In sexually size-dimorphic species, physiological constraints derived from differences in body size may determine different food requirements and thus a trophic niche divergence between males and females. These relationships between sexual size dimorphism (SSD) and dietary overlap are not well understood in birds. We analyzed differences between the sexes in diet composition, dietary diversity, diet selection, and volume and density of droppings, as well as the dietary overlap between sexes, in the Great Bustard (Otis tarda), the species showing the highest SSD among birds. We discuss the differences that we found in relation to various predictions derived from ecological and physiological differences between the sexes, under the hypothesis that these differences are ultimately determined by the strong SSD of this species. As expected, our best linear mixed-effects food selection models included sex as a main factor explaining differences in diet composition and dietary diversity of Great Bustards throughout the annual cycle. Both sexes were mostly herbivorous, consuming legumes when they were available. Males consumed fewer arthropods, but of significantly larger size, than females. The droppings of males were larger, heavier, and slightly denser than those of females. Males showed higher dietary diversity than females, except during the postmating season. The mean dietary overlap between the sexes was 0.7, one of the smallest values among birds. Overall, our results suggest that the species' extreme SSD along with the distinct reproductive role of each sex might explain the trophic niche divergence in the Great Bustard.
Birds are important seed dispersers for fleshy fruits through their transportation of ingested seeds. The seeds of many species germinate faster and in greater proportions after passing through a digestive tract, although the effects of this passage vary amongst bird and plant species. Many factors determine the germination success of ingested seeds, such as seed scarification during the digestion process, the fertilizing effect of droppings and the removal of pulp surrounding the seeds. In central Spain, the great bustard (Otis tarda) may act as a disperser of European black nightshade (Solanum nigrum). We analysed the germination success of ingested and non-ingested S. nigrum seeds. The fertilizing effect of bustard droppings and the disinhibition effect of the removal of Solanum pulp on final germination percentage, germination speed and viability were also assessed. Although ingested seeds germinated faster than non-ingested seeds, the former showed a lower germination percentage than the latter: 80–87% versus 99%. Droppings and fruit pulp showed no effect on germination enhancement, except in one aspect: the germination speed of non-ingested seeds decreased when they were sprayed with a fruit extract. We confirm that seeds ingested by great bustards had lower germination success than non-ingested seeds. Although seed ingestion by great bustards reduced seedling emergence, the number of emerged seedlings was still quite large. Thus, great bustards may play a role as a S. nigrum seed dispersal vector.
Sexual size dimorphism (SSD) may favour physiological peculiarities in diet, behaviour and home-range size both across species and within species. Sex-specific differences in diet and behaviour have been reported in several bird species but there are fewer studies of foraging area size in sexually dimorphic bird species. Foraging area size should be greater in the bigger sex according to home-range size predictions based on body mass. We tested this prediction in a winter study of foraging area size in the Great Bustard Otis tarda, the most sexually size-dimorphic bird species, which forages in unisexual flocks. In this species the temporal pattern of a flock's feeding intensity; the proportion of birds actively feeding (FI) and the size of the morning foraging area (MFA) of each sex are unknown. We recorded the behaviour and movements of unisexual flocks of Great Bustards during winter mornings and sampled food availability to take into account its effect on FI and MFA. FI increased and then decreased through the morning in both sexes, and was lower in males than in females. This sexual difference was greater where legume availability was smaller. Legumes were the most preferred substrate type. Consequently, MFA sizes were smaller in sites with more legume availability. We did not find sexual differences either in the size of MFA or in the selection of the two preferred substrate types: legumes and stubble fields. MFA and FI were determined to a greater extent by ecological factors such as food availability than by metabolic requirements derived from body size differences. These results obtained from a short-term study do not preclude an effect of sexual size dimorphism on MFA size and FI of Great Bustards over longer periods but show that the body size effect on foraging behaviour may be smaller than predicted only by SSD.
