Composition of the diet of Netta peposaca (Birds: Anseriformes) in Santa Fe province, Argentina

SHORT COMMUNICATION

^IInstituto Nacional de Limnología (CONICET). Paraje El Pozo, Ciudad Universitaria, CP 3000, Santa Fe, Argentina. E-mail: [e-mail]

^IIFacultad de Bioquímica y Ciencias Biológicas, Universidad Nacional del Litoral (CONICET, UNL). El Pozo, CP 3000, Santa Fe, Argentina

^IIIFacultad de Ciencia y Técnica, Universidad Autónoma de Entre Ríos. CP 3100, Entre Ríos, Argentina

^IVFacultad de Ciencias Veterinarias, Instituto Multidisciplinario sobre Ecosistema y Desarrollo Sustentable (UNICEN). Buenos Aires, Argentina

^VFacultad de Humanidades y Ciencias, Universidad Nacional del Litoral. Paraje El Pozo, Ciudad Universitaria, CP 3000, Santa Fe, Argentina

ABSTRACT

We examined the diet of the Rosy-billed Pochard, Netta peposaca (Vieillot, 1816), (n = 59) during the autumn of 2010 in Santa Fe province, Argentina. Were analyzed a total of 59 ducks (n = 37 females and n = 22 males) to assess their gastrointestinal contents. The trophic spectrum was composed of 16 taxa (plant fraction: animal fraction 9 and 7). The contribution of each food category estimated using the IRI was: Echynochloa sp. (Poaceae) = 4301, Polygonum sp. (Polygonaceae) = 956, and unidentified graminoid species = 53 (females: Echynochloa sp. = 4313, Polygonum sp. = 1095 and males: Echynochloa sp. = 5522, Polygonum sp. = 814). No significant differences were found between females and males in diet composition (Fisher's test, p < 0.05). Moreover, the diversity index (H') was significantly (Mann-Whitney, p < 0.05) different between sexes. The Rosy-billed Pochard showed a predominantly phytophagous feeding habit in autumn. Likewise, this is the first contribution on the sexual differences in the feeding biology of the Rosy-billed Pochard.

Keywords: Stomach contents analysis; Rosy-billed Pochard; trophic composition.

An important approach to determining bird community structure is the study of trophic ecology (BELTZER 2003, REALES et al. 2009). Knowing how different avian species share and exploit resources helps us to understand of their distribution, abundance and diversity (OLGUÍN et al. 2013). The Rosy-billed Pochard, Netta peposaca (Vieillot, 1816) is a member of the Anatidae and suffers great pressure from hunting in Argentina (BLANCO et al. 2002). This bird nests among reeds in wetlands, mainly in an area that extends from the central provinces of Córdoba, Santa Fe, Entre Ríos, and Buenos Aires, southwards to Río Negro (NAROSKY & RUDA VEGA 2009).

In Argentina the Rosy-billed Pochard migrates to northern provinces and the bordering countries Brazil and Uruguay in autumn, (BELTZER & MOSSO 1992). The present study examined the feeding habits of the Rosy-billed Pochard. Specifically, we asked the following questions: (1) what is the composition of the diet of this bird in the Santa Fe province, Argentina? (2) are there any sex differences with regards to diet composition?

The study area is located in mid-eastern Argentina (Fig. 1), a part of the Espinal ecoregion (BURKART et al. 1999). Average annual rainfall in the area is 800 mm and the mean annual temperature is 18°C. The area is dominated by wetlands, remnants of fluvial forests, and intensively managed agricultural lands (cultivated with soybean, maize, wheat and rice) (ARTURI 2006). We selected two sampling sites with similar characteristic within the San Javier department, Santa Fe Province: Cañada de Blanche (30°31'18"S, 60°02'51"W) n = 12 (9 females and 3 males) and La Brava (30°27'19"S, 60°09'28"W) n = 35 (28 females and 19 males) (ARTURI 2006, PELTZER et al. 2006).

We surveyed the contents of fifty-nine stomachs obtained from Rosy-billed Pochard individuals (37 females and 22 males). Individuals were donated by tourist hunters during May and June 2010 (autumn period in Argentina). We studied the gastrointestinal contents using a stereomicroscope (Nikon 10x). After that, the contents were stored and preserved in 70% ethanol and were deposited in the collection of the Instituto Nacional de Limnología (INALI-CONICET-UNL), Santa Fe, Argentina. Food items were identified to the most detailed taxonomic level possible using a binocular microscope; the number of items per digestive tract was also recorded (BELTZER 2003, OLGUÍN et al. 2013). The trophic diversity (H) was calculated for each individual according to the method of HURTUBIA (1973) and using the formula of BRILLOUIN (1965): H = (I/N) (log₂ N! - S log₂ Ni!), where N is the total number of taxonomic entities found in the stomach of each individual and Ni is the total number of prey species i in each stomach. Individual estimations were added randomly to obtain the cumulative dietary diversity (Hk). Trophic niche breadth was calculated using LEVINS' (1968) index: Nb = (S Pij²) - 1, where Pij is the probability of the presence of item i in sample j. To represent the contribution of taxonomic categories to the diet, an index of relative importance (IRI) was used, following PINKAS et al. (1971): IRI = FO% (N%+V%), where FO% is the occurrence frequency, N% is the numeric percentage and V% is the volumetric percentage of each food category. The weight of each bird was recorded with a near balance Pesola^® scale Micro-Line. Food differences between sexes were analyzed by comparing the presence or absence of each prey taxon, according to Fisher's probability test (ATTADEMO et al. 2007). In addition, the rarefaction method (MAGURRAN 2004) was used to calculate richness (S) and the Shannon-Weaver Diversity Index (H') of species consumed by sexes using 1,000 random permutations in the EcoSim program (GOTELLI & ENTSMINGER 2007). Moreover, Mann-Whitney (U) tests were used to compare differences between the sexes according to the diversity (H') and richness (S) indexes.

