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Breeding Success of Rostrhamus sociabilis (Snail Kites) at Two Florida Lakes
James A. Rodgers, Jr.

Southeastern Naturalist, Volume 6, Number 1 (2007): 35–46

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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. Introduction 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. Study Area 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. Methods 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 Kochert 1982). 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 Results Clutch size 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 Kissimmee was: 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 (Table 3). 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 Okeechobee was: 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 Lake level # 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 Lake level # 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 Discussion 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. Acknowledgments 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 the Florida Fish and Wildlife Conservation Commission’s studies 7520 and 7524. 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