CONICET | Buscador de Institutos y Recursos Humanos

The Snowy-crowned Tern (Sterna trudeaui), an endemic species of southern South America, can be found throughout the year at Mar Chiquita Reserve showing great affinity to estuarine and freshwater systems. The information on the biology of this species is very scarce in spite of its wide distribution. Studies conducted on different Sternidae species have shown that the activity patterns can be affected by environmental factors. In this study, we present novel information on the diet of the Snowy-crowned Tern and analyze the effect of environmental variables on its foraging behavior at Mar Chiquita Reserve. The study was conducted between December 2003 and May 2004. Behavioral observations were periodically taken with binoculars (10x50), scope (20-30x), and tape recordered. Observations of randomly taken individual birds were made as focal sampling. In each observation, date, starting time, tide level, cloud cover, water surface condition, and wind intensity were recorded. The observations on foraging effort were split in three periods: morning (before 11:00 h local time), noon (11:00 - 15:00 h) and afternoon (after 15:00 h). Foraging effort was estimated for all samples (N = 147) and the success rate was estimated excluding the samples where there was uncertainty about prey capture (N = 42). Prey length was estimated by comparing the fish length with the bill length. Our results are in agreement with previous studies describing Snowy-crowned Tern eating small fish. From the 84 fish prey observed, 24% were identified as Odontesthes sp. (Silversides), 5% as Paralichthys sp. (Flatfish), while the rest remained undetermined. Most of fish prey (55.7%) were smaller than 35 mm, 25.7% were between 35 and 50 mm in length, and the remaining 18.6% were larger than 50 mm. The average foraging effort was 1.26 ± 0.93 plunges min-1, and the average success rate was 0.76 ± 0.52 captures min-1. Foraging effort differed significantly between daytime periods (F2-144 = 18.82 P < 0.001), being significantly higher during the morning (1.77 ± 1.15 plunges min-1) compared to noon (1.01 ± 0.64 plunges min-1) and to afternoon (0.84 ± 0.38 plunges min-1) (Tuckey post-hoc comparisons, P< 0.05). Foraging effort and success rates tended to increase with tide level (F2-144 = 0.4, P = 0.67 for foraging effort and F2-39 = 0.903, P = 0.41 for success rate). This trend can be at least partially related to variations in fish availability through the tidal cycle due to differences in their behavior. A negative trend was observed in the foraging effort under different cloud cover conditions (F2-144 = 2.63, P = 0.07). This trend was less evident when the capture rates were analyzed, but a slight decrease in foraging success was also observed under overcast conditions (F2-39 = 0.18, P = 0.83). Previous studies on foraging ecology of other piscivorous bird species addressing the effect of cloud cover showed that in birds targeting fish from the air an increment in cloud cover could affect visibility. While foraging effort tended to increase with increasing wind intensity, success rate tended to decrease. Nevertheless, no significant differences were observed (F2-144 = 1.23, P = 0.3, and F2-39 = 1.6, P = 0.21, respectively for foraging success, and foraging effort). As wind intensity was highly correlated with water surface roughness (r2= 0.88), the decrease in foraging success rate could be associated with lower prey visibility beneath a rippled water surface. Although some particular references about the Snowy-crowned Tern actually exist for estuaries in Argentina, this information is anecdotical. The information given in this paper contributes to the basic knowledge of this species, emphasizing the need of further studies. The preference of this tern for estuarine and inland freshwater bodies as foraging areas during breeding and non-breeding seasons shows the importance of the wetlands conservation for the species.

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