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2007 SOUTHEASTERN NATURALIST 6(1):35–46
Breeding Success of Rostrhamus sociabilis (Snail Kites) at
Two Florida Lakes
James A. Rodgers, Jr.*
Abstract - Rostrhamus sociabilis (Snail Kite) nesting success and productivity
were studied at the recently colonized Lake Kissimmee and historically occupied
Lake Okeechobee in Florida during 1987 to 1993. Mean (± SD) clutch size for all
lake-years was 2.79 ± 0.49, with 3-egg clutches (75.2%) being most frequent. No
significant difference was detected in the overall distribution of 2-, 3-, and 4-egg
clutches between lakes. The mean number of fledglings for all lake-years was 0.79
± 0.99 fledgling/nest. Only 44.1% of all nests were successful in fledging birds.
Fledging success did not differ significantly among years at Lake Kissimmee.
However, significant differences occurred among years at Lake Okeechobee due to
the high frequency of failed nests in 1990 (72.2%) and 1991 (74.3%) during low
lake levels. Productivity at Lake Kissimmee (0.88 ± 1.00 fledgling/nest) was significantly
greater than at Lake Okeechobee (0.74 ± 0.98 fledgling/nest).
Estimated nest survivorship (Sday: nest age in days) combined for all years and
both lakes was as follows: S28 (hatching) = 0.67 ± 0.14; S42 (2 weeks) = 0.58± 0.14;
S56 (4 weeks) = 0.46 ± 0.20, and S70 (fledging) = 0.45 ± 0.03 (n = 641 nests). There
was a higher probability of nest survivorship to hatching and fledging at Lake
Kissimmee compared with Lake Okeechobee (S28 = 0.74 ± 0.11 and S70 = 0.53 ± 0.03
versus S28 = 0.63 ± 0.16 and S70 = 0.40 ± 0.02, respectively). Lake level and breeding
date during early incubation had a significant effect on nest survivorship at both Lake
Kissimmee and Lake Okeechobee.
Rostrhamus sociabilis plumbeus Ridgway (Florida Snail Kite) is an
endangered species that has been monitored during the last four decades
using midwinter-population and summer-nest surveys in Florida (Bennetts
and Kitchens 1997a, b; Rodgers et al. 1988; Stieglitz and Thompson 1967;
Sykes 1979, 1983). Kite populations and their distribution have undergone
substantive changes during that time, including a major range shift northward
from the water conservation areas (i.e., Everglades) and Lake
Okeechobee to recolonize the Kissimmee chain of lakes in the early 1980s
during drought conditions (Beissinger and Takekawa 1983, Takekawa and
Beissinger 1989). Replicate-year and site studies comparing the fates of
young hatched throughout the breeding season can identify regions of a
species’ range where individuals are experiencing reproductive stress
(Cooke et al. 1984, Ricklefs 1980, Thompson et al. 1986, Vander Werf 1992,
Wrege and Emlen 1991). My primary objective in this paper is to compare
the fecundity of Snail Kites nesting at the recently recolonized Lake
*Florida Fish and Wildlife Conservation Commission, 4005 South Main Street,
Gainesville, FL 32601; james.rodgers@MyFWC.com.
36 Southeastern Naturalist Vol. 6, No. 1
Kissimmee and historically occupied Lake Okeechobee. I also examine
within-lake variation in kite productivity among years and correlate fledging
success with lake ecological variables.
Lakes Kissimmee (surface area = 135 km2, littoral zone = 21 km2) and
Okeechobee (surface area = 1732 km2, littoral zone = 400 km2) are two
eutrophic lakes in central and south Florida characterized as having mostly
shallow, turbid water with broad littoral marshes, especially at Lake
Okeechobee (Phlips et al. 1995). Nests were monitored on Lake Okeechobee
from Taylor Creek south to Ritta Island, including Moonshine Bay and
Observation Shoal. Nests on Lake Kissimmee were primarily located around
Bird, Rabbit, Strum, and Brahma Islands, and along the western shoreline of
the northern two-thirds of the lake. These two lakes are about 82 km apart
and lake levels are managed by both state and federal agencies for water
resources and flood-control purposes.
