Recruitment of mottled ducks (Anas fulvigula) on the upper Texas gulf coast
Rigby, Elizabeth Anne
MetadataShow full item record
Surveys of the mottled duck (Anas fulvigula) population in Texas have shown steep declines over the last 20 years. The causes of the population decline are not well understood, but low recruitment has recently been hypothesized. Female survival during the breeding season has not been widely discussed as a contributing factor, even though it plays an important role in population dynamics. I investigated mottled duck recruitment as well as female survival during the breeding season and gathered information on habitat use to guide management decisions. My objectives were to (1) estimate weekly survival rates of mottled duck females throughout the breeding season, (2) estimate the daily movement and home range of mottled duck females during the breeding season, (3) estimate daily survival rates of ducklings and estimate mottled duck recruitment, and (4) estimate daily movements and home range of mottled duck broods. My study area was the Texas Chenier Plain National Wildlife Refuge (NWR) Complex, which was comprised of Anahuac, McFaddin, Texas Point, and Moody NWRs, with most research occurring on Anahuac NWR. I caught female mottled ducks in decoy traps and baited swim-in traps on Anahuac NWR from 1 February to mid-April or early May, 2006-2008. Females were fitted with back-mounted radio transmitters and tracked through the end of the breeding season, 30 June. I used 2 tracking methods: remote locations, where I used observer coordinates and bearings to the female to triangulate female locations, and visual locations where I tracked the female until seen. I captured 15 females in 2006, 12 in 2007, and 20 in 2008. Of these, 2 died in each of the first 2 years and 3 in the third year due to subsequent trapping stress and were removed from analyses. Survival of a female in each week was documented either with a visual location or a remote location that indicated obvious movement since the last location. I estimated weekly female survival using the Known Fate procedure of Program MARK (White and Burnham 1999) and tested 5 a priori models with a body condition index (body mass (g) / wing chord (mm) ) included as a covariate. Models were evaluated based on Akaike’s information criterion corrected for small sample size (AICc) values. Female survival estimates for the entire 19-week breeding season were 75.5% for the top model, range 63.3% -87.2% for other models with ÄAICc less than 4. Mottled duck nesting propensity was similar for 2006 and 2007 but increased in the third year. There is some evidence that breeding season survival of female mottled ducks decreased when nesting effort increased. Females showed the highest nesting propensity (62.5%) in 2008, the year with the lowest female survival rate. For all years, weekly survival for females was the same during the breeding season as it was for the remainder of the year. Although nesting may carry increased risk, as signaled by the decrease in female survival when nesting effort increased, nesting rates were so low that the weekly survival rate was not reduced significantly during the breeding season. Low nesting propensity appeared to be linked with poor habitat conditions such as drought and high marsh salinity. The links among nesting propensity, female survival, and habitat suitability, suggests that a “trade-off” is at play for female mottled ducks. I also used visual and remote locations of mottled duck females to calculate minimum daily movements. Large dispersal movements (> 10 km) were excluded from daily movement and home range calculations. I calculated home ranges for females using 95% adjusted-kernel density estimates. In 2006, 7 females dispersed > 10 km from the study area, moving an average of 31.9 km. Dispersing females moved to inland freshwater impoundments in a year of extreme drought, indicating dispersal is influenced by weather and habitat conditions. Dispersal was precluded during drought in 2008 by presence of new local refugia. Females that nested moved less than females that did not. Home range size did not differ between breeding versus non-breeding females or among years, suggesting female home ranges were limited by habitat patch size. To estimate duckling and brood survival, movement, home range, and habitat use, I tracked radio-transmittered ducklings and radio-transmittered females with broods. I caught mottled duck broods from airboats in 2006, 2007, and 2008. I randomly selected 2 ducklings from each brood and fitted them with radiotransmitters. I captured 7 ducklings from 4 broods in 2006, 43 ducklings from 23 broods in 2007, and 9 ducklings from 5 broods in 2008. I used duckling visual and remote location data to estimate daily duckling and brood survival using the Known Fate procedure of Program MARK (White and Burnham 1999). The top models for both brood and duckling survival were constant survival among days and years. My duckling survival estimate was greater than expected (66.9% for the 30-day period) and was high compared to a previous estimate of 41.1% in Texas, as well as compared to estimates of duckling survival for closely related species. Although duckling survival was high in my study, recruitment estimates were low. To maintain a stable population, I estimated that a recruitment rate of R = 0.91 is needed. When recruitment was calculated using existing nest success estimates, all but 1 recruitment estimate were smaller than R = 0.91, implying negative population growth. By comparing mottled duck recruitment parameters to mallard parameters, I confirmed that low hen success, determined by nest success and nesting propensity, was largely responsible for low mottled duck recruitment. Duckling locations and female with brood locations were also used to calculate minimum daily brood movements and brood home ranges using 95% adjusted-kernel density estimates. Brood movements differed among years, but brood home range size did not because habitat patch size was not limited by water level. Instead, human created barriers (roads, levees, bayous), dry zones, and areas dominated by heavy, impassable vegetation formed physical barriers to duckling movement. There was only 1 observed brood movement between habitat patches. The isolation of broods within habitat patches may increase risk to broods in years of severe drought. Movements were greatest in 2007, the year with the greatest rainfall and the only year without a drought period. Movements and home range were similar for broods in moist soil units (MSUs) and broods in coastal marsh. The cornerstone of mottled duck management must be the maintenance of a trio of habitats necessary for their life cycle: nesting habitat, brood rearing habitat, and adult habitat. Low observed nesting propensity and previously low reported nest success indicate increasing nesting habitat area and quality should be a prominent goal of habitat management. Location of nesting areas is as important as habitat quality. Nesting areas need to be adjacent to areas suitable for broods. Predator control in mottled duck nesting areas has not been widely pursued on the gulf coast but should be carefully considered. Current recommendations for desired brood habitat characteristics, such as disturbed, broken marsh, were largely supported by habitat use of mottled duck broods in my study. Managers should encourage connectivity of brood areas to give broods the widest possibilities for habitat selection. In managing habitats for stages of the mottled duck life cycle, managers should be particularly attuned to the effects of drought. Local refugia during drought are critical as adult mottled duck habitat and can reduce the need for dispersal of adult mottled ducks from coastal refuges.