1
Diet and Selectivity of Porphyrio porphyrio (Purple
Swamphen) in Florida
Corey T. Callaghan1,* and Dale E. Gawlik1
Abstract - We tested whether Porphyrio porphyrio (Purple Swamphen) in South Florida
selected particular types of food and whether their diets differed among 3 geographically
separate wetlands (northern Everglades, a stormwater treatment marsh, and Lake
Okeechobee littoral zone). We found that the Purple Swamphens we collected from the
treatment marsh were larger than those from the other sites. The primary food item of
the Purple Swamphen at all 3 sites was Eleocharis cellulosa (Gulf-coast Spikerush),
comprising 79%, 72%, and 49% mean dry weight of total gut contents for the northern
Everglades, littoral zone, and treatment marsh, respectively. Accounting for availability,
Purple Swamphens were strongly selective for Gulf-coast Spikerush, which is a common
plant in the southeastern US. The availability of this plant is not likely to be a factor limiting
the spread of this bird northward.
Introduction
The spread of nonnative and invasive species is a major problem for US policy
makers (USFWS 2006), costing billions of dollars nationally every year (USFWS
2012). Aware of the limited funds available for control efforts, scientists have responded
to threats posed by the growing number of invasive species by developing
screening tools to focus management actions on the most harmful species. Screening
tools typically require basic ecological and life-history information about the
invasive species and its effects on the invaded ecosystem. We conducted this study
to fill gaps in basic information for Porphyrio porphyrio L. (Purple Swamphen;
hereafter Swamphen) in south Florida.
The Swamphen is a member of the Rallidae family, which ranges widely across
Europe, Australia, Asia, Africa, and New Zealand (Pranty 2012, Pranty et al. 2000).
Like other rallids, Swamphens are secretive and spend the majority of their time in
marshes. However, they occupy a wide diversity of habitats, including freshwater
and brackish wetlands dominated by emergent vegetation, pastures, and disturbed
areas (del Hoyo et al. 1996, Freifeld et al. 2001, Sanchez-Lafuente et al. 2001).
In 1996, a population of Swamphens was discovered in Pembroke Pines in Broward
County, FL (Pranty et al. 2000). In the subsequent 2 decades, the Swamphen
expanded its range northwest, through the northern Everglades, and Lake
Okeechobee (Pearlstine and Ortiz 2009, Pranty 2013), a distance of approximately
60 km (Fig. 1). Individual Swamphens have been documented moving more than
300 km to colonize new habitats and territories within their native range (Sanchez-
Lafuente et al. 2001). Additionally, their widespread occupation of oceanic islands
demonstrates their capability as dispersers (Garcia-Ramirez and Trewick 2015).
1Environmental Science Program, Florida Atlantic University, Boca Raton, FL, 33431.
*Corresponding author - ccallaghan2013@fau.edu.
Manuscript Editor: Paul Leberg
Everglades Invasive Species
2016 Southeastern Naturalist 15(Special Issue 8):1–14
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Diet studies from other continents suggest Swamphens are generalists that can
exploit a variety of local plant species (Johnson and McGarrity 2009). Swamphens
are known to be predominantly herbivorous (Balasubramaniam and Guay 2008),
Figure 1. A map of south Florida, showing the the initial introduction location, 3 locations
where Purple Swamphens were collected, and the anticipated general spread of the
Swamphens, 2014.
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but are also opportunistic, consuming a wide range of taxa, including birds, amphibians,
reptiles, fish, eggs, insects, arthropods, and mollusks (Balasubramaniam
and Guay 2008, del Hoyo et al. 1996). Although little is known about their diet in
Florida, Swamphens are generally found there in places dominated by herbaceous
wetland plants (Pranty 2012).
Swamphens invading novel habitats provide a unique opportunity to study processes
such as habitat selection and range expansion (Duncan et al. 2003), which
is especially relevant in our rapidly changing climate. Phenotypic divergence from
source populations, as well as any divergence among different Florida populations
could provide insight into how these invaders are adapting to their new environment.
One way to study this expansion is to compare morphological changes that
may have taken place among populations.
In Florida, the Swamphen is likely still at an early stage in its invasion trajectory
(sensu Simberloff 2001); thus, wildlife management agencies need additional
information on the resources used by this species. In particular, more detailed information
is needed on the basic biology and life history of the Swamphen in its
invaded ecosystem. Our goals were to support management of the Swamphen in
Florida by (1) quantifying the diet of Swamphens, (2) determining the selectivity
of food items by Swamphens, and (3) comparing morphology of Swamphens in 3
regions of South Florida.
