Pablo García Borboroglu

HABITAT REQUIREMENTS AND SELECTION BY KELP GULLS (LARUS DOMINICANUS) IN CENTRAL AND NORTHERN PATAGONIA, ARGENTINA

Abstract We analyzed habitat requirements and selection of Kelp Gulls (Larus dominicanus) breeding at 68 colonies along 2,500 km of coastline in Patagonia, Argentina. Kelp Gulls use a wide range of nesting habitats that include areas with shrub, grass, and herbs which provide different degrees of cover; open ground of various substrate types; and slopes of widely different gradients, confirming the plasticity of the species with respect to habitat recorded elsewhere in the Southern Hemisphere. Eighty percent of the colonies were on islands, the probability of occupation of which increased with distance from mainland and availability of vegetation. Vegetation cover was a key component of nesting habitat for Kelp Gulls in Argentina. Gulls occupied areas with higher cover than random areas; but within vegetated areas, they showed preference for sparser cover that increased visibility and facilitated escape. Kelp Gulls often nested with other species of seabirds and seals and had a positive association with the Olrogs Gull (L. atlanticus).

HABITAT USE AND BREEDING DISTRIBUTION OF MAGELLANIC PENGUINS IN NORTHERN SAN JORGE GULF, PATAGONIA, ARGENTINA

Abstract We evaluated distribution and size of Magellanic Penguin (Spheniscus magellanicus) colonies in northern San Jorge Gulf, Argentina, characterizing nesting habitat at 14 islands with colonies (IC) and 28 islands without nesting penguins (INC). Digital analysis of a Landsat TM satellite image and field measurements were used to assess vegetation and substrate. The 14 breeding colonies ranged in size from 13 to 96,300 nests, and number of breeding pairs was estimated at 218,460. Colonies were only located on islands. Digital analysis of the satellite image showed that the proportion of area with shrub–steppe vegetation was significantly higher at IC than at INC (38.6 vs. 4.9%), whereas percentage of rocky substrate was lower at IC (33.6 vs. 64.8%). Percentage of vegetation cover was positively correlated with island area and with number of breeding pairs. Most nests (98.6%) were located under bushes, whereas the rest were burrows dug into the substrate. Mean size of bushes used for nesting was higher at IC (4.47 vs. 0.99 m2) as was the percentage of silt–clay substrate (47.31 vs. 1.98%). Coast minimum slopes at INC were steeper than the slopes of entrance routes at IC (27.6 vs. 5.7°). Probability of occupation of islands increased with amount of shrub-type vegetation cover and silt–clay substrate, and decreased with increasing slopes and amount of sandy substrate. Magellanic Penguins selected islands with shrub-type vegetation and soil that allow building nests with adequate cover. These results are consistent with other studies showing vegetation cover is an important nesting requirement for Magellanic Penguins.

Population regulation in Magellanic penguins: what determines changes in colony size?

Seabirds are often studied at individual colonies, but the confounding effects of emigration and mortality processes in open populations may lead to inappropriate conclusions on the mechanisms underlying population changes. Magellanic penguin (Spheniscus magellanicus) colonies of variable population sizes are distributed along the Argentine coastline. In recent decades, several population and distributional changes have occurred, with some colonies declining and others newly established or increasing. We integrated data of eight colonies scattered along ∼ 600 km in Northern Patagonia (from 41°26´S, 65°01´W to 45°11´S, 66°30´W, Rio Negro and Chubut provinces) and conducted analysis in terms of their growth rates, production of young and of the dependence of those vital rates on colony age, size, and location. We contrasted population trends estimated from abundance data with those derived from population modeling to understand if observed growth rates were attainable under closed population scenarios. Population trends were inversely related to colony size, suggesting a density dependent growth pattern. All colonies located in the north—which were established during the last decades—increased at high rates, with the smallest, recently established colonies growing at the fastest rate. In central-southern Chubut, where colonies are the oldest, the largest breeding aggregations declined, but smaller colonies remained relatively stable. Results provided strong evidence that dispersal played a major role in driving local trends. Breeding success was higher in northern colonies, likely mediated by favorable oceanographic conditions. However, mean foraging distance and body condition of chicks at fledging were influenced by colony size. Recruitment of penguins in the northern area may have been triggered by a combination of density dependence, likely exacerbated by less favorable oceanographic conditions in the southern sector. Our results reaffirm the idea that individual colony trends do not provide confident indicators of population health, highlighting the need to redefine the scale for the study of population changes.

Breeding Habitat Requirements of the Imperial Cormorant and Rock Shag in Central Patagonia, Argentina

Abstract We studied the habitat requirements and nest site characteristics of Imperial Cormorants (Phalacrocorax atriceps) and Rock Shags (P. magellanicus) in northern Golfo San Jorge, Argentina. Colonies of both species were only located on islands. We characterized nesting habitat at 15 and 31 islands with and without Imperial Cormorant colonies, and 19 and 27 islands with and without Rock Shag colonies, respectively. The probability of finding nesting cormorants and shags was independent of island size. Both cormorants and shags nested on only 18% of islands or islets closer than one km to the mainland, but nested in between 35% and 52% of islands located further offshore. All Imperial Cormorant and Rock Shag colonies were located on bare, rocky substrate. All colonies of both species were located more than 50 m from shrubs. The mean slope of the substrate where colonies were located was significantly greater for Rock Shags than for Imperial Cormorants (73° vs. 3°). Orientation of cliffs and steep slopes where Rock Shags nested was mostly to the east. Over 86% of Imperial Cormorant colonies were located on the side of the island facing the open sea. Both height and depth of the nest were significantly larger for the Rock Shag, while internal diameter was significantly larger for Imperial Cormorant nests. The distribution patterns of cormorants and shags at the northern sector of Golfo San Jorge appears to be largely determined by the presence of islands, with availability of open rocky substrate and low shrub vegetation cover.

