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Survival, Recovery, and Reproductive Success of Mottled Ducks on the Upper Texas Coast
Trey McClinton, Heather A. Mathewson, Stephen K. McDowell, and Jared D. Hall

Southeastern Naturalist, Volume 18, Issue 1 (2019): 53–64

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Southeastern Naturalist 53 T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 22001199 SOUTHEASTERN NATURALIST Vo1l8.( 118):,5 N3–o6. 41 Survival, Recovery, and Reproductive Success of Mottled Ducks on the Upper Texas Coast Trey McClinton1,*, Heather A. Mathewson1, Stephen K. McDowell2, and Jared D. Hall1 Abstract - Anas fulvigula (Mottled Duck) has experienced long-term population declines due to habitat loss and other anthropogenic factors. Our objectives were to (1) generate annual survival and recovery estimates, while examining the influence of age and sex, and (2) examine the influence of rainfall and drought on reproductive success. We followed the Brownie approach using the RMark package in R to analyze 4967 bandings and 705 recoveries from 2004–2015. We examined linear and curvilinear relationships between precipitation variables and a reproductive index. Hatch-year (HY) males had the highest annual recovery probability, while after-hatch-year (AHY) females had the lowest. Annual survival varied predominately by sex but also with age. Hatch-year females had the lowest estimate of survival, while after-hatch-year males had the highest. Total rainfall during peak nesting season showed a weak negative relationship with our reproductive success index (β = -0.0085, 95% CI: -0.0240, 0.0070), and was our only competitive model besides the null. Annual survival and recovery estimates were similar to other studies on Mottled Ducks. Our reproductive success analysis was inconclusive in that either there is no effect of precipitation or the measures we used for the reproduction index or the predictor variables were inadequate. Introduction During the 20th century, many North American waterfowl species experienced population decline in response to anthropogenic factors such as overexploitation and habitat loss through wetland conversion. Considering these declines, federal, state, and non-governmental organizations established regulations and began managing wetlands to promote waterfowl prosperity (Anderson et al. 2018). Most populations have recovered well, yet the status of others remain uncertain or below long-term goals (US Fish and Wildlife Service 2017). Today researchers utilize various modeling methods and surveys to monitor nest and brood-rearing success, as well as annual survival. These monitoring techniques provide data to determine the success of current management practices and suggest changes that may be needed to insure future prosperity. Anas fulvigula Ridgway (Mottled Duck) is a medium-large waterfowl species similar to Anas platyrhynchos L. (Mallard), and most closely related to Anas diazi Ridgway (Mexican Duck). Its continental range is isolated to 2 endemic populations, along the Western Gulf Coast (i.e., Texas, Louisiana, Mississippi, 1Department of Wildlife, Sustainability, and Ecosystem Sciences, Tarleton State University, Stephenville, TX 76402. 2Texas Parks and Wildlife Department, Port Arthur, TX 77640. *Corresponding author - mcclin73@msu.edu. Manuscript Editor: Frank Moore Southeastern Naturalist T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 2019 Vol. 18, No. 1 54 and Alabama) and Florida (McCraken et al. 2001), and an introduced population along coastal South Carolina (Shipes et al. 2015). Mottled Ducks exhibit a unique non-migratory characteristic that requires them to meet all life-cycle needs in the habitats and associated marshlands of these areas. This year-round dependency exposes the species to seasonal stresses that may not normally be associated with birds that migrate to and from breeding and wintering grounds. Potentially due to its limited range and additional stressors, the Mottled Duck is a species of historically small population size, relative to other dabbling ducks (e.g., Mallard; Wilson 2007). In addition, multiple factors have led to long-term population declines on the upper Texas coast in recent decades; including habitat loss due to urban development and changes in land use (Stutzenbaker 1988), sea-level rise and changes in hydrology (Moon 2014), low nest success in adjacent regions (Durham and Afton 2003), hybridization with Mallards (Stutzenbaker 1988), continued susceptibility to lead poisoning decades after the use of lead shot was banned for waterfowl hunting (McDowell et al. 2015, Wilson 2007), and low recruitment (Johnson 2009, Stutzenbaker 1988, Wilson 2007). Considering these influencing factors and historically low numbers, long-term population-monitoring protocols are in place so that we can observe the effectiveness of our adaptive management plans. Ballard et al. (2001) suggested that there might be substantially more Mottled Ducks in Texas than previously thought. However, significant negative trends have been observed in the number of Mottled Ducks detected in the US Midwinter Survey (1970–2003) and in the Breeding Bird Survey (1966–2002; Sauer et al. 2003), the number of Mottled Ducks killed per hunter per day (1966–2002; Martin and Padding 2002), and the number of Mottled Duck breeding pairs per square mile on national wildlife refuges along the upper Texas Gulf Coast (1985–2004; Wilson 2007). Banding programs are an important part of waterfowl conservation in North America (Blohm 2006, Haukos 2015, Smith et al. 1989). Though waterfowl banding efforts have been taking place since the early 20th century, it was not until 1994 and 1997 that extensive and standardized Mottled Duck banding programs were implemented in Louisiana and Texas, respectively (Haukos 2015). Banding data is used to observe movements, monitor harvest pressure and vulnerability for age classes, estimate species recruitment, and estimate annual survival rates (Haukos 2015). Due to mild weather conditions found within their range, the timing of Mottled Duck breeding is highly variable. Nests have been documented as early as February, with peak activity from March to May (Finger et al. 2003, Stutzenbaker 1988), and renesting attempts through August (Stutzenbaker 1988). Grand (1992) discussed the influences of weather conditions on Mottled Duck nest initiation. He noted initiation delays in association with years of low autumn and winter precipitation. Multiple studies (Durham 2001, Engling 1950, Grand 1992) have noted that in addition to affecting initiation date, heavy rainfall events cause nest failures through flooding. By monitoring rainfall and analyzing it in conjunction with banding data, managers can determine the effects of precipitation on recruitment. Considering these declines and influencing factors, the objectives for this project were to (1) determine survival and recovery probabilities for Mottled Ducks banded Southeastern Naturalist 55 T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 2019 Vol. 18, No. 1 on the J.D. Murphree Wildlife Management Area (WMA), using the best predicting variables, and (2) examine the influence of rainfall and drought on Mottled Duck reproductive success. We predicted that Mottled Duck recovery probabilities on the property would be higher in hatch-year (HY) birds, but not significantly affected by sex because the species is relatively monomorphic. We predicted that Mottled Duck survival probabilities would be influenced by sex due to additional hazards faced by females in nest incubation and brood rearing. We also hypothesized that years of excessively high and excessively low rainfall totals during peak nesting season would reduce recruitment. High rainfall results in flooding of nests, whereas low rainfall results in drought conditions that could negatively impact brood survival. Field-site Description The focal point for our study was the J.D. Murphree WMA near Port Arthur, TX. The WMA is ~9915 ha of fresh, intermediate, and brackish coastal marshes that have long been an important wintering habitat for various species of waterfowl and year-round habitat for Mottled Ducks. The J.D. Murphree WMA is located in the Chenier Plain where Mottled Duck population densities along the western gulf coast are the highest (Stutzenbaker 1988), and roughly 85% of all annual bandings have occurred since the early 2000s (Haukos 2015). Some data used in this study was also gathered from neighboring marshes, including the Anahuac and McFaddin National Wildlife Refuges, and a few privately owned properties. Methods Data acquisition We obtained banding data from the Texas Parks and Wildlife Department (TPWD) gathered by the J.D. Murphree WMA staff from 2001 to 2016. Biologists captured Mottled Ducks using rocket-nets (Dill and Thornsberry 1950), portable swim-in traps (Szymczak and Corey 1976), and nightlight airboat captures (Stutzenbaker 1988) every summer from June to September. Nightlight capture was the most successful method for catching birds. Due to the considerable amount of food resources available during the summer, the large number of flightless molting adults and juvenile birds have no need to utilize bait at trap and net sites. Upon capture, biologists aged and sexed Mottled Ducks by observing morphological and cloacal characteristics (Hochhaum 1942, Stutzenbaker 1988). They fitted birds with a uniquely numbered United States Geological Survey (USGS) size-7 aluminum leg band. After banding, they immediately released birds to minimize stress associated with being captured. We obtained all recoveries from USGS Bird Banding Lab in Laurel, MD, at the Patuxent Wildlife Research Center, using TPWD’s Master Banding Permit for Mottled Ducks banded from 2001 to 2016. We included only healthy, uninjured, wild-caught birds from banding surveys and dead recoveries obtained from the USGS Bird Banding Lab. Since the waterfowl hunting season falls on 2 calendar years, we set each hunting season from the fall of the year it began until 15 February of the following year to allow for late band-reporting from hunters. Southeastern Naturalist T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 2019 Vol. 18, No. 1 56 We obtained daily rainfall data from the National