Evaluation of the High Plains Mallard Management Unit in the Central Flyway

Prepared by
P. Joseph Gabig, Nebraska Game and Parks Commission, Lincoln
Howard D. Funk, Colorado Division of Wildlife, Fort Collins
James H. Gammonley, Colorado Division of Wildlife, Fort Collins
James K. Ringelman, Colorado Division of Wildlife, Fort Collins

July, 1997
Report to the Central Flyway Council

Executive Summary

History ox pulling a plow During the early 1960's, when duck seasons were very restrictive, the Central Flyway Council (Council) began an effort to identify if additional hunting opportunity could be provided. Data analysis demonstrated that several components of mallard population dynamics were different east and west of the 100th longitude. Experimental hunting seasons were conducted in the portion of the Flyway west of that line in 1968 and 1969 including a new way to establish the daily bag limit called the Point System. The results of these experiments and further analysis culminated in a report entitled Justification for the Central Flyway High Plains Mallard Management Unit . Beyond expansion of the use of the Point System, that report caused two other major things to occur in 1972: duck hunting regulations were established in the Central Flyway separately from the Mississippi Flyway and additional days for duck hunting were provided in the Central Flyway west of the 100th longitude on an operational basis. This additional hunting opportunity was directed at drake mallards. The High and Low Plains Mallard Management Units were established within the Central Flyway.

In 1988, in association with the publication of theSupplemental Environmental Impact Statement on The Sport Hunting of Migratory Birds (USFWS 1988), the US Fish and Wildlife Service (Service) asked the Central Flyway Council (Council) to review the High Plains Mallard Management Unit (HPMMU). The purpose of the review was to verify the original conclusions using current and historical data and new analysis methods and determine if the original objectives had been met. It was clear that the review would also serve to determine if the Unit and its associated hunting opportunity could be justified in the future. The Council responded in March, 1989 (Central Flyway Council meeting minutes) with a Position Statement and associated justification which included some basic harvest data analysis. In 1990, The Service asked the Council to conduct, with Service collaboration, a comprehensive analysis of all data pertinent to the HPMMU. This report is the final product of that analysis.

math symbols Underpinning this summary document are two separate, detailed reports with a similar title but with the additional annotation of "Part A " and "Part B ". Part A is sub-titled "Duck Population Parameters " and Part B "Duck Wintering Populations, Harvest and Hunting Effort " This summary document will briefly describe data types, sources and analytical approaches and will point the reader to Parts A and B for details. The emphasis here will be on the results of analysis and their relationship to the original justifications.

In general terms, this report has two objectives: to test all but one of the seven conclusions of the original justification using over twenty-five years of data and; to bring data not considered in the original justification to bear on the effect of the additional hunting opportunity provided in the High Plains on the mallard and other duck resource. But it goes beyond that to include analysis of data sets for the entire mid-continent mallard population and the primary areas where these birds are harvested. Some data sets for all ducks are included. The one conclusion of the original justification that was not considered for re-examination is the boundary between the High and Low Plains.

The area from which data were collected essentially encompasses the entire range of mid-continent mallards. Therefore, data were collected during and regarding all major aspects of their life cycle and includes measures of breeding population size and reproduction, banding data from both pre-season and winter, determination of migration routes, harvest characteristics, and winter population size, composition and distribution.

Band records from 578,000 mallards and 54,000 associated hunt season recoveries from the years 1963-89 for mallards banded in the winter in the Central Flyway and west tier states of the Mississippi Flyway were used to estimate survival rates of males and females. Band records from 588,000 mallards and 86,000 associated hunt season recoveries from the years 1962-89 for mallards banded pre-season for a large area of mid-continental North America were used to make additional, independent survival rate estimates. Breeding duck indices from strata 15-50 and the years 1951-1991 and duck production data from 1966-90 were analyzed for temporal changes. Duck population data from the Coordinated Mid-winter Surveys conducted in the Central and Mississippi flyways for the years 1964-91 were analyzed. Analysis of duck harvest data from 1963-90 from the Central and Mississippi flyways were conducted. The number of ducks taken per active adult hunter in a season for the years 1963-90 was considered. Hunter participation for the same years was considered using the number of active adult duck hunters and days hunted per active adult hunter.

two hunters standing in a marsh The general analytical approach for all analysis was to compare data values between various periods of regulatory changes both within and between harvest areas. The regulatory periods considered were: pre-High Plains (1963-68); stabilized regulations (1979-84) and; restrictive regulations (1985-90). Some data are presented for a post-High Plains period (1972-78) and the entire series of years. The hunting regulations that were in place during the stabilized regulations period are often considered to have been "liberal." Where possible, data are linked to harvest area. Generally, data are presented for four harvest areas: High and Low Plains of the Central Flyway and west tier states and the remainder of the Mississippi Flyway.