I surveyed Lakes Kissimmee and Okeechobee via transects using airboats
or a helicopter during January through September 1987–93 to locate
kite nests and map breeding locations. Snail Kite nests were visited every 4–
7 days during 1987–1989 and 1–2 weeks during 1990–93. Care was taken to
reduce researcher effects on the breeding kites by minimizing nest monitoring
during pair-formation and early egg-laying periods (Steenhof and
Data are represented as mean ± 1 SD and analyzed as a lake-year unit.
Fledging rate is defined as the average number fledglings per nest and
fledging success is defined as the percent of nests that fledged 1 bird. I
followed the method of Snyder et al. (1989) and designated 20 nests that
were about to fall into the water (and which I supported with poles) as whole
nest failures and used their categories for other sources of nesting failure.
However, I followed the recommendations of Steenhof (1987) for measuring
nest success and productivity of raptors by not designating nest starts (i.e.,
partially completed structures initiated by unpaired males that never contained
eggs) as a breeding effort. Numerous raptor species exhibit alternate
or supernumerary nests (Palmer 1988a, b). This problem is further complicated
by sequential monogamy or ambisexual mate desertion characteristic
of Snail Kites (Beissinger 1987, Beissinger and Snyder 1987). The omission
of nests failing before egg-laying can result in over-estimating nest success
(Beissinger and Snyder 2002).
Statistical analyses were performed using the SAS System (SAS Institute,
Inc. 2000) on datasets with 10 nests. Nonparametric statistics were
used on clutch-size and fledging variables because the data were discrete,
exhibited a small range of 1 to 5, and often were not normally distributed. A
2007 J.A. Rodgers, Jr. 37
Pearson chi-square test using the FREQ procedure was used to analyze
differences in clutch and fledging variables. If a chi-square test indicated
significant among-year and among-lake differences, then pairwise samples
of the data were compared with a Fisher’s exact test. Low frequency (< 10)
of expected cell counts for some analyses necessitated lumping clutch sizes
into small (1 and 2 eggs), medium (3 eggs), and large (4 and 5 eggs)
categories. Nestlings were considered fledged at 6 weeks of age (42 days
post-hatching), and a nest was designated successful at 10 weeks (28-day
incubation period + 42-day nestling period = 70 days) if it fledged 1 bird.
Fledging success was analyzed for 0-, 1-, 2-, and 3-fledgling categories
because kites rarely fledged > 3 young (Dreitz and Shannon 2001).
Nesting date (Julian day of the year when the clutch was completed)
was an easily determined chronological reference point for breeding
cycles since nest building and egg-laying can be quite variable in length
(Snyder et al. 1989), and I did not always know the egg-laying dates
because I avoided causing disturbance early in the pair-formation period.
Nest survival was analyzed for hatching at 28 days (S28), nestling age of 2
weeks (S42), nestling age of 4 weeks (S56), and fledging age (S70). I employed
the logistic-exposure model of Shaffer (2004) using the
GENMOD procedure to estimate the daily nest survival (S) with nest
failure times that tended to be interval and right censored. The logisticexposure
model is similar to logistic regression, but the former method
employs a link function (1/exposure time) to account for the probability
that a nest surviving an interval is dependent on the length of the monitoring
period. Thus, S28 represents the probability of at least 1 egg in a
nest surviving from egg-laying to hatching, and S70 is the probability of a
nest surviving from egg-laying to fledging at least 1 young. I also used
the GENMOD procedure to investigate the relationship between the
binary response (nest success or failure) and a set of potentially explanatory
lake variables. Terms in the full model specified the binary proportion
nest success as the response variable and year, hatch date of the nest,
water depth below the nest, lake water levels during early incubation, and
type of nesting substrate (woody or non-woody species) as explanatory
variables. Type of nesting substrate has been suggested as an important
variable in the fledging success of kites (Sykes et al. 1995). Water depth
was measured beneath the nest during the early incubation period. I used
the daily lake levels taken from the Water Resources Data for Florida, US
Geological Survey, Tallahassee, FL, for each nest. Lake levels were in
meters above mean sea level (MSL). Woody species included Salix
caroliniana Michaux (southern willow) and Cephalanthus occidentalis
Linnaeus (buttonbush), while non-woody species included Typha latifolia
Linnaeus (cattail) and Scirpus spp. (bulrush). Explanatory variables and
various interaction terms were eliminated from the model when P > 0.10
and using Akaike’s Information Criterion (AIC) calculated using the deviance
plus twice the number of parameters in the model.
