Nico Varo

Grit Ingestion and Size-related Consumption of Tubers by Graylag Geese

Abstract In herbivorous birds the processing rate of food is constrained by gizzard capacity. To enhance digestive processes, many species ingest grit to grind the food. Grit ingestion, however, may further limit the capacity of the gizzard. Graylag Geese (Anser anser) wintering in SW Spain fed mainly on Alkali Bulrush (Scirpus maritimus) tubers, showing a preference for small tubers. This preference may be due to a faster disintegration of small tubers than larger ones inside the gizzard. As larger tubers are likely coarser than smaller tubers, more grit would be necessary to process larger tubers. However, the ingestion of more grit to grind large tubers would be at the expense of ingesting additional tubers because of gizzard capacity limitations. Under these circumstances, there may be an inverse relationship between tuber size and amount of grit ingested to optimize food ingestion. Indeed, we found such a relationship. Grit facilitated the disintegration of tubers. This suggests that relying on some amount of grit to facilitate the grinding of food should outweigh the loss of gizzard capacity to the amount of food ingested.

Behavioural and population responses to changing availability of Artemia prey by moulting black-necked grebes, Podiceps nigricollis

We examined how availability of brine shrimps, Artemia parthenogenetica, influenced temporal aspects of foraging behaviour and population dynamics of moulting black-necked grebes, Podiceps nigricollis, from late August to early December in four salt ponds in the Odiel marshes, southern Spain, in 2008 and 2009. The moulting grebe population was higher in 2009, coinciding with an increase in shrimp biomass, with a peak of 2,500 birds in October. Grebes increased their time spent foraging as the season progressed, coinciding with decreases in shrimp biomass and water temperature. Foraging activity was lower in 2009, when shrimp biomass was greater. Diving was the most frequent feeding method, especially as the season progressed. Brine shrimps at the bottom of the water column were larger than those near the surface. Differences between years in grebe body mass suggest that changing shrimp availability and water temperature had an influence on body condition. The grebe population consumed an estimated 0.2–2.0% of the standing crop of Artemia per day, with this fraction increasing as the season progresses, thus contributing to the decline in the Artemia population. Our results suggest that moulting grebes are probably only able to adjust foraging effort within certain limits, especially at the end of moulting period when thermal stress is greatest and food supply is lowest. They may leave the study area when they can no longer meet their energy requirements.

Differences in food assimilation between two coot species assessed with stable isotopes and particle size in faeces: Linking physiology and conservation

The red-knobbed coot Fulica cristata experienced a dramatic population decline in Spain, where the common coot F. atra does not face conservation problems. This is puzzling because both species have similar ecologies. It has been suggested that habitat alterations affected the quality of food plants, and this impacted differentially both coots. To verify this, we conducted experiments to determine the assimilation efficiency of both species in relation to food quality. Two types of diets differing in fibre content (commercial food and Potamogeton pectinatus) were offered to captive red-knobbed and common coots, during both spring and autumn. We examined variations in faecal particle size among coot species and diets, indicative of the facility with which food can be assimilated, and used the stable isotope technique to study differences between coot species in stable isotope fractionations from consumption to excretion. Faecal particle size was larger in red-knobbed than in common coots when fibre content was high, but was similar when it was low. Faecal particle sizes were larger in autumn, when fibre content was higher, than in spring. In general, delta(15)N in faeces of red-knobbed coots was greater than in faeces of common coots. These results suggest that the digestive efficiency of the red-knobbed coot was lower than that of the common coot, and that the differences increased when the fibre content in food plants increased. Managers should try to make available to coots wetland habitat with high quality food, which may be facilitated by prolonging the hydroperiods.

Cestodes change the isotopic signature of brine shrimp, Artemia, hosts: implications for aquatic food webs.

To reach the final host (greater flamingos), the cestode Flamingolepis liguloides alters the behaviour of its intermediate host, the brine shrimp, Artemia parthenogenetica, causing it to spend more time close to the water surface. During summer 2010, we showed that the prevalence of this cestode was consistently higher at the top of the water column in the Odiel salt pans in south-western Spain. We used stable nitrogen (N) and carbon (C) isotopic analysis to test the hypothesis that cestodes also alter resource use by Artemia. In early summer, we compared stable isotopes in infected hosts at the surface with those from uninfected hosts at the bottom of the water column. In late summer, we compared infected and uninfected Artemia from the bottom. δ(15)N was consistently enriched in infected individuals compared with uninfected hosts, especially in Artemia with multiple infections of F. liguloides (family Hymenolepididae) and those with mixed infections of F. liguloides and cestodes of the family Dilepididae. Infected individuals from the surface were enriched in δ(13)C compared with uninfected ones from the bottom, but the opposite was found when comparing uninfected and infected Artemia from the same depth. This may be caused by the increase in lipid concentration in infected Artemia. Isolated cysticercoids of F. liguloides were significantly enriched in δ(13)C compared with cysticercoids in infected hosts, but surprisingly were not enriched in N. Our findings illustrate the way cestodes can alter food webs and highlight the importance of considering the parasitic status of prey in studies of trophic ecology in saline wetlands.