Methods
From January to March 2014, Florida Fish and Wildlife Conservation Commission
(FWC) personnel used shotguns and steel shot to collect sample birds with
from 3 geographically separate wetlands across south Florida—water conservation
area 2B in the northern Everglades (WCA2B), stormwater treatment area 1W
(STA1W), and the littoral zone of Lake Okeechobee (Fig. 1). The birds were collected
from areas of emergent marshes at all 3 sites.
Diet
We removed and stored in 70% ethanol the stomach contents from the sample
birds. Prior to analysis of stomach contents, we created macro- and micro-level reference
collections of plant material from the WCA2B site. We prepared slides (Dusi
1949) to create a reference collection at the microscopic (cellular) level. We identified
food items in a hierarchical manner through a macroscopic and microscopic
level of sorting and identification (Ward 1968). We sorted stomach contents at the
macroscopic level by aggregating items with the same texture and structure visible
to the naked eye. We retained the remaining smaller particles, termed homogenate,
for subsequent microscopic analysis. We conducted the microscopic analysis by
spreading the homogenate evenly across one hundred 0.8 cm x 0.8 cm cells arranged
in a 10 x 10 grid. We randomly selected 10 cells and identified the contents
by their cellular structure.
After sorting and identification, we placed food items in a drying oven at 55 °C
for ~48 h until they reached a constant weight (Free et al. 1971); each macroscopic
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and homogenate sample was then weighed. We used the proportion of each food
item identified in the subsamples to determine its dry mass in t he homogenate.
Swanson et al. (1974) recommended using an aggregate percentage approach
rather than an aggregate volume approach. Therefore, we employed the former
method, and we present the diet data as (1) the average percent of dry weight,
(2) the percent occurrence of food items, and (3) the percent occurrence in the Swamphens
(i.e., the number of Swamphens that consumed a particular item from that
particular area; Prevett et al. 1979). The average percent of dry weight is defined
as ΣWi / n, where Wi is the weight of the ith food item expressed as a percentage of
all food items in the sample, and n is the total number of Swamphen samples for a
particular site. The percent occurrence of food items is defined as ΣFi / ΣFs and the
percent occurrence in the Swamphens is defined as ΣFi / n, where Fi = occurrence
of food item i in a sample, and Fs = number of food items in a sample.
We investigated differences in diet by performing a multidimensional scaling
(MDS) ordination with a Bray-Curtis similarity matrix and an analysis of similarity
(ANOSIM) to test for significant differences among sites. ANOSIM provides
a global R-value that indicates the degree of discrimination among sites. We also
conducted a similarity percentages procedure (SIMPER) to determine the percentage
each food item contributed to any differences among sites. All techniques were
performed in PRIMERv6 software (Clarke and Gorley 2006).
Selectivity
We employed Chesson’s index of selectivity (Chesson 1978) to determine
whether Swamphens in WCA2B showed a preference for any particular plant species.
Plant-availability data were not available for the other 2 study sites. Chesson’s
index quantifies selectivity and determines food preference by comparing the proportions
and distribution found in the environment to those found in the diet. This
technique assumes that prey abundance is large compared to the amount of food
consumed. It also assumes that the ability of the organism to consume a particular
item is equal for each item (Chesson 1983). The index is calculated by using the
formula:
n
αi = (ri / pi) / ( Σ [ri / pi]), i = 1, ... , m
i = 1
where αi is the selectivity index for prey type i; ri is the relative abundance of prey
type i consumed by the Swamphen; pi is the percent of prey type i in the environment
calculated from the vegetation surveys; and m is the number of prey types
available in the environment (m = 7 prey types encountered during the vegetation
surveys). In order to interpret Chesson’s index, values of ai are related to 1/m.
Random feeding occurs when αi = 1/m. Preferential selection of a prey type occurs
when αi > 1/m, and avoidance of a prey type occurs when αi < 1/m. We calculated
the ai at the individual level and then we arrayed the mean indices of all individuals
to create a mean selectivity index (Rudershausen et al. 2005); 95% confidence
intervals surrounding the mean selectivity index were also calculated.