We present evidence of a possible case of self-medication in a lekking bird, the great bustard Otis tarda. Great bustards consumed blister beetles (Meloidae), in spite of the fact that they contain cantharidin, a highly toxic compound that is lethal in moderate doses. In addition to anthelminthic properties, cantharidin was effective against gastrointestinal bacteria that cause sexually-transmitted diseases. Although both sexes consumed blister beetles during the mating season, only males selected them among all available insects, and ingested more and larger beetles than females. The male-biased consumption suggests that males could use cantharidin to reduce their parasite load and increase their sexual attractiveness. This plausibly explains the intense cloaca display males perform to approaching females, and the meticulous inspection females conduct of the male’s cloaca, a behavior only observed in this and another similar species of the bustard family. A white, clean cloaca with no infection symptoms (e.g., diarrhea) is an honest signal of both, resistance to cantharidin and absence of parasites, and represents a reliable indicator of the male quality to the extremely choosy females. Our results do not definitely prove, but certainly strongly suggest that cantharidin, obtained by consumption of blister beetles, acts in great bustards as an oral anti-microbial and pathogen-limiting compound, and that males ingest these poisonous insects to increase their mating success, pointing out that self-medication might have been overlooked as a sexually-selected mechanism enhancing male fitness.
We update and present relevant information regarding the abundance and distribution of the great bustard Otis tarda in Castilla y León (Spain) in 2008, compare it with previous census results, and analyse the effects of agricultural changes on the provincial abundance and distribution of the species. The study area was surveyed from four-wheel drive vehicles driven at low speed (20-30 km/h) along predetermined transects, stopping frequently at prominent spots. The intention was to detect all the great bustards present in the study area. The great bustard population in Castilla y León during the breeding season of 2008 was 14,025 birds: 5,637 males, 7,760 females and 628 individuals whose sex could not be determined. The population was 34% greater in 2008 than in 1998 when the area surveyed in both censuses is compared. Population increases were recorded in all provinces except for Soria, where no birds were observed. Not all local populations within provinces increased: we detected population declines of over 10% in 43 (27%) of the 160 polygons surveyed both in 1998 and 2008, population increases of over 10%in 93 (58%) polygons, and largely stable populations (changes less than 10%) in 24 (15%) polygons. Provincial increases in bustard numbers were positively correlated with increases in the extent of unirrigated legume crops. The great bustard population in Castilla y León in 2008 was the largest in the world, accounting for 27-32% of the global population and about 47% of the estimated Spanish population. The spread of unirrigated legumes, crops that are promoted by agri-environment schemes, may have fostered the observed population increase between 1998 and 2008. A survey of the region every ten years is proposed to monitor future population trends.
. Actualizamos y aportamos nueva información sobre la abundancia y distribución de la avutarda común Otis tarda en Castilla y León (España) en 2008, comparamos dicha información con la de censos anteriores, y analizamos los efectos de la transformación agrícola sobre la abundancia provincial de la especie. El área de estudio fue censada en recorridos predefinidos con vehículos todo terreno a baja velocidad (20-30 km/h) y realizando paradas frecuentes en lugares con amplio campo visual. El objetivo fue detectar y contabilizar todos los individuos presentes en el área de estudio. La población reproductora de avutarda censada en 2008 fue de 14.025 aves: 5.637 machos, 7.760 hembras y 628 aves cuyo sexo no pudo determinarse. El número de avutardas contabilizadas en 2008 fue un 34% superior al de 1998 teniendo en cuenta el área muestreada en ambos censos. Se registraron incrementos poblacionales en todas las provincias excepto en Soria donde, al igual que en la primavera de 1998, no se detectó la especie. Las poblaciones locales no se incrementaron en todos los lugares ni en la misma proporción: en 43 (27%) de los 160 polígonos visitados en 1998 y 2008 se produjeron descensos poblacionales superiores al 10%, 93 (58%) polígonos experimentaron un aumento poblacional superior al 10%, y en 24 (15%) no se observaron cambios superiores al 10%. El incremento en el número de avutardas censadas en cada provincia se correlacionó positivamente con el incremento en la superficie de leguminosas de secano. Concluimos que la población de avutardas de Castilla y León censada en 2008 fue la mayor del mundo, representando alrededor del 47% de la estima poblacional de España y el 27-32% de la mundial. La expansión de cultivos adecuados para la especie como las leguminosas de secano, que son subvencionadas por programas agroambientales, han debido favorecer el aumento poblacional observado entre 1998 y 2008. Sin embargo, para confirmar la tendencia al alza en la población de Castilla y León y sus causas, es necesario mantener un programa de censos de la especie con una periodicidad al menos decenal.