Three of the original justifications referred to a tendency of mallards to return to a wintering area and differences in the distribution of mallard band recoveries between harvest areas. Data are presented that affirm these original justifications. Mallards 'home' to the High Plains at a high rate: nearly 80% of males banded there and recovered / reported were reported to have been taken in the High Plains. Over 70% of the females banded there and recovered / reported were reported in the unit. For both sexes, less than 7% of the recoveries from High Plains bandings were in the Mississippi Flyway. There is also a clear affinity for birds to the Low Plains banding area: more than 60% of the recoveries occur within the banding area for both sexes. A similar statement can be made about mallards banded in the west Mississippi Flyway. Data show the larger relationship between the west Mississippi Flyway and the Low Plains than between the west Mississippi Flyway and the High Plains. There is evidence from pre-season bandings that the proportion of total recoveries that occurred in the High Plains and total Central Flyway is lower in a post-High Plains period than earlier.

One original justification addressed recovery and survival rates, indicating that "harvest pressure" was low in the High Plains and survival rates were high. This analysis showed that direct recovery rates from pre-season banded mallards (i.e. indices to harvest rates) were lower during the period of Stabilized Regulations than during a pre-High Plains period on a mid-continental scale. A more detailed analysis of long-term (1962-85) direct recovery rates showed rates for adult male mallards banded and mostly recovered in the Mississippi Flyway were 1.5 times higher than for adult males banded and mostly recovered in the High Plains. For adult females, the same calculation produced a value of 1.9: for immature males and females, the values are 1.9 and 2.1, respectively. During the 1962-75 period, the ratio of mean direct recovery rates between mallards oriented to the Mississippi Flyway and those oriented to the High Plains for adult females, immature males and immature females were 1.7, 1.6 and 1.9, respectively. During the period 1976-85, these same ratios were 2.2, 2.7 and 2.7 for adult females and immature males and females, respectively. It has been demonstrated that harvest rates of mallards during the 1962-85 period were substantially lower in the Central Flyway than in the Mississippi Flyway.

Two methods, both using winter banding data, were employed to test for differences and changes in survival rates of mallards. When the mean 1964-84 survival rate was compared, a significantly higher rate was found for males than females banded in the High Plains and Low Plains of the Central Flyway and the west tier of states in the Mississippi Flyway. Using Method 1, tests between survival rates during a pre- and a post-High Plains period indicated a significant decline in those rates for males banded in the High and Low Plains and for females banded in the High Plains. Similar results were obtained by using the composite z-statistic from Method 2 to test pre- and post-High Plains survival rates except that a significant decline was also detected for females banded in the Low Plains. Using the weighted t-statistic from Method 2, a significant decline in survival rates was also detected for females banded in the west Mississippi Flyway. Using survival rates from Method 1, no significant differences were found in survival rates when comparing mean survival rates for the three periods 1964-68, 1971-84 and 1979-84 between the same banding areas above for either sex (e.g. mean survival rates for males banded in the High Plains between 1971-84 were not significantly lower than the 1971-84 mean rate for males banded in either of the two other areas).

silhouette of a mallard in flight

Long term (1963-1988) survival rates using pre-season banding data from areas encompassing southwest Alberta, south central Colorado, Michigan/Illinois and southwest Manitoba were calculated. The long term, broadly based, average survival rate for adult male mallards (67.4%) was significantly higher than the rate for adult females (59.2%). Immature male and female mallards had long-term, average survival rates (58.1% and 57.6%, respectively) that were similar to one another. For males, the long-term, average survival adult rate (67.4%) was significantly greater than that for immatures (57.9%). The same rates when compared for adult and immature females (59.5 and 57.6%, respectively) were also significantly different.

Average survival rates for four age/sex cohorts for a pre-High Plains (1963-70) period were compared to rates for the Stabilized Regulations period (1979-84). The composite z value that tested for higher rates during the pre-High Plains period was not significant for either adult (67.7% vs. 67.0%) nor immature males (56.4 vs. 60.0). Similar results were obtained for adult females (60.3% vs. 60.6%) and immature females (58.1% vs. 55.8%). Similar tests were conducted between the pre-High Plains period and a Restrictive Regulations period (1985-88) with a similar null hypothesis. Similar results were obtained: no test produced a significant composite z value. The mean survival rate values for the two periods were: for adult males, 67.7% vs. 68.3; for immature males, 56.4% vs. 66.9%); for adult females, 60.3% vs. 59.8%; and for immature females, 58.1% vs. 63.1%. Survival rates were not greater during a pre-High Plains period than either of two latter periods for any age/sex cohort of mallards.