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We defined a vegetation sampling area as the approximate spatial ranges
of the Swamphens collected for our study. We determined the spatial range
by plotting the coordinates of the locations from which each bird in the study
was initially flushed. We applied a 1.03-ha buffer to each location, which represents
the average home-range size of the Porphyrio martinicus L. (Purple
Gallinule; West and Hess 2002), a congener of the Swamphen; the home range
of the Swamphen in Florida is unknown. We created a minimum convex polygon
around the buffered locations to delineate the extent of the area from which to
sample vegetation. We generated random points within this defined area such
that each point represented the northeast corner of 3 nested vegetation-sampling
plots. The 3 plots were 5 m x 5 m, 3 m x 3 m, and 1 m x 1 m in size (Ross et al.
2003). We used a modified Braun-Blanquet scale to determine the percent cover
of each species within each of these subplots (Mueller-Dombois and Ellenburg
1974). We sampled vegetation at 10 random points, but added no new species in
the last 4 plots; thus 6 random points were adequate to characterize the available
plant species (Cain 1938).
We calculated vegetation available to Swamphens in the environment as the plot
averages for the 3 nested-plot sizes at each of the 10 random points. We determined
percent cover of each plant type by converting each Braun-Blanquet value to the
midpoint of the corresponding percentage range.
Morphology
FWC staff collected 30 birds from Lake Okeechobee, of which 25 were intact
enough to be used for morphometric analysis. Twenty-nine birds were collected
from STA1W, of which 28 were intact and included in the morphometric analysis.
Thirty-two birds were collected from WCA2B, all of which were included
in the morphometric analysis. Only 2 of the birds collected were juveniles, both
from Lake Okeechobee; these were excluded from all analyses.
We measured body mass, bill length to gape, exposed culmen, bill width, bill
depth, tarsus length, wing chord, and tail length of each bird carcass (e.g., Pyle et
al. 2008). Swamphens are sexually dimorphic (Marchant and Higgins 1993), and
therefore, sex determination is an important factor in considering morphologic
differences among sites. Hence, we determined sex with a genetic analysis of feathers
plucked from each individual. Sex could not be determined for 2 of the birds,
and we excluded them from the morphology analysis. We employed PRIMERv6
software for multivariate statistics (Clarke and Gorley 2006) to quantify morphometric
differences among sites. We used multi-dimensional scaling (MDS) with a
Euclidian-distance similarity matrix to visualize similarities or differences among
sites, and analysis of similarity (ANOSIM) to determine if there were significant
differences among groups (Clarke and Gorley 2006). The data were normalized via
a base-10 log-transformation before performing the analysis to account for the difference
in morphological measurement types.
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Results
Diet
The macroscopic-level sorting procedure showed a low diversity of food types,
which we confirmed with microscopic analysis. Eleocharis cellulosa Torr. (Gulfcoast
Spikerush) was the dominant plant consumed (Table 1), comprising more
than 70% of the average dry weight of birds’ diets from Lake Okeechobee and
WCA2B, and about 50% from STA1W. Gulf-coast Spikerush also occurred in
100% of Swamphen samples from both WCA2B and Lake Okeechobee, and 96%
of samples from STA1W. Birds from STA1W had a more diverse diet than birds
from other sites. Only 3.3% of the average dry weight was unidentified (Table 1).
Additionally, we observed no grit in the stomachs of birds from WCA2B, whereas
25% and 59% of samples from Lake Okeechobee and STA1W, respectively,
contained grit. Six birds consumed insects, but all specimens were small and
presumed to have been consumed incidentally. Two birds consumed lepidopterans.
Fifty percent of Swamphens from WCA2B had mollusks in their stomachs,
whereas only 1 sample from STA1W had a mullosk, and we found no mollusks in
samples from Lake Okeechobee.
Purple Swamphen diets differed among the 3 sites (R = 0.525, P < 0.001; Fig. 2).
The SIMPER analyis uses dissimilarity to demonstrate the degree to which food
items contribute to the difference of diet among sites; therefore, 2 pairwise tests
Figure 2. An MDS plot demonstrating the similarity/dissimilarity of Purple Swamphen diets
for STA1W = Stormwater Treatment Area 1W, WCA2B = Water Conservation Area 2B, and
LKO = Lake Okeechobee in south Florida of samples collected from January to March 2014.
The ordination was performed using a Bray-Curtis similarity matrix.