Oceanic and Atmospheric Administration’s (NOAA) Port Arthur City station (~5 km from the WMA) from 2001 to 2016. We obtained standardized precipitation–evapotranspiration index (SPEI) data from a digital CSIC database (Vicente-Serrano et al. 2010) for the same timeframe. SPEI data can be used to measure the intensity and duration of droughts and how they vary over time, so we used this as an index of marsh condition while Mottled Ducks were attempting to nest and rear broods. Data analyses We used the RMark package (Laake 2013) of R (Version 3.3.2, R Development Core Team, Vienna, Austria), which is an interface for Program MARK (White and Burnham 1999), to conduct survival and recovery analyses using the Brownie deadrecovery approach (Brownie et al. 1985). We lacked data to support a live–dead approach as we only had a total of 15 individuals recaptured outside of their banding year. Variables considered for both analyses included year, sex (male, female), and age class (hatch-year [HY], after-hatch-year [AHY]). The HY cohort includes both HY birds (first-year bird capable of flight) and local birds (first-year bird incapable of sustained flight) (Haukos 2015, Johnson et al. 1995). The AHY cohort includes any bird of at least 1 year of age. We used an information-theoretic approach for model selection (Burnham and Anderson 2002). We evaluated support for models using Akaike’s information criterion adjusted for small sample size (AICc). We derived a candidate set of 30 a priori models to test specific biological hypotheses. Our candidate model set included various additive and interactive combinations of the aforementioned variables, as well as main effects, a null model, and a general model. We adjusted for overdispersion using median ĉ (Cooch 2017). We considered models with ΔQAIC of less than 2 as competitive (Burnham and Anderson 2002). We used the top model to generate estimates of annual survival and recovery and respective standard errors. We also generated estimates by age class and sex. To determine how rainfall affects recruitment through reproductive success, we analyzed J.D. Murphree banding data in conjunction with rainfall data and SPEI data. We used the proportion of HY birds banded relative to AHY birds banded as an index of reproductive success (Peery et al. 2007). We tested correlations between the proportion of HY birds and the total birds banded and banding days to rule out any bias in our index with effort. We used linear regression and an information-theoretic approach to evaluate candidate models representing our a priori predictions about climatic influences on reproductive success. We determined support for a model using the AICc. We identified competitive models as those models with a ΔAICc of less than 2. We conducted these analyses in R (Version 3.3.2). Our prediction is that reproductive success would be low when conditions are excessively dry and excessively wet, resulting in a curvilinear relationship between some measure of precipitation and reproductive success. Thus, we evaluated 9 models that included the null model, and linear and curvilinear models that included 4 measures of precipitation: (1) total rainfall during the nest season (March–July), (2) total rainfall Southeastern Naturalist 57 T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 2019 Vol. 18, No. 1 during the peak nesting season (March–May), (3) the number of significant rainfall events during each peak nesting season, and (4) drought index during the nesting season. We defined significant rainfall events as any event (day) in which the measured rainfall was greater than the long-term average daily rainfall (1.32 centimeters for 2001–2016 at our study location). Results For our survival and recovery analyses, we omitted 3 years (2001–2003) due to small sample size (e.g., only 1 AHY bird was recovered in 2001) and 1 year (2016) because recoveries from the 2016–2017 hunting season was not available at the time of data acquisition (i.e., recoveries were truncated after the 2015–2016 hunting season). Ultimately, we analyzed 4967 bandings and 705 recoveries (Table 1) spanning 12 years (2004–2015). We adjusted for overdispersion using a correction factor of 2.99 as determined by median ĉ. Model selection uncertainty resulted in 5 competitive models, all of which included various combinations of the age and sex variables. The additive effect of bird age and sex best explained bird recovery (Table 2); probability of recovery was greater for HY birds, and for males within the age categories (Fig. 1). The main effect of sex was most influential on annual survival (Table 2), and our estimate of annual survival was greatest for males (0.628; Fig. 2). Using the additive effect of sex and age, survival is highest for AHY males (0.637) and lowest for HY females (0.387), but there is overlap in standard errors for all other age and sex categories (Fig. 3). For our reproductive success analyses, we used the 16 years of banding data acquired from TPWD. Since we were using proportions derived from banding data, the lack of recoveries would have no effect on our results, thus there was no need to