Feeding Ecology and Development of Juvenile Black Ducks in Maine

-Data from 41 juvenile Black Ducks (Anas rubripes) collected in the Penobscot River valley of Maine from June through August 1974-76 were used to estimate the proportion of aquatic invertebrates in the prefledging diet and the allometric growth rates of the tarsi, flight muscles, and alimentary system. The proportion of aquatic invertebrates in the diet of downy and partially feathered juveniles averaged 88 and 91% of dry weight, but decreased to 43% for fully feathered young. The most important invertebrate food organisms for juvenile Black Ducks were asellid isopods, molluscs, nymphs of Ephemeroptera and Odonata, and larvae of Coleoptera, Trichoptera, and Diptera. A high proportion of invertebrates was consumed during the period of fastest absolute and relative growth. Estimation of allometric growth rates with the power formula (Y = a X&) showed that (1) the legs were relatively large at hatching and developed slowly; (2) the flight muscles, which were relatively small at hatching, grew slowly until the 4-week period preceding fledging, when they increased as the 4.75 power of body weight; and (3) growth of the liver and gizzard was approximately proportional to body weight. The data support Ricklefs' thesis that delayed functional maturity of the wings permits an increase in the overall growth rate of waterfowl. Received 12 January 1979, accepted 6 August 1979. BECAUSE the components of avian reproductive strategy (e.g. nest site, clutch size, mode of development) are interdependent, it is necessary to consider the requirements of the young with those of the parent birds (Cody 1971: 473). The same is true of energy relationships; adults and young must be treated as a single unit during the period of parental care (Ricklefs 1974: 268). These concepts have stimulated increased interest in the growth and energy requirements of young birds (e.g. Ricklefs 1975). Before such a holistic approach is possible for the Black Duck (Anas rubripes) or other waterfowl, additional data must be collected. Information on the food requirements and development of the young is essential. Early reports on the foods used by juvenile waterfowl were based on limited collections (Collias and Collias 1963) or on techniques biased toward the less digestible food items (Swanson and Bartonek 1970). Nevertheless, the importance of aquatic invertebrates for the young of several species of dabbling (Anatini) and diving (Aythyini) ducks has been established (Bartonek and Hickey 1969, Sugden 1973). Limited data available for juvenile Black Ducks suggest that aquatic invertebrates may be an important food (Mendall 1949: 82-83), but the timing and extent of invertebrate use are unknown. The objectives of the present paper are to quantify the proportion of freshwater invertebrates in the diet of juvenile Black Ducks and to describe the allometric growth of the tarsi, wing musculature, and alimentary system. STUDY AREA AND METHODS Juvenile Black Ducks were collected in the lower Penobscot River valley, within 55 km of Orono, Maine. Undeveloped areas of the Penobscot valley currently support spruce-fir and maple-beech-birch 1 Migratory Bird and Habitat Research Laboratory, U.S. Fish and Wildlife Service, Laurel, Maryland 20811 USA. 737 The Auk 96: 737-745. October 1979 This content downloaded from 157.55.39.253 on Wed, 08 Jun 2016 06:23:48 UTC All use subject to http://about.jstor.org/terms 738 KENNETH J. REINECKE [Auk, Vol. 96 forest types (Ferguson and Kingsley 1972). The soils of this area developed in a stony till of variable thickness overlying relatively insoluble Paleozoic rocks (Doyle 1967). As a result, waters of the lakes and ponds are dilute, and total alkalinities rarely exceed 20 ppm (as CaCO3) (Mairs 1966). Approximately 25 beaver ponds and sedge-meadow streams were used as collection sites during the study. These wetlands are classified as Types 3 and 4 (Martin et al. 1953), or as subclasses of deep and shallow marsh associated with seasonal flats, shrub swamps, and bogs (Golet and Larson 1974). Although I tried to observe a period of feeding activity before each collection (Swanson and Bartonek 1970), this was at times impossible because of low Black Duck densities and poor visibility in Maine wetlands. Of the 41 juveniles collected from June through August 1974-76, 88% contained more than 5 food items in the esophagus and were included in the analysis. Approximately equal numbers (n = 13, 12, 11) of downy (class I), partially feathered (class II), and fully feathered (class III) juveniles were collected. The class III sample included both flying and flightless young because flighted juveniles were impossible to distinguish when feeding. The food from each esophagus was preserved in 75% ETOH in the field, and the carcasses frozen. In the laboratory, I identified the food items using Martin and Barkley (1961), Pennak (1953), and a reference seed collection. The dry weight of each food was measured to the nearest 0.1 mg after drying for 48 h in a convection oven at 50-55?C. I chose dry weight as the principal measure of food importance in this study because differences in the moisture content of seeds and invertebrates bias volumetric and wet weight data (Sugden 1973), and because dry weight has a more direct nutritional and energetic interpretation. Statistical treatment of food-use data is often difficult; an alternative is to present the results in several formats, each of which reduces certain biases. For this study I calculated food importance as (1) percentage occurrence, (2) aggregate percentage, and (3) aggregate dry weight. Swanson et al. (1974b) reviewed the use of these terms. Aggregate dry weight is the total weight of a food item in a sample of birds divided by the weight of all such food items; aggregate percentage is the proportion of a given food item in each bird averaged over all birds in a sample. Specimens frozen during the summer were later dissected. The age of each bird was estimated by reference to plumage descriptions (Gollop and Marshall 1954). To quantify juvenile development, I obtained wet weights to the nearest 0. 1 g for the gizzard, liver, flight muscles (pectoralis, coracobrachialis, and supracoracoideus), and whole body. I compared the differential growth of organs and tissues with allometric growth constants as described by Ricklefs (1975) and Brody (1945: Ch. 15). RESULTS AND DISCUSSION Juvenile food use.-The importance of aquatic invertebrates for three age classes of juvenile Black Ducks is summarized in Table 1. Of the three statistics, percentage occurrence is least informative because of the use of both animal and plant foods by nearly all birds. Agreement between the aggregate percentage and aggregate dry weight estimates suggests that the analysis has not been seriously biased by large food volumes in a small number of birds. All three statistics show a transition from consistently high invertebrate consumption in classes I and II to a diet of mixed composition during class III. The significance of the decline in the aggregate percentage use of invertebrates during class III was tested with a one-way ANOVA. The F-statistic was significant (F = 9.55, df = 2,33, P < 0.001), and a Student-Newman-Keuls test (Sokal and Rohlf 1969: 239) showed that invertebrate usage by class III young was less (P < 0.01) than that of classes I and II; 95% confidence limits for invertebrate consumption were: class I, 83.6-100%; class II, 87.4-100%; and class III, 10.3-80.6%. Important invertebrate foods consumed by class I birds were Diptera, Trichoptera, and Ephemeroptera (Table 2). The dipterans were larval, pupal, and adult midges (Chironomidae) and mosquitoes (Culicinae). For about 10 days after the ducklings hatched, most of their prey were adult or emerging insects captured at or above the water surface. Dry weight consumption of trichopteran larvae and ephemeropteran nymphs This content downloaded from 157.55.39.253 on Wed, 08 Jun 2016 06:23:48 UTC All use subject to http://about.jstor.org/terms October 1979] Juvenile Black Duck Feeding Ecology 739 TABLE 1. The importance of aquatic invertebrates in the diet of juvenile Black Ducks collected in Maine.