed for sora during autumn migration 2012–2016. CA: Conservation Area, state land managed
by the Missouri Department of Conservation; NWR: National Wildlife Refuge, federal land managed by the US Fish and Wildlife Service.
Table 1. Survey start and end dates for each year of autumn surveys of sora (Porzana carolina) in Missouri, USA.
year
start date
end date
visits per
state
property
2012
17 August
7 October
3
visits per
federal
property
number of
impoundments
number of
sora
detected
3
40
1895
number of
available
vegetation
points
900
number of
sora-used
vegetation
points
909
.........................................................................................................................................................................................................................
2013
11 August
27 October
3
4
39
1876
1890
624
2014
12 August
22 October
4
4
33
1268
2268
589
2015
12 August
23 October
4
4
33
1063
1710
522
2016
10 August
20 October
4
4
33
1803
2124
664
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
experimental unit (figure 2). We measured water depth at (i) the centre of the plot and (ii) 5 m from
the centre in the four cardinal directions (figure 2); we used the mean of these five measurements to
characterize water depth. We visually estimated and created a sketch in the field to record the per
cent cover of plant groups in each plot and placed them in the following categories: (i) annual moist
soil plants, (ii) perennial moist soil plants, (iii) upland vegetation, (iv) woody vegetation, and (v) open
areas [23,24].
Annual moist soil plants are those that fall below the water surface at the end of the growing season,
including Polygonum spp. and Echinochloa spp. [2]. Perennial moist soil plants are those that stay above
the water surface at the end of the growing season, including Typha spp. and Sparganium spp. Woody
vegetation was predominantly Salix spp. and Cephalanthus occidentalis. Upland vegetation was composed
of a wide suite of terrestrial annual plants, including upland grasses, Solidago spp., Asclepias spp. and
Helianthus spp. Open areas are those without any vegetation and could be either bare earth or open
water surface.
2.4. Analysis
We examined sora habitat use by comparing sora-used versus available habitat plots in a binomial mixed
model framework. Our response variable was a binary of whether a point was used (1) or available (0).
25
5
metres
10
5
0
0
5
10
15
metres
20
25
Figure 2. Each habitat plot was 25 m in diameter; the space within this 25 m diameter circle is the area over which the coverage of our
vegetation variables was estimated. The five points are the five places where water depth was measured to the nearest centimetre, the
centre of the plot, and 5 m away from the centre in each cardinal direction.
Our observations of sora among visits were not independent because of the sora’s long stopover duration
(40+ days, [25]). Owing to the lack of independence, we nested visit within year in a repeated-measures
framework as a part of a binomial mixed model in the R package ‘lme4’ ([18,19], v. 1.1–12, R v. 3.3.2). We
included mean water depth squared, and per cent cover of annual moist soil plants, perennial moist soil
plants, woody vegetation, upland vegetation and open area, as fixed effects with random effects (random
intercepts) of year. We included mean water depth squared because we expected habitat use to increase
at some medium depth and decrease thereafter. We include a random effect of year because of the
variation in weather and precipitation conditions among years. We considered a fixed-effect coefficient
to be significant if the estimated coefficient and its 95% confidence interval did not overlap with 1. All
data and code to reproduce these analyses are available in DataDryad [26].
3. Results
We detected 7905 sora during August to October 2012–2016 (for the number of sora detected per year,
table 1). We found significant positive use (sora using annual moist soil vegetation at higher percentages
than is available; figure 3 and table 2) for annual moist soil vegetation and average water depth squared.
On average, sora used water depths approximately 12 cm (yearly averages between 1.5 and 19.1 cm;
electronic supplementary material, table S2); on average, sora used annual moist soil vegetation per cent
cover of 37% (yearly averages ranging from 21 to 70%; electronic supplementary material, table S2),
showing that they are positively using habitat with shallow water depths and annual plant cover.
We found significant negative use (sora using lower percentages than available) with upland
vegetation, woody vegetation and open areas (figure 3 and table 2; electronic supplementary material,
table S2). We found no significant relationship with the rate at which sora used locations characterized
as perennial moist soil vegetation (6.2% average sora used; electronic supplementary material, table S2).