In summary, it is shown that harvest rates for mallards are lowest in the High Plains and greatest in the Mississippi Flyway and intermediate in the Low Plains. Survival rates from winter banded mallards were shown to be similar for the High and Low Plains. A temporal change was detected in the survival rates of winter banded male and female mallards that were banded in the High and Low Plains. No change in survival rates was detected for four mallard cohorts banded pre-season in a large area of North America.

Although no formal test was conducted, the estimate for female survival rate from winter banding data for the pre-High Plains period reported in the original justification document of 73% is considerably higher than the 61% reported here from Method One and higher than the 68% from Method Two. The likely reason for these differences is that different methods were used to make the estimates. Taken literally, Justification 3 was only partially verified. Most likely, one reason is the use of different analytical techniques. In the original justification, it was suggested that mallards winter banded in the Low Plains had a lower survival rate than those banded in the High Plains during a pre-High Plains period. This was not found to be the case using formal tests here and Method 1 survival rates. In fact, no difference in survival rates within any of the three periods investigated was found between mallards banded in any of the three band areas (e.g. average survival for pre-High Plains banded mallards in the High Plain was not different for birds banded in the same period in the other two harvest areas included). The analysis of data that was not included in the original justification (pre-season bandings) indicates that mallard survival rates are relatively constant over time and landscapes. A discussion of these data and related studies is provided. A conclusion of that discussion is that survival rates of mallards do likely vary temporally and geographically, particularly for males, but are generally constant across the broad region used by mid-continent mallards. Further, it is concluded that mallard harvest rates are temporally and geographically variable and do not change in association with survival rates in most cases.

Beyond the larger picture painted for Mid-continent mallards, a more detailed analysis of direct recovery rates for mallards oriented to the Central and Mississippi Flyway was conducted. It was determined that direct recovery rates (indices to harvest rates) were lowest for mallards oriented to the west Central Flyway and highest for mallards oriented to the Mississippi Flyway. Further, it was shown that as the influence of hunting in the Mississippi Flyway increased, so did the direct recovery rate. It was concluded that the different recovery rates reflect considerable differences in harvest rates. It was shown that the hunting regulations in the Central Flyway and in particular in the High Plains portion, were considerably more liberal than in the Mississippi Flyway. It follows that there are factors beyond hunting regulations (days and daily bag limit) that affect mallard harvest rates.

a prairie marsh

One justification pointed to "about 75%" of the duck hunters in the Central Flyway occurring in the Low Plains but the "distribution" of mallards was similar between the High and Low Plains. It further indicated that the proportion of the total duck harvest that was mallards was higher (60%) in the High Plains than in the Low Plains (40%). Duck stamp sales, numbers of active adult hunters, January mallard inventories, percentage of the annual duck harvest composed of mallards, and wintering mallards per hunter were compared for the High Plains and Low Plains portions of the Central Flyway, and the west tier and east tier of the Mississippi Flyway, during four regulatory periods. The four periods were: pre-HPMMU (1963-67), early-HPMMU (1970-78), Stabilized Regulations (1979-84), and Restrictive Regulations (1985-90). Temporal trends in these measures during the 1970-90 HPMMU period were examined using linear regression.

During each period, the Low Plains accounted for >70% of duck stamp sales and numbers of active adult waterfowl hunters in the Central Flyway. During 1970-1990, numbers of active adult hunters declined in all areas but declined at a steeper rate in the Low Plains, West Tier, and East Tier than in the High Plains.

Average January indices of mallards did not differ between the High Plains and Low Plains during any period. January indices of mallards in the East Tier also did not differ from those in the High Plains and Low Plains, but January indices of mallards were higher in the West tier than in other areas during all periods. During 1970-90, January indices of mallard populations declined in the High Plains, Low Plains, and East Tier but slopes did not differ among these areas. The January inventory of mallards showed no linear trend in the West Tier.