38 Southeastern Naturalist Vol. 6, No. 1
Mean clutch size pooled for all lake-years was 2.79 ± 0.49 (n = 560
nests); mean clutch size among lake-years ranged from 2.47 ± 0.64 to 2.94 ±
0.71 (Table 1). The most frequent clutch size was 3 eggs (75.2%, n = 421
nests), followed by 2 eggs (21.4%, n=120 nests), 4 eggs (2.3%, n = 13 nests),
1 egg (0.9%, n = 5 nests), and 5 eggs (0.2%, n = 1 nest). Clutches of 3 eggs
ranged from 47.6% to 87.0% among lake-years (Fig. 1).
No significant (2 = 1.45, 2 df, P = 0.49, n = 560 nests) difference was
detected in the overall distribution of 2-, 3-, and 4-egg clutches between
Lakes Kissimmee and Okeechobee. Nor were there significant (Fisher’s
exact tests, P > 0.05) differences between lakes within individual years.
However, kites exhibited significant among-year differences (Table 1)
Figure 1. Distribution of Snail Kite clutch sizes for Lakes Kissimmee and
Okeechobee, FL, 1987–1993. Sample sizes are given in Table 1.
2007 J.A. Rodgers, Jr. 39
within Lakes Kissimmee (2 = 23.23, 12 df, P = 0.03) and Okeechobee (2 =
23.29, 12 df, P = 0.03).
Fledging and nest success
Mean fledging success for all lake-years was 0.79 ± 0.99 fledgling/nest
(range = 0-4, n = 661 nests). Fledging success ranged from 0.43 ± 0.79
fledgling/nest to 1.14 ± 1.12 fledglings/nest among lake-years (Table 1).
Only 44.9% of nests fledged 1 nestling; 16.0% fledged 1 nestling, 22.9%
fledged 2 nestlings, 5.8% fledged 3 nestlings, and 0.2% fledged 4 nestlings
(Fig. 2). Among years, 1990 (0.97 ± 1.08 fledgling/nest) exhibited the
highest fledging rate and 1991 (0.55 ± 0.84 fledgling/nest) the lowest rate.
No significant (2 = 22.12, 18 df, P = 0.23) difference in fledging success
was found among years at Lake Kissimmee (Fig. 2). However, significant
(2 = 48.83, 18 df, P = 0.001) differences occurred among years at Lake
Okeechobee due to the large frequency of nests in 1990 (72.2%) and 1991
(74.3%) that fledged no kites during low lake levels (Fig. 2). Kites nesting at
Lake Kissimmee (0.88 ± 1.00 fledgling/nest) produced a significantly (2 =
12.41, 3 df, P < 0.001) greater number of fledglings than Lake Okeechobee
(0.74 ± 0.98 fledgling/nest) due to the greater frequency of nests on Lake
Okeechobee that produced either 0 (59.4%) or 1 (12.3%) fledgling (Fig. 2,
Table 1). Considering within-year comparisons between the two lakes, productivity
was significantly different in 1991 (2 = 12.72, 3 df, P = 0.005),
1992 (2 = 10.12, 3 df, P = 0.018), and 1993 (2 = 9.80, 3 df, P = 0.020).