subset. There was variation among years on the WMA in the number of birds (HY and AHY; min–max =117–1068), the number of days banding (min–max = 9–45), and the proportion of HY to AHY (min–max = 0.33 to 0.97). Model selection indicated that the null model outcompeted our predictive models (Table 3). Table 1. Banding and recovery totals for Mottled Ducks in J.D. Murphree Wildlife Management Area during years included in survival and recovery analyses. HY = hatch year, AHY = after hatch year. Year HY banded AHY banded HY recovered AHY recovered 2004 282 103 49 9 2005 401 63 64 10 2006 441 121 60 18 2007 99 18 24 10 2008 157 178 29 17 2009 697 371 64 29 2010 662 140 65 18 2011 141 45 47 15 2012 155 46 26 14 2013 257 97 46 8 2014 129 49 31 8 2015 235 81 33 11 Southeastern Naturalist T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 2019 Vol. 18, No. 1 58 Table 2. Model selection results for the top 12 Brownie-approach survival and recovery models of Mottled Duck banded pre-hunting season from 2004 to 2015 at the J.D. Murphree Wildlife Management Area in Texas. QAICc = Akaike’s information criterion with a correction for finite sample sizes and over dispersion (median ĉ correction), ΔQAICc = change in QAICc relative to the model with the smallest QAICc value, ω = model weight relative to the other models considered in this model set, K = number of parameters, sex = male or female, age = AHY or HY, and t = year. Model QAICc Δ QAICc ω K -2Log(L) S(sex) f(age + sex) 1919.599 0.000 0.271 5 5710.427 S(age + sex) f(age + sex) 1920.417 0.818 0.180 6 5706.878 S(sex) f(age) 1920.644 1.045 0.161 4 5719.546 S(age + sex) f(age) 1921.525 1.926 0.104 5 5716.188 S(sex) f(age*sex) 1921.588 1.989 0.100 6 5710.381 S(age*sex) f(age + sex) 1921.659 2.060 0.097 7 5704.595 S(age*sex) f(age) 1923.351 3.753 0.042 6 5715.654 S(age) f(age + sex) 1924.750 5.152 0.021 5 5725.832 S(sex) f(age + t) 1926.505 6.907 0.009 15 5671.018 S(age + sex) f(sex) 1927.063 7.464 0.006 5 5732.748 S(age + sex) f(age + t) 1927.532 7.933 0.005 16 5668.069 S(sex) f(sex) 1929.603 10.005 0.002 4 5746.338 Figure 1. Mottled Duck recovery es t i - mates (AHY F: 0.046, AHY M: 0.060, HY F: 0.080, HY M: 0.104) and standard errors, derived from the best-fitting model, by age and sex class from 2004 to 2015 at the J.D. Murphree Wildlife Management Area in Texas. AHY = after hatch year, H Y = h a t c h year, F = female, M = male. Southeastern Naturalist 59 T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 2019 Vol. 18, No. 1 Table 3. Model selection results for precipitation variables influencing an index of reproductive success (proportion of HY to AHY birds banded) of Mottled Ducks from 2001 to 2016 at the J.D. Murphree Wildlife Management Area in Texas. AICc = Akaike’s information criterion with a correction for finite sample sizes, ΔAICc = change in AICc relative to the model with the smallest AICc value, ω = AICc model weight, K = number of parameters, Peak season rainfall = total rainfall during peak nesting season (March–May), Season rainfall = total rainfall during nesting season (March–July), Significant event = number of days when rainfall total was greater than the long-term daily average (1.32 cm), and SPEI = standardized precipitation–evapotranspiration index. Model AICc Δ AICc ω K -2Log(L) Null -9.515 0.000 0.394 2 7.219 Peak season rainfall -7.972 1.543 0.182 3 7.986 Season rainfall -7.048 2.467 0.115 3 7.524 Significant event -6.686 2.829 0.096 3 7.343 SPEI index -6.510 3.005 0.088 3 7.255 Season rainfall non-linear -4.790 4.725 0.037 4 8.213 Significant event non-linear -4.649 4.865 0.035 4 8.143 Peak season rainfall non-linear -4.404 5.111 0.031 4 8.020 SPEI index non-linear -3.755 5.760 0.022 4 7.696 Figure 2. Mottled Duck survival estimates (F: 0.473, M: 0.628) and standard errors, derived from the best-fitting model, by sex class from 2004 to 2015 at the J.D. Murphree Wildlife Management Area in Texas. F = female, and M = male. Southeastern Naturalist T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 2019 Vol. 18, No. 1 60 The second competitive model suggested a linear, negative relationship between reproductive success and peak nesting season rainfall (β = -0.0085), but the 95% confidence interval overlapped zero (95% CI: -0.024, 0.007) suggesting uncertainty in the direction of the effect. Discussion Our study supported our predictions that recovery primarily differed by age, while survival primarily differed by sex. The influence of age on recovery is potentially due to the fact that older birds have been exposed to hunting, so they may be wiser to avoid suspicious bird congregations (i.e., decoy spreads), poor or excessive calling, and heavily hunted areas all together. We predicted that sex would not influence recovery because sexual dimorphism is less apparent in Mottled Ducks than other species, so targeting of males by hunters may not be as common. This targeting theory is supported in Johnson et al. (1992), where the males of obviously sexual dimorphic species are said to be subjected to greater hunting mortality. However, the slight influence of sex on recovery (Table 2) could perhaps be indicative of