Mallards comprised over half of the average annual duck harvest in the High Plains, and about one third of the average annual duck harvest in the Low Plains. Excluding Texas data from the Low Plains, mallards comprised an average proportion of 43% of the total duck harvest during the pre-HPMMU period, 48% during the early HPMMU period, 51% during the Stabilized Regulations period, and 53% during the Restrictive Regulations period. Within the High Plains, Low Plains, and West Tier, mallards comprised similar proportions of the total annual duck harvest among all periods; within the East Tier, mallards comprised a smaller proportion of total annual duck harvests during the pre-HPMMU period than during other periods. During each period, mallards comprised a larger proportion of total duck harvests in the High Plains than in all other areas. During 1970-90, the proportion of mallards in the total seasonal duck harvest increased linearly in the East Tier and showed no linear trend in the High Plains, Low Plains or West Tier.

a black and white of a mallard pair

The average number of wintering mallards per hunter was greater during the pre-HPMMU period than during all other periods in the High Plains and Low Plains. However, there were more wintering mallards per hunter in the High Plains than in the Low Plains and other areas during all periods except for the West Tier during the Restrictive Regulations period. The number of wintering mallards per hunter declined in the High Plains and the Low Plains, increased in the West, and showed no linear trend in the East Tier. Slopes of decline did not differ between the High Plains and the Low Plains.

Data not included in the original justification document was investigated to determine if effect from establishment of the High Plains Unit occurred. Analysis considered Breeding Population Indices (BPI) between 1965 and 1991 and Production Indices between 1966 and 1990. Breeding populations of mallards that could be affected by harvest in the High Plains of the Central Flyway have not been negatively affected by the provision of a greater amount of hunting opportunity in the High Plains than occurred in the Low Plains. In addition, the daily bag limit for mallards in the entire Central Flyway (which was more liberal than in the Mississippi Flyway for most of the period studied) had no greater affect on the mallard BPI than the daily bag limit in other harvest areas. Data for the Mississippi Flyway offers little evidence that the mallard breeding population index for areas that provide mallards for harvest there responded in a different manner than for areas that provide mallards to the High Plains or the entire Central Flyway.

The greater hunting opportunity provided in the High Plains when compared to the Low Plains - opportunity intended to be directed toward mallards - did not have a negative effect on the Breeding Population Index for other duck species considered as a group (ducks other than mallards). Further, the relatively liberal daily bag limit for mallards provided in the entire Central Flyway for most of the period studied did not have a negative effect on other duck species.

A decrease in the number of ducklings was detected for all data sets and there is evidence that it changed in a similar manner for data sets associated with all harvest areas. The slope of the regression line for nine data sets is significant and negative. T-tests comparing the two ten-year means (1966-75 and 1981-1990) showed that the latter mean was significantly different for all data sets. The percent change between the two means ranged from -30 to -60 percent. It is concluded that the number of ducklings in survey strata associated with the High Plains Mallard Management Unit did not change in a manner differently than in strata not associated with the area. Further, a similar statement can be made for the Low Plains portion of the Central Flyway and for the Flyway as a whole.

Brood size was shown to have decreased across nearly all areas considered and the change has been similar in areas associated with the High Plains and Low Plains data sets. The slope of the regression line for nine data sets is significantly different than zero: slopes are negative for eight of the nine data sets. T-tests comparing the two ten-year means (1966-75 and 1981-1990) showed that the latter mean was significantly different for eight data sets. For these eight data sets, the percent change ranged from -7 to -16 percent. It is concluded that brood size associated with areas that contribute ducks to the High Plains Mallard Management Unit has not changed in a manner different than the brood size from most other areas. Further, the conclusion is extended to the Low Plains portion of the Central Flyway and the Flyway as a whole.

One justification in the original report, which was based on data and methods not accessible at this time, was unable to be addressed. Further, the affirmation of this justification is not considered critical to the affirmation of the effect and value of the High Plains Unit. Another justification in the original report is a statement about the value of establishing the High Plains Mallard Management Unit. It is based as much or more on philosophy than on data. That philosophy is restated and affirmed in this report.

Due to the large number of data types that relate to the entire life cycle of mid-continent mallards, many detailed conclusions were made in preparation for this final (summary) report. There isn't a good set of criteria to determine which conclusion might be more important than another. In many instances, it was simply determined that the same processes are going on within the mid-continent mallard population indicated by other researchers in the past. However, most conclusions contained in this report relate to a temporal change - specifically to a comparison of a pre-High Plains period to various post-High Plains periods.

All these findings, when viewed collectively, clearly point to an affirmation of the original justifications for establishment of the High Plains Mallard Management Unit. In summary, those justifications indicated that additional hunting opportunity could be provided in the High Plains than was available in the Low Plains without jeopardizing the health or status of the duck and, in particular, the mallard resource. This, as it turns out, is the Goal of Central Flyway species and population management plans.

A sunset

While this and the associated documents affirm the integrity and value of the High Plains Mallard Management Unit, the authors believe that they also show that some recent duck hunting seasons may have been more restrictive than they needed to be in the Central Flyway. In addition, this work has suggested a series of recommendations for future actions and research.