Nest survival varied with nest age. The greatest incidence of nest failure
occurred prior to hatching. The post-hatch failure rate was nearly constant
until the nestlings were about 4-weeks old, and few nests failed after this
time. Estimated nest survival (Sday) combined for all years and both lakes
was as follows: S28 (hatching) = 0.67 ± 0.14; S42 (2 weeks) = 0.58 ± 0.14; S56
(4 weeks) = 0.46 ± 0.20, and S70 (fledging) = 0.45 ± 0.03 (n = 641 nests).
Lake Kissimmee (n = 227 nests) exhibited a higher probability of nest
survivorship to hatching (S28 = 0.74 ± 0.11) and fledging (S70 = 0.53 ± 0.03)
Table 1. Among year comparison of mean (± 1 SD) clutch sizes and fledging rates (number of
fledglings/nest) of Snail Kites breeding at Lakes Kissimmee and Okeechobee, FL, 1987–1993.1
Lake Kissimmee Lake Okeechobee
Year2 Clutch size Fledging rate Clutch size Fledging rate
1987 2.64 ± 0.50 (14)BC 0.87 ± 1.06 (15)A 2.87 ± 0.34 (23)A 1.00 ± 1.20 (29)A
1988 2.94 ± 0.71 (19)A 0.90 ± 1.04 (21)A 2.47 ± 0.64 (15)BC 0.73 ± 0.88 (15)B
1989 2.48 ± 0.51 (21)C 0.81 ± 0.90 (26)A 2.58 ± 0.58 (45)B 0.74 ± 0.91 (58)BC
1990 2.80 ± 0.49 (50)AB 1.14 ± 1.12 (58)A 2.71 ± 0.49 (7)ABC 0.44 ± 0.78 (18)BCD
1991 2.74 ± 0.49 (47)ABC 0.79 ± 0.91 (53)A 2.85 ± 0.47 (87)ABC 0.43 ± 0.79 (109)D
1992 2.76 ± 0.54 (21)ABC 0.65 ± 1.03 (23)A 2.85 ± 0.49 (54)AB 0.79 ± 0.95 (63)ABC
1993 2.87 ± 0.34 (39)AB 0.80 ± 0.98 (41)A 2.89 ± 0.39 (118)A 0.95 ± 1.10 (132)ABC
Total 2.77 ± 0.51 (211) 0.88 ± 1.01 (237) 2.81 ± 0.48 (349) 0.74 ± 0.98 (424)
1Sample sizes are given in parentheses.
2Among year comparisons with similar superscripted letters within the same lake are not
significantly (Fisher’s exact test, P > 0.05) different.
40 Southeastern Naturalist Vol. 6, No. 1
compared with Lake Okeechobee (n = 414 nests) to hatching (S28 = 0.63 ±
0.16) and fledging (S70 = 0.40 ± 0.02).
Sources of nest failure
Approximately 28% of all Snail Kite nest initiations (n = 278 of 1003
nests) did not proceed past the nest-building stage. The frequency of these
nest starts ranged from 11.8 to 47.5% of all nests initiated within lake-years.
Only 44.1% (n = 667 nests, range = 14.3–55.0% among 14 lake-years) of all
Snail Kite nests that contained at least one egg fledged birds (Table 2). Most
failed nests were found abandoned (38.0%) prior to or immediately following
egg hatching for unknown reasons. Nest collapse was the second highest
Figure 2. Distribution of Snail Kite fledglings per nest for Lakes Kissimmee and
Okeechobee, FL, 1987–1993. Sample sizes are given in Table 1.
2007 J.A. Rodgers, Jr. 41
source of nest failure (18.6%), whereas predation was detected at only 0.7%
of nests. Predation was difficult to detect and may have occurred more
frequently than my data suggest.