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Table 1. Biomass estimates of food items in Purple Swamphen stomachs from Stormwater Treatment Area 1W, Water Conservation Area 2B, and Lake
Okeechobee in south Florida January to March 2014. The numbers in parantheses indicate the sample size for the corresponding area. % of PUSW = the
percent occurrence in the Swamphens (i.e., the number of Swamphens that consumed a particular item from that sampling site). + indicates that the item
was present but comprised less than 1.0% of the mass.
Water Conservation Area 2B (n = 32) Lake Okeechobee (n = 24) Stormwater Treatment Area 1W (n = 27)
Average % occurrence % of Average % occurrence % of Average % occurrence % of
Items in diet % dry wt of food item PUSW % dry weight of food item PUSW % dry weight of food item PUSW
Plant material
Eleocharis cellulosa 79.6 27.8 100.0 72.5 44.4 100.0 49.3 32.9 96.3
Typha sp. 1.2 19.0 55.6
Cladium jamaicense seeds 9.4 22.6 81.3 + 3.7 8.3 + 3.8 11.1
Panicum spp. seeds 5.3 14.8 53.1
Eleocharis spp. seeds + 2.6 9.4 21.8 33.3 75.0
Typha seeds 3.6 2.5 7.4
Insecta spp. + 4.3 15.6 + 1.9 4.2
Lepidoptera spp. + + 3.1 + 1.3 3.7
Mollusk sp. 3.3 13.9 50.0 + 1.3 3.7
Grit 5.1 11.1 25.0 35.3 20.3 59.3
Shot pellets 7.1 2.5 7.4
Unknown plant matter 2.1 13.0 46.9 + 5.5 12.5 3.3 16.5 48.1
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were conducted for each site location (Table 2). Panicum spp. (panic grasses)
seeds, only present in WCA2B, accounted for 46% of the dissimilarity between
WCA2B and Lake Okeechobee, whereas they accounted for 30% of the dissimilarity
between STA1W and WCA2B. Likewise, grit, only present in STA1W and Lake
Okechobee, accounted for 42% of the dissimilarity between WCA2B and STA1W.
Selectivity
We identified 6 species of emergent aquatic plants—Gulf-coast Spikerush,
panic grasses, Nymphaea odorata Aiton (American White Water-lily), Cladium
jamaicense (Crantz.) Kük. (Jamaican Swamp Sawgrass), Typha spp. (cattails), and
Pontedaria cordata L. (Pickerelweed)—and 1 submerged aquatic plant (Utricularia
spp. [bladderwort]) in the 10 plots. The 2 most-abundant emergent species
were Gulf-coast Spikerush (10.5%, 9.3%, and 9.0% cover) and American White
Water-lily (7.3%, 7.0%, and 8.0% cover) at the 5 x 5-m, 3 x 3-m, and 1 x 1-m plots,
respectively.
Swamphens at WCA2B selected Gulf-coast Spikerush at each of the 3 hierarchical
levels at which we carried out the vegetation surveys (Table 3). We also found
that Swamphens selected Jamaican Swamp Sawgrass seeds, but that selection was
weaker than selection for Gulf-coast Spikerush. Swamphens consumed Jamaican
Swamp Sawgrass much less often than Gulf-coast Spikerush.
Table 3. Mean food-type selectivity (Chessons’s index, ai; 95% CI) across all 32 individuals from Water
Conservation Area 2B for each of the 3 plot sizes. All values for Gulf-coast Spikerush are greater
than 1/m, which indicates selection of this prey type at all levels. Refer to the methods for a further
explanation of how to interpret Chesson’s index.
Plot size
5 m x 5 m 3 m x 3 m 1 m x 1 m
Food item Mean (95% CI) Mean (95% CI) Mean (95% CI) 1/m
Eleocharis cellulosa 0.587 (0.463–0.680) 0.554 (0.429–0.680) 0.350 (0.232–0.468) 0.143
Cladium jamaicense 0.290 (0.185–0.394) 0.385 (0.265–0.505) 0.534 (0.404–0.665) 0.143
Panicum spp. 0.124 (0.048–0.199) 0.061 (0.011–0.111) 0.115 (0.037–0.194) 0.143
Table 2. Dissimilarity in food items between pairs of study sites from samples collected from January
to March 2014. STA1W = Stormwater Treatment Area 1W, WCA2B = Water Conservation Area 2B,
and LKO = Lake Okeechobee, in south Florida. Food items are listed in order of decreasing contribution.