The trophic spectrum of the Rosy-billed Pochard diet (n = 47) comprised 16 prey items, nine of vegetal origin and seven of animal origin (Table I). The trophic diversity per stomach ranged between 0 and 2.19 and cumulative trophic diversity was 1.23 (Fig. 2), with the curve reaching an asymptote at stomach number 11. The trophic niche breath was 0.98. The contribution of each food category estimated using the IRI, was: Echynochloa sp. (Poaceae) = 4301, Polygonum sp. (Polygonaceae) = 956, and unidentified graminoid species = 53. For females, food categories included were: Echynochloa sp. = 4313, Polygonum sp. = 1095, and unidentified graminoid species = 81, whereas for males, they were: Echynochloa sp. = 5522, Polygonum sp. = 814, and Setaria italic (Beauv, 1812) (Poaceae) = 86. The mean body weight ± SD of females and males ± was 1078.93 ± 90.36 g, and 1154.74 ± 86.59 g, respectively. The frequency of occurrence of all the prey items did not differ between males and females (Fisher's exact probability test, p < 0.05). Moreover, the diversity index (H') was significantly (Mann-Whitney, p < 0.05) different between the sexes (H' = 0.66 ± 0.40 for females and H' = 0.11 ± 0.30 for males). By contrast, species richness (S) was not different (Mann-Whitney, p < 0.05) by males and females (were S = 0.69 ± 0.40 and S = 0.79 ± 0.30).

The Rosy-billed Pochard showed a predominantly phytophagous feeding habit in autumn, with Echinochloa sp. and Polygonum sp. being the main food items. These results are consistent with findings reported by BELTZER & MOSSO (1992), who studied the diet of this species in a natural area of the Middle Paraná River. The dominant consumption of seeds in autumn is a behavior associated with the need to obtain carbohydrate-rich food, which is essential for meeting thermophysiological requirements in that season. Seeds readily provide a high energy supply, which is essential for alleviating temperature stress during that season (JORDE & OWEN 1988). Although the Rosy-billed Pochard has been indicated as a pest in rice fields (BUCHER 1984), no seeds of this crop were found in stomach contents in the present work. The absence of rice seeds may be related to the sampling period, which does not encompass the time of rice heading. The asymptote reached by the curve of cumulative trophic diversity indicates that the sample size was enough to identify the prey items consumed by the species during the study period. Niche breadth value was lower than the reported by ROZZATTI et al. (1994) because the items consumed were homogeneous and Echinochloa sp. was a numerically high item. Although that study was conducted in rice fields, the difference in the presence of rice reveals the species plasticity relative to the offer of other food resources. ROZZATTI et al. (1994) identified only eight items, whereas in the present study, the spectrum comprised 16 taxonomic entities. Furthermore, no significant differences in the frequency of occurrence of prey items between males and females were observed, suggesting that the prey items are equally available to both sexes in the environment. However, using rarefaction to control for the differences in the number of prey showed higher diversity of prey-items for females than for males. This finding may be attributed to the fact that females were more affected than males and needed to use other sources to complement their diet, but the information obtained is not enough to formulate a hypothesis. Further studies encompassing the entire annual cycle are necessary to complement the present data. The variety of food consumed in autumn reflects the plasticity in the Rosy-billed Pochard to exploit diverse food items according to habitat conditions and resource offer. Finally, the results of our study enhance the knowledge on the composition of the diet of Netta peposaca, which is important to conservation management to ensure stable and continuous population levels. Likewise, this is the first contribution to the feeding biology of the Rosy-billed Pochard discriminated by sex.

ACKNOWLEDGMENTS

We thank Walter Boeger for critical reading of the manuscript as well his helpful observations. We thank the hunters, who operated under the argentine national decree ratified by Law 4830 in Article 4, subsection a, Resolution 036, for donating the samples of the species studied. Our research was conducted according to the guidelines for use of wild animals in accordance with the established by the law "Protection and Conservation of Wild Fauna" (Argentina National Law 22421) and with authorization of the Ministerio de Aguas, Servicios Públicos y Medio Ambiente, from Santa Fe province, Argentina.

LITERATURE CITED

Submitted: 03.V.2013

Accepted: 26.IX.2013

Editorial responsibility: Diego Astúa de Moraes

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Composition of the diet of Netta peposaca (Birds: Anseriformes) in Santa Fe province, Argentina

Pamela F. Olguín^I; Andrés M. Attademo^II; Adolfo H. Beltzer^I; Ana L. Ronchi Virgolini^III; Rodrigo Lorenzón^I; Andrea Caselli^IV; Sofía Arce^V; Alejandro Percara^III

Publication Dates

• Publication in this collection
  21 Mar 2014
• Date of issue
  Feb 2014

History

• Accepted
  26 Sept 2013
• Received
  03 May 2013