Nesting success and lake parameters
Overall nest success (fledge 1 nestling) in woody species (49.2%) was
significantly greater (2 = 11.88, P = 0.008) than that in nonwoody species
(41.3%). Logistic analysis indicated water depth beneath the nest during the
incubation period (P = 0.87), year (P = 0.24), and nesting substrate (P = 0.62)
had no significant effect on probability of nest survival at Lake Kissimmee.
However, lake level during early incubation (P < 0.09) and nesting date (P =
0.06) was suggestive of a significant association with nest survivorship
(n = 227 nests). The equation for the estimated daily nest survival at Lake
S = 14.02 - 0.57 lake level - 0.01 nest date.
At Lake Kissimmee, 33 of 53 nests (62.3%) fledging 2 kites and 10 of 17
nests (58.8%) fledging 3 kites were initiated at lake levels > 15.00 m
Neither water depth beneath the nest (P = 0.11) nor type of nesting
substrate (P = 0.62) had a significant effect on the probability of nest
survival at Lake Okeechobee, whereas year (P = 0.08), lake level (P <
0.001), and hatch date (P < 0.06) were associated with nest survivorship (n =
414 nests). The equation for the estimated daily nest survival at Lake
S = 180.85 - 0.09 year + 0.82 lake level + 0.01 nest date.
At Lake Okeechobee, 62 of 99 nests (62.6%) fledging 2 kites and 17 of 20
nests (85.0%) fledging 3 kites were initiated at lake levels > 4.25 m (Table 3).
Table 3. Number of fledglings/nest for Snail Kites by 0.25-meter lake intervals (m above MSL)
at Lakes Kissimmee and Okeechobee, FL, 1987–1993. # = number of fledglings per nest.
a. Lake Kissimmee
# 14.00–14.25 14.26–14.50 14.51–14.75 14.76–15.00 15.01–15.2515.26–15.5015.51–16.00
0 3 3 5 27 31 34 13
1 2 0 2 14 15 15 3
2 6 0 4 10 8 18 7
3A 2 0 1 4 5 5 0
b. Lake Okeechobee
# 3.25–3.50 3.51–3.75 3.76–4.00 4.01–4.25 4.26–4.50 4.51–4.75 4.76–5.00
0 14 24 86 6 26 44 53
1 5 7 14 0 4 15 7
2 7 2 28 0 11 35 16
3 0 0 3 0 2 12 3
A1 nest produced 4 fledglings.
42 Southeastern Naturalist Vol. 6, No. 1
Mean clutch size of Snail Kites at Lakes Kissimmee (2.77) and
Okeechobee (2.81) was intermediate to previous reports (Beissinger 1986
[2.71], Snyder et al. 1989 [2.66], Sykes 1987a [2.92]). Whereas Beissinger
(1986) and Snyder et al. (1989) reported similar yearly clutch size despite
large annual variations in water levels, I found mean clutch size varied
among years at both lakes. I did find that within-year clutch size was less
variable between Lakes Kissimmee and Okeechobee. Similar within-year
clutch sizes at two lakes separated by 82 km may be due to female kites
responding to similar ecological conditions at these lakes or movement
among wetlands prior to breeding (Bennetts 1993, Bennetts and Kitchens
1997b, Rodgers and Stangel 1996). Both Snyder et al. (1989) and I found
little correlation between clutch size and lake levels.
Whereas clutch size exhibited little variation within each year, it did vary
among years, suggesting yearly responses to different feeding conditions at
Lakes Kissimmee and Okeechobee. Larger clutch sizes at Lake Kissimmee
were not associated with either early or late breeding seasons. However, the
larger clutch sizes associated with earlier breeding seasons at Lake
Okeechobee is similar to other findings (Murphy 1986, Perrins and
McCleery 1989). Because Snail Kites often exhibit mate desertion and are
capable of producing multiple broods during the same season (Beissinger
and Snyder 1987), renesting female kites may have obscured a seasonal
decline in clutch size during my study as noted in many avian species
(Conrad and Robertson 1993, Dijkstra et al. 1982, Järvinen 1993, Rowe et al.