Cum. % dis. = cumulative percent dissimilarity.
STA1W vs. LKO WCA2B vs. LKO STA1W vs. WCA2B
Food items Cum. % dis. Food items Cum. % dis. Food items Cum. % dis.
Grit 41.94 Panicum seeds 46.57 Panicum seeds 30.41
Eleocharis 64.34 Eleocharis 63.10 Grit 57.66
Shot pellets 75.46 Eleocharis seeds 73.81 Eleocharis 82.25
Eleocharis seeds 83.86 Grit 82.69 Shot pellets 90.24
Cladium seeds 88.91 Cladium seeds 88.50
Typha flower 93.67 Unknown 93.75
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Morphology
In total, we sampled 83 Swamphens across the 3 sites; STA1W (n = 27), WCA2B
(n = 31), and Lake Okeechobee (n = 25). The morphology of adult Swamphens differed
significantly among study sites (Global R = 0.164, P < 0.001; Fig. 3), and all
pairwise differences were significant. However, based on estimates of R, the magnitude
of the differences was much larger between birds from STA1W and WCA2B
(R = 0.236, P < 0.001) than between those from STA1W and Lake Okeechobee (R
= 0.154, P < 0.008) or WCA2B and Lake Okeechobee (R = 0.098, P < 0.01). Birds
in STA1W had the largest mean body mass, bill length to gape, exposed culmen,
bill depth, bill width, and wing chord (Table 4).
Discussion
Diet
Prior to this study, the diets of Swamphens in Florida had not been quantified.
Pranty (2012) reported that the birds were predominantly herbivorous but that they
also took some small invertebrate prey. A diet study of Swamphens from their native
range in Australia found that they primarily ate plants from the Poaceae (59%),
Cyperaceae (17%), and Hydrocharitaceae (11%) families (Norman and Mumford
1985). Our study confirmed that Swamphens in Florida are also predominantly
herbivorous. However, as opposed to a generalist diet, we found a strong selection
for Gulf-coast Spikerush (Cyperaceae). At 2 of the 3 study sites, Swamphens
Figure 3. An MDS plot showing the morphometric similarity/dissimilarity of individual
Purple Swamphens from STA1W = Stormwater Treatment Area 1W, WCA2B = Water
Conservation Area 2B, and LKO = Lake Okeechobee in South Florida of samples collected
from January to March, 2014. The ordination was performed using a Euclidean distance
similarity matrix.
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Table 4. Morphological characteristics of 85 Purple Swamphens collected from Stormwater Treatment Area 1W, Water Conservation Area 2B, and Lake
Okeechobee in south Florida, Janurary to March 2014. The number of decimal places present in the table corresponds to the level of accuracy that was
achieved while measuring that particular morphological characteristic.
Morphological Water Conservation Area 2B (n = 31) Lake Okeechobee (n = 25) Stormwater Treatment Area 1W (n = 27)
characteristic Min Max Mean SD Min Max Mean SD Min Max Mean SD
Body mass (g) 505 730 621 57 555 850 689 81 570 815 702 66
Bill length to gape (mm) 21.48 32.47 25.28 2.57 21.73 27.72 24.84 1.74 23.34 29.61 26.07 1.61
Exposed culmen (mm) 25.59 39.85 33.68 2.87 32.49 38.41 34.73 1.72 32.02 39.41 35.45 1.91
Bill depth (mm) 20.13 24.52 22.40 1.24 20.79 25.71 23.33 1.57 21.42 27.17 24.32 1.32
Bill width (mm) 10.86 15.74 13.47 1.32 12.01 15.84 14.19 0.82 11.63 16.17 14.39 1.23
Tarsus length (mm) 84.93 109.12 96.32 6.28 84.38 106.04 95.77 5.63 93.16 112.54 102.90 4.88
Wing chord (mm) 22.3 25.9 24.0 0.8 22.4 25.0 23.7 0.8 21.5 26.7 24.5 1.3
Tail length (mm) 7.3 9.2 8.2 0.6 6.9 8.7 7.7 0.5 7.2 9.4 8.2 0.5
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consumed predominantly 1 plant species rather than a more even mix of species.