4. Discussion
Sora are often considered to be a habitat generalist among the rails [8], and our findings support this
portrayal. Although we found sora in a wide variety of water depths (dry to over 50 cm) that speaks to
their adaptability, including their ability to swim and dive [27], most sora used shallow water depths
when available. The median water depth used by sora was similar to the depth typically associated with
the breeding season [28], deeper than previous autumn migration work [3,10–12] and overlaps with that
used by migratory teal during autumn migration [3,29]. Other rails (Virginia rail, Rallus limicola and
yellow rail, Coturnicops noveboracensis) use drier habitats during autumn migration [30].
Our work supports previous characterizations of sora autumn migration habitat, which showed
selection for annual seed-producing plants, with seeds being their primary autumn food source, in
combination with tall dense cover [10,31]. Sora diet diversifies in the spring and includes a higher
proportion of aquatic invertebrates [8]; this shift is probably in preparation for the breeding season when
proteins, lipids and minerals are needed for egg production [32,33] and other life-history events (i.e.
................................................
15
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20
6
1.44 ***
0.99
perennial moist soil vegetation
woody vegetation
open water
................................................
1.07 ***
water2
upland vegetation
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annual moist soil vegetation
0.80 ***
0.71 ***
0.70 ***
0.75
1.00
1.25
fixed effect coefficient
1.50
Figure 3. Fixed effects coefficients from binomial mixed models comparing sora (Porzana carolina) used versus available habitat from
wetland impoundments surveyed during 2012–2016 in Missouri, USA. Line represents the 95% confidence interval around the point. ***
indicates a significant difference because the 95% confidence interval does not overlap 1. Items to the left of the dotted line showed
negative selection (selected less often than available); items to the right showed positive selection (selected more often than available).
Table 2. Model results of binomial mixed model comparing sora (Porzana carolina) used habitat points with available points from moist
soil wetlands in Missouri, USA from 2012 to 2016.
p-value
covariate
estimate
standard error
intercept
−1.26
0.28
<0.001
open
−0.40
0.04
<0.001
upland
−0.30
0.04
<0.001
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
perennial
0.002
0.03
0.94
average water depth squared
0.05
0.01
<0.001
annual moist soil vegetation
0.28
0.05
<0.001
woody vegetation
−0.38
0.04
<0.001
random effects
visit nested within year
variance
1.01
standard deviation
1.006
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
year
0.01
0.113
.........................................................................................................................................................................................................................
molting). Similarly, a shift in sora habitat use occurs in spring as plant structure and cover for nesting
and raising young become more important, whereas during autumn migration plants that provide the
greatest energetic resources (i.e. seeds) are needed.
Many of the wetland impoundments we surveyed were largely dry during our first visits in early
August. While sora are selecting deeper water when it is present, it is not widely available early in the
autumn migration season. The limited area of inundated wetlands could impact the quality of autumn
5n7t4) [26].
Authors’ contributions. A.M.V.F. conducted the fieldwork, did the data analysis, drafted the first version of this paper
and led the writing of the manuscript. D.C.M. helped obtain funding, assisted with fieldwork, gave input on the data
analysis and helped with the writing of the manuscript. D.G.K. helped obtain funding, gave input on the data analysis
and helped with the writing of the manuscript.
Competing interests. We have no competing interests.
Funding. A.M.V.F. was supported by a combination of US Fish and Wildlife Service, Missouri Department of
Conservation and University of Arkansas funds during her PhD. D.C.M. is a full-time employee of the Missouri
Department of Conservation. D.G.K. was, at the time of this work, a full-time employee of the US Geological Survey,
now retired.
Acknowledgements. Special thanks are expressed to M. Boone, L. Brinkman, D. Datlof, J. Fournier, J. Lehman, H. Pavisich
and N. Seeger for their diligent work in the field. Thanks are also expressed to E. Lawton and A. Bond for their helpful
comments on an earlier draft. Any use of trade, firm or product names is for descriptive purposes only and does not
imply endorsement by the US Government.
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