1994, Snyder and Wiley 1976). Beissinger (1986) and Snyder et al. (1989)
have suggested that Snail Kites maintain a small clutch size (modal size of 3
eggs) to facilitate mate desertion of smaller broods during good years.
Overall mean fledging success of Snail Kites in my study (0.79 fledgling/
nest) was less than reported by Beissinger and Snyder (1987: 2.00), Snyder
et al. (1989: 1.96), and Sykes (1987b: 0.99). However, my observation that
44.1% of nests fledged 1 nestling is greater than the 21% reported by
Beissinger (1986) and 32.0% reported by Snyder et al. (1989). Adjustment
of brood size relative to current nutrient availability allows breeding birds to
fledge the maximum number of adequately nourished young. Most studies of
Snail Kite demography report variable fledging success among wetlands or
years (Beissinger 1986, Snyder et al. 1989, Sykes 1987b, this study), which
typically is lower during periods of low water (Sykes 1987b, this study).
However, Snyder et al. (1989) suggested that the lack of a relationship
between lake levels and fledging success in their study was because Lake
Okeechobee did not completely dry out as Everglade habitats farther south
did, which allowed more rapid re-population of the littoral region by
Pomacea paludosa Say (apple snails).
Snail Kites exhibited higher fledging rates and nest survivorship at the
recently colonized Lake Kissimmee compared to historically occupied Lake
Okeechobee. While there are no data for the amount of suitable foraging
2007 J.A. Rodgers, Jr. 43
habitat or snail populations for either lake, Lake Kissimmee is a smaller lake
and exhibits a narrower littoral zone compared to Lake Okeechobee. Perhaps
the more stable water levels on Lake Kissimmee during my study were
responsible for the higher fledging rates. Other avian studies have demonstrated
among-site (Coulson and Thomas 1985, Järvinen 1993), among-year
(Bancroft 1986, Järvinen 1989), or within-season (Skagen 1987) differences
in nestling survivorship and have suggested food resources as the proximate
factor in reproductive success (Martin 1987).
While Dreitz et al. (2001) concluded that kite nest success was not correlated
with water levels except with low-water conditions in a meta-analysis of
multiple studies and wetlands, Beissinger (1995), Beissinger and Snyder
(2002), and this study did find nest success was correlated with lake levels. I
found a greater frequency of nests with 2 and 3 fledglings at higher lake levels
at both Lakes Kissimmee and Okeechobee. This inconsistency among studies
may be due to differences in either model specification or covariates used
(Beissinger and Snyder 2002). I suggest that there may be differing effects of
water levels on productivity in lake versus Everglade habitats.
Snail Kites have nested at Lake Okeechobee since records have been kept
(Sykes et al. 1995, Sykes 1984), but they have only returned to nest at Lake
Kissimmee since the early 1980s (Beissinger and Takakawa 1983). The lack
of among-year variation in fledging rates at Lake Kissimmee may be due to
little among-year variation in lake levels maintained by water managers and
in habitat availability, while the variable fledging rates at Lake Okeechobee
reflects the more variable lake levels and resulting variation in available
foraging and nesting habitat among years. Future research should attempt to
identify the specific ecological factors (e.g., littoral zone vegetation, apple
snail abundance and accessibility, amount of foraging habitat) responsible
for the more variable among-year fledging success at Lake Okeechobee and
the basis for higher fledging rates at Lake Kissimmee.
Numerous individuals assisted with the field collection of data, especially S.
Schwikert, H. Smith, M. Delany, and B. Millsap. Statistical consultation and assistance
with computer analyses was provided by S. Linda and P. Kubilis. I thank T.
Shaffer for SAS codes and macros for estimating daily nest survival rates with the
logistic-exposure model. R. Bennetts provided information regarding Snail Kite
dispersal patterns in south Florida. I thank S. Nesbitt, B. Millsap, K. Meyer, and an
anonymous referee for reviewing earlier drafts of this paper. This study was part of
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