The higher use of Cyperaceae in Florida than in Australia likely reflects the availablity
of plants in the environment. However, the narrower range of plant species
consumed by birds in Florida demonstrates that across their range, Swamphens
have the ability to specialize on specific plant species to dif ferent degrees.
Although we found a small percentage of animal matter in the diet of
Swamphens, caution in interpretation is warranted because the birds were collected
during a single dry season; inferences about diet should be restricted to that period.
It is possible that we missed seasonal switches from one food item to another.
For instance, Balasubramaniam and Guay (2008) noted that in their native range,
Swamphens consumed Cygnus atratus (Latham) (Black Swan) eggs. Indeed, the
closest relative of the Swamphen here in south Florida, the Purple Gallinule, has a
diet that varies greatly with seasonality and locality (West and Hess 2002). More
than 50% of the diet of Gallinules during spring and summer is animal material
such as arthropods, annelids, and mollusks (Mulholland and Percival 1982). We did
not investigate spring and summer diets.
A broad diet is a trait associated with succesful establishment by exotic species
(Blackburn et al. 2009). Swamphens have successfully established in a number of
locations and, although they are reported to be generalists, diets in our study had a
narrow breadth. There are several possible reasons for this apparent inconsistency.
Swamphens in Florida are already well established, which makes it possibile that
the species initially had a wider diet breadth which narrowed once they were successfully
established (Overington et al. 2011). If true, this pattern would support
Wright et al.’s (2010) “adaptive flexibility hypothesis” in which they predicted a
decline in behavioral diversity during the establishment of a population due to successful
strategies being learned and taught.
Similar to diets of Purple Gallinule and Gallinula galeata (Lichtenstein) (Common
Gallinule), the diet of the Swamphens in our study was dominated by plant, not
animal matter (Bannor and Kiviat 2002, West and Hess 2002). Both of these gallinule
species have been known to feed on exotic plants (Mulholland and Percival
1982), demonstrating that they are generalists and can shift their diet in response
to the available plant community. The Swamphen’s and gallinules’ shared ability
to be generalists and to sometimes specialize on certain species could lead to high
diet overlap.
Selectivity
Resource selection occurs in a hierarchical fashion. First-order selection is
the physical or geographical range of a species, 2nd-order selection represents the
home range of an individual or group of individuals, 3rd-order selection is the use of
habitat components within a home range, and 4th-order selection is the use of particular
food items (Johnson 1980). Based on anecdotal evidence that showed large
“eat-outs” of Gulf-coast Spikerush from Lake Okeechobee (T. Beck, FWC, pers.
comm.), we hypothesized a priori, and subsequently confirmed, that Swamphens
selected the spikerush in the WCA2B site at the 4th-order selection level. We
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utilized 3 different-sized plots due to the hierarchical process of habitat selection.
However, we did not sample vegetation at lower-order levels of selection, so we do
not know whether Swamphens were selecting Gulf-coast Spikerush at those levels
as well. In addition, we found weak selection for Cladium seeds, which we would
expect to vary seasonally because of plant phenology, and thus, availability.
Morphology
Body size in birds is often related to habitat quality (Johnson 2007), suggesting
that the STAs may provide better habitat for Swamphens than the other sites.
Given how quickly Swamphens have expanded their range across the region, it is
surprising to find significant differences in morphological measurements among the
study sites. This pattern is puzzling because Swamphens were clearly selecting for
Gulf-coast Spikerush, but they were largest in STA1W, where the spikerush made
up the smallest proportion of their diet. The strong selection for this plant in the
area where the birds were smallest suggests that factors other than plant species
may play a role in habitat quality, or alternately, the benefit of the spikerush is not
reflected in body size but rather some demographic response, such as productivity.
If Swamphen habitat quality is determined by plant-community characteristics and
trophic status, then quantification of these effects could be used to model future
range-expansion in Florida. Alternatively, body size differences could result from
some form of character displacement (Grant and Grant 2006); there may have been
slight differences in the avifauna present at each of the 3 study sites.
Conclusion
This study provides a quantitative basis for the perception that the Swamphens
in south Florida utilize Gulf-coast Spikerush as a main food resource. Given that
the spikerush is widespread and fairly abundant throughout Florida and the southeast
US, it is not likely to limit the Swamphen’s distribution. It is uncertain how
this preference for Gulf-coast Spikerush and the likely expansion of Swamphens
throughout Florida might impact native species. Potential effects of resource competition
could be evident for other species that rely heavily on this plant species.