Distribution and abundance of Chubut Steamerducks: an endemic species to Central Patagonia, Argentina

Summary We documented the breeding distribution and estimated abundance of Chubut Steamerducks Tachyres leucocephalus, a flightless waterbird endemic to a relatively small section of coastline in Patagonia, Argentina. The distribution of Chubut Steamerducks is restricted to approximately 700 km of coast. We counted 1,703 adult steamerducks at a subset of shorelines within their range and estimated 1,841 adults after correcting for visibility for shore-based surveys. To estimate adult densities in unsurveyed areas, we used two different methods of extrapolation, resulting in estimates of 1,587 and 1,832 adults. Combined with numbers from surveyed shorelines, the total breeding population size is estimated to be between 3,428 and 3,673 adults. In addition, we counted 1,899 juvenile steamerducks, which occur in irregular aggregations. The Interjurisdictional Marine Park in San Jorge Gulf contains about 46% of the entire population, which may provide some protection from disturbance and habitat destruction. However, oil pollution, other anthropogenic activities, and invasive species still pose potential threats to the population.

Variation in the Size of Eggs of Kelp Gulls (Larus dominicarius) at Two Colonies in Patagonia, Argentina

Abstract Although several studies have reported basic information about the size of Kelp Gull (Larus dominicarius) eggs, no attention has been paid to the comparison of patterns of intra-clutch variation at different breeding sites. Here, the variation in egg size of Kelp Gulls at two colonies from Patagonia, Argentina, was analyzed. The objectives were to characterize egg size variation in Kelp Gulls breeding at a recently established colony in Punta Loma that primarily relies on natural prey; and to evaluate if patterns of intra-clutch variation in that colony differ from those at Vernaci Sudoeste Island, a larger and well-established colony with access to supplementary food in the form of fishery waste. In two-egg clutches, first-eggs were larger than second-eggs. Neither average egg size nor intra-clutch patterns differed between study sites. Also, in two-egg clutches, egg size decreased as date of laying increased, probably due to younger birds laying smaller eggs later in the season. The pattern of intra-clutch variation in three-egg clutches differed between sites. Three-egg clutches at Vernaci Sudoeste Island presented similar sizes for first- and second-eggs and these were larger than third-eggs. This pattern contrasted with the steady decrease in relation to laying order that was observed at Punta Loma. Also, second-and third-eggs from Vernaci Sudoeste Island were larger than corresponding eggs from Punta Loma. Our data suggest that differences in egg size between colonies could be the consequence of enhanced food availability at Vernaci Sudoeste Island in the form of discards generated by trawl fisheries. Future studies to evaluate age of birds and inter-annual environmental stochasticity are needed to elucidate the effect of these factors on egg size variation.

Trophic Ecology of Breeding White-Headed Steamer-Duck (Tachyeres leucocephalus)

Abstract. White-headed Steamer-Ducks (Tachyeres leucocephalus) are flightless waterfowl endemic to a small section of coastline in Patagonia, Argentina. This study provides the first detailed information on White-headed Steamer-Duck diet composition, foraging behavior and prey availability. This study was conducted in the northern San Jorge Gulf, Patagonia, during 2007. Fifteen feeding territories were identified, and food availability was quantified within them and also within unused areas. A total of 45 feces were analyzed to determine diet composition, and foraging behaviors of females were monitored. Benthic community diversity differed between territories and non-territories. Overall, diversity and invertebrate abundance tended to be higher in territories. Ten invertebrate prey taxa were identified in adult diets, and the most frequent prey in feces were crabs, mussels, and ragworms. Head-neck dipping was the most common method of feeding. Our results improve the understanding of the relationship between diet, selection of areas with special invertebrate availability and foraging techniques used to access and exploit the resources.

Variation in the size of eggs of Chubut Steamer Ducks (Tachyeres leucocephalus)

Abstract Although several studies have analysed spatial or temporal variation in the size of eggs in waterfowl (Anseriformes), no study has analysed variation throughout the breeding range of a species. Chubut Steamer Ducks (Tachyeres leucocephalus) are flightless marine waterfowl endemic to a small section of coastline in Patagonia, Argentina. We partitioned and analysed sources of variation in the size of 989 Chubut Steamer Duck eggs, from 175 clutches, obtained during the breeding seasons of 1998 and 2004–08 at 31 islands throughout the range of the species. In relation to other Anseriformes, we found a low level of variation (coefficient of variation = 6.4%) at the species level. Most variation in the size of eggs were within clutches (59.0%) and among clutches (34.5%); variation between islands within geographical areas accounted for only 6.5% of variation, and there was no variation among geographical areas. Variation in size of eggs was not related to year, apparent clutch-size or latitude. The low variation at the species level and the lack of variation in size of eggs between geographical areas could be a consequence of the low genetic diversity and restricted distribution of the species. This study highlights the potential importance of variation in egg-size for understanding ecological processes linked to the natural history of avian species.

Current status of the threatened Olrog’s Gull Larus atlanticus : global population, breeding distribution and threats

Fil: Yorio, Pablo Martin. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Nacional Patagonico; Argentina