an underlying ecological factor that makes males more inclined to subject themselves to hunting danger. Survival was most influenced Figure 3. Mottled Duck survival estimates (AHY F: 0.495, AHY M: 0.637, HY F: 0.387, HY M: 0.531) and standard e r r o r s , d e - rived from the second bestfitting model, by age and sex class from 2004 –2015 at the J.D. Murphree Wildlife Management Area in Texas AHY = after hatch year, HY = hatch year, F = female, M = male. Southeastern Naturalist 61 T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 2019 Vol. 18, No. 1 by sex. This finding is likely due to the energetic costs and additional hazards that hens are exposed to during nest care and brood rearing (Johnson et al. 1992) and is commonly shown in studies comparing duck survival by sex (Franklin et al. 2002). Additional life experience leading to better survival likelihood could explain the moderate effect of age on survival (Table 2), and why Johnson et al. (1995) found age class to be an influential factor on Mottled Duck survival i n Florida. Our recovery estimates (Fig. 1) are similar the recovery rates exhibited in another western Gulf Coast population study (Haukos 2015). In Johnson et al. (1995), the authors found both AHY and HY male recovery probabilities that are comparable to our work; however, they found AHY and HY female recovery probabilities that are significantly lower than our estimates. Nichols et al. (1987) found relatively similar recovery probabilities in Mallards and American Black Ducks. However, Bartzen and Dufour (2017) calculated Anas Acuta L. (Northern Pintail) recovery probabilities that are significantly lower than our estimates. Our survival estimates (Figs. 2, 3) are relatively similar to those found in previous studies of the western Gulf Coast Mottled Duck population (Haukos 2015, Wilson et al. 2003) and other common Mallard-like species (Nichols et al. 1987). However, Northern Pintail and Anas Americana Gmelin (American Wigeon) exhibited higher estimates of survival in Bartzen and Dufour (2017) and Lake et al. (2006), respectively. These findings could potentially be attributed to the slow life-history strategies of these species leading to more cautious behavior (Ackerman et al. 2006). Our data did not support our hypothesis that low and high amounts of precipitation influenced reproductive success as measured by our reproductive index. Instead, there was a trend, although weak, suggesting a relationship between increased rainfall during peak nesting season and increased reproductive success. The lack of support for our hypothesis and for the linear trend could have been due to the broad range of values in our data set, inadequacy of our precipitation measures to capture the effects of precipitation, or bias in our reproductive index such that the index did not represent reproductive success. Directly monitoring nesting success and water levels would provide a more direct approach to addressing these hypotheses; however, the time and effort required to accumulate this data set over many years is often a limitation. Haukos (2015) included data from the J.D. Murphree WMA in his analyses of the Western Gulf Coast Population’s survival and recovery probabilities, but to our knowledge, this study is the first to exclusively examine a long-term data set from this area. So, this work provides estimates most indicative of the Mottled Ducks on the J.D. Murphree WMA. That being said, the daily Mottled Duck movements between the associated marshlands make these estimates plausible indices for all of the upper Texas Coast. The comparison of our study results to those encompassing larger regions (e.g., the western gulf coast) indicate that Mottled Ducks on the J.D. Murphree WMA and surrounding areas are potentially representative of other populations. However, declaring our results to be anything more than an index or representation of larger scale populations would be extrapolating beyond the scope of our study area. Southeastern Naturalist T. McClinton, H.A. Mathewson, S.K. McDowell, and J.D. Hall 2019 Vol. 18, No. 1 62 Also, to our knowledge, this is the first study to model the proportion of HY birds banded as an index of reproductive success against various measure of precipitation to predict recruitment in Mottled Ducks. Though we did not find a suitable model, we provided an avenue for future works to explore different precipitation measures or other indices of reproductive success. Upon finding a suitable variable and index combination, the J.D. Murphree WMA managers will be able to determine yearly Mottled Duck reproductive success and trends more precisely than current monitoring allows. Acknowledgments We thank Shaun Oldenburger, Master Bander with TPWD, for use of the banding data collected under his permit. We thank Michael Rezsutek, the TPWD Waterfowl Program, and the many TPWD biologists, fish and wildlife technicians, and interns who spent countless hours in the field collecting data. We thank Brendan Shirkey of the Winous Point Marsh Conservancy, for his advice and guidance with manuscript composition. 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