Gulf-coast Spikerush is generally known to provide habitat and food for a variety
of fish, invertebrates, and waterfowl. Additionally, our study presents observations
that suggest that divergent evolution may have taken place among 3 different
populations of Swamphens in a short period of time. The combination of the
Swamphens’ diet as well as this potential divergent evolution make this species
an excellent candidate for future studies of potential impacts to the native fauna in
south Florida.
Acknowledgments
Funding for this study was provided by the Florida Fish and Wildlife Conservation Commission
(FWC). We thank Jennifer Eckles (FWC) for overseeing the project, and J. Edwards
(FWC), and N. Larson (South Florida Water Management District) for collecting birds.
Diane Harshbarger assisted with vegetation surveys, which ultimately led to her becoming
C.T. Callaghan’s wife. Comments from 2 anonymous reviewers improved this manuscript.
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Literature Cited
Balasubramaniam, S., and P.J. Guay. 2008. Purple Swamphens (Porphyrio porphyrio) attempting
to prey upon Black Swan (Cygnus atratus) eggs and preying upon a cygnet
on an urban lake in Melbourne, Australia. Wilson Journal of Ornithology 120:633–635.
Bannor, B.K., and E. Kiviat. 2002. Common Gallinule (Gallinula galeata). No. 685, In
A. Poole (Ed.). The Birds of North America Online. Cornell Lab of Ornithology, Ithaca,
NY. Available online at http://bna.birds.cornell.edu/bna/species/685/. Accessed 25
April 2015.
Blackburn, T.M., J.L. Lockwood, and P. Cassey. 2009. Avian Invasions: The Ecology and
Evolution of Exotic Birds. Oxford University Press, New York, NY. 320 pp.
Cain, A.S. 1938. The species-area curve. American Midland Naturalist 19:573–581.
Chesson, J. 1978. Measuring preference in selective predation. Ecology 59:211–215.
Chesson, J. 1983. The estimation and analysis of preference and its relationship to foraging
models. Ecology 64:1297–1304.
Clarke, K.R., and R.N. Gorley. 2006. PRIMER v6: User manual/tutorial. PRIMER-E,
Plymouth, UK. 192 pp.
del Hoyo, J., A. Elliot, and J. Sargatal (Eds.). 1996. Handbook of the Birds of the World.
Volume 3: Hoatzins to Auks. Lynx Edicions, Barcelona, Spain. 821 pp.
Duncan, R.P., T.M. Blackburn, and D. Sol. 2003. The ecology of bird introductions. Annual
Review of Ecology, Evolution, and Systematics 34:71–98.
Dusi, J.L. 1949. Methods for the determination of food habits by plant microtechniques
and histology and their application to Cottontail Rabbit food habits. Journal of Wildlife
Management 13:295–298.
Free, J.C., P.L. Sims, and R.M. Hansen. 1971. Methods of estimating dry-weight composition
in diets of steers. Journal of Animal Science 32:1003–1007.
Freifeld, H.B., D.W. Steadman, and J.K. Sailer. 2001. Landbirds on offshore islands in
Samoa. Journal of Field Ornithology 72:72–85.
Garcia-Ramirez, J.C., and S.A. Trewick. 2015. Dispersal and speciation in Purple Swamphens
(Rallidae: Porphyrio). The Auk 132:140–155.
Grant, P.R., and B.R. Grant. 2006. Evolution of character displacement in Darwin’s Finches.
Science 313:224–226.
Johnson, D.H. 1980. The comparison of usage and availability measurements for evaluating
resource preference. Ecology 61:65–71.
Johnson, M.D. 2007. Measuring habitat quality: A review. The Condor 109:489–504.
Johnson, S.A., and M. McGarrity. 2009. Florida’s introduced birds: Purple Swamphen
(Porphyrio porphyrio). University of Florida IFAS Extension Service publication WEC
270. Gainesville, FL.
Marchant, S., and P.J. Higgins. 1993. Handbook of Australian, New Zealand and Antarctic
Birds. Oxford University Press, Melbourne, Australia. 984 pp.
Mueller-Dombois, D., and H. Ellenburg. 1974. Aims and Methods of Vegetation Ecology.
John Wiley and Sons, New York, NY. 530 pp.
Mulholland, R., and H.F. Percival. 1982. Food habits of the Common Moorhen and Purple
Gallinule in north-central Florida. Proceedings of the Annual Conference of the Southeastern
Association of Fish and Wildlife Agencies 36:527–536.
Norman, F., and L. Mumford. 1985. Studies on the Purple Swamphen, Porphyrio porphyrio,
in Victoria. Wildlife Research 12:263–278.
Overington, S.E., A.S. Griffin, D. Sol, and L. Lefebvre. 2011. Are innovative species ecological
generalists? A test in North American birds. Behavioral Ecology 22:1286–1293.
Southeastern Naturalist
C.T. Callaghan and D.E. Gawlik
2016
14
Vol. 15, Special Issue 8
Pearlstine, E.V., and J.S. Ortiz. 2009. A natural history of the Purple Swamphen (Porphyrio
porphyrio). University of Floirda IFAS Extension publication WEC 272, Gainesville,
FL.
Pranty, B. 2012. Population growth, spread, and persistence of Purple Swamphens (Porphyrio
porphyrio) in Florida. Florida Field Naturalist 40:1–12.
Pranty, B. 2013. Introducing the Purple Swamphen. Birding May:38–45.
Pranty, B., K. Schnitzius, K. Schnitzius, and H.W. Lovell. 2000. Discovery, origin, and
current distribution of the Purple Swamphen (Porphyrio porphyrio) in Florida. Florida
Field Naturalist 28:1–40.
Prevett, J.P., I.F Marshall, and V.G. Thomas. 1979. Fall foods of Lesser Snow Geese in the
James Bay region. Journal of Wildlife Management 43:736–742.
Pyle, P., S. Howell, D. DeSante, and R. Yunick. 2008. Identification Guide to North American
Birds. Slate Creek Press, Point Reyes Staion, Bolinas, CA. 836 pp.
Ross, M.S., D.L. Reed, J.P. Sah, P.L. Ruiz, and M.T. Lewin. 2003. Vegetation: Environment
relationships and water management in Shark Slough, Everglades National Park.
Wetlands Ecology and Management 11:291–303.
Rudershausen, P.J., J.E. Tuomikoski, J.A. Buckel, and J.E. Hightower. 2005. Prey selectivity
and diet of Striped Bass in western Albemarle Sound, North Carolina. Transactions
of the American Fisheries Society 134:1059–1074.
Sanchez-Lafuente, A.M., F. Valera, A. Godino, and F. Muela. 2001. Natural and humanmediated
factors in the recovery and subsequent expansion of the Purple Swamphen
Porphyrio porphyrio L. (Rallidae) in the Iberian Peninsula. Biodiversity and Conservation
10:851–867.
Simberloff, D. 2001. Biological Invasions: How are they affecting us and what can we do
about them? Western North American Naturalist 61:308–315.
Swanson, G.A., G.L. Krapu, J.C. Bartonek, J.R. Serie, and D.H. Johnson. 1974. Advantages
in mathematically weighting waterfowl food-habits data. Journal of Wildlife Management
38:302–307.
US Fish and Wildlife Service (USFWS). 2006. Lacey Act. 18 USC 42–43. Available online
at https://www.fws.gov/le/pdffiles/Lacey.pdf. Accessed 15 March 2015.
USFWS. 2012. The cost of invasive species. US. Fish and Wildlife Service. Available
online at https://www.fws.gov/verobeach/PythonPDF/CostofInvasivesFactSheet.pdf.
Accessed 15 March 2015.
Ward, A.L. 1968. Stomach content and fecal analysis: Methods of forage identification. Pp.
146–158, In H.A. Paulesn Jr., E.H. Reid, and K.W. Parker (Eds.). Range and Wildlife
Habitat Evaluation: A Research Symposium. US Department of Agriculture, Flagstaff
and Tempe, AZ. 220 pp.
West, R.L., and G.K. Hess. 2002. Purple Gallinule (Porphyrio martinicus). No. 626, In
A. Poole (Ed.). The Birds of North America Online. Cornell Lab of Ornithology, Ithaca,
NY. Available online at http://bna.birds.cornell.edu/bna/species/626/. Accessed 25
April 2015.
Wright, T.F., J.R. Eberhard, E.A. Hobson, M.L. Avery, and M.A. Russello. 2010. Behavioral
flexibility and species invasions: The adaptive flexibility hypothesis. Ethology, Ecology,
and Evolution 22:393–404.