Differences in foraging behaviour of sympatric coots with different conservation status

Red-knobbed coot, Fulica cristata, was formerly distributed across several countries in the south-west Palaearctic. Remnant populations are now located in Morocco and Spain. In the latter, the red-knobbed coot is considered critically endangered, and its decline is attributed to changes in agricultural practices in the basins of wetlands, which have affected the quality and quantity of food. However, red-knobbed coot is currently more abundant in Morocco. Foraging behaviour reveals information about an animal’s perception of the place that it currently occupies. Thus, we hypothesised that if habitat quality is lower in Spain than in Morocco, this should be expressed in the foraging activity of coots, which should be greater in Spain to compensate for the lower food availability. Our results supported this prediction: red-knobbed coots spent more time foraging in Spain than in Morocco. Furthermore, we compared the foraging behaviour of red-knobbed and common coots, Fulica atra, in southern Spain, where common coot does not face conservation problems, and found that red-knobbed coots spent more time foraging than common coots to meet daily energetic needs. Overall, our results suggested that constraints of food quality and quantity on habitat use were more severe for red-knobbed coots than for common coots, and that this may be a key factor for the conservation of red-knobbed coot in Spain. Managers should try to facilitate longer hydroperiods in wetlands, which would ensure higher quality food during longer periods for coots.

Breeding biology of two sympatric coots with contrasting conservation status

Capsule Red-knobbed Coots Fulica cristata and Common Coots Fulica atra have similar breeding biology except in chick survival, which was significantly lower in Red-knobbed Coots. Aims To provide information on the breeding biology of Red-knobbed Coots in Spain and to compare this with similar data for Common Coots. Methods During two consecutive breeding seasons, clutch size, egg size, brood size and chick survival of Red-knobbed and Common Coots were studied in nearly all of the lagoons in which Red-knobbed Coots were breeding. Results Clutch size was similar in the two species. Egg volume varied between years within species. Both species had similar brood sizes but chick survival differed significantly, being lower in Red-knobbed Coots (36% and 55% in 2002 and 2003, respectively) than in Common Coots (93% and 91% in 2002 and 2003, respectively). The number of fledglings per pair was lower in Red-knobbed Coots (0.9 and 2.1 in 2002 and 2003, respectively) than in Common Coots (2.2 and 3.0 in 2002 and 2003, respectively). Conclusion The disparity in productivity between Red-knobbed and Common Coots in the lagoons of southern Spain is mainly due to differential chick survival rates. These differences could be the key factor that has led to their currently different conservation status in Spain.

Female-Biased Sex Ratio in Moulting Black-Necked Grebes Podiceps nigricollis in Southern Spain

We tested if Black-necked Grebe, a species in which both sexes undertake moult-migration, have an unbiased sex ratio at a moulting site in Europe, as previously found in North America and as was expected for a species with biparental care. For this we used a unique long-term dataset of 5821 grebes captured for ringing throughout the moulting seasons of 2006–2012 in the Odiel salt-marshes (SW Spain). The grebes were sexed and classified as adults (74%) or juveniles (26%). Birds ringed at Odiel were recovered over a wide area up to central Russia and south to the Canary Islands and Morocco. We report on a unique case of a strongly biased sex ratio in a moult-migrating bird species with biparental care, in which adult females were significantly more abundant than adult males in all 7 years (1.6–4.2 females per male). Biased sex ratios were not found among juveniles. Differences between North America and Europe in the sex ratios of adult Black-necked Grebes at moulting sites may be explained by the much larger American moulting sites, which would facilitate an unbiased sex ratio in North America, but not in Europe. Moulting sites in Europe may reach carrying capacity because of their smaller size, forcing the late migrating individuals (adult females and juveniles) to move longer distances to sites farther from breeding areas, such us the Odiel salt-marshes.

Exposure of black-necked grebes (Podiceps nigricollis) to metal pollution during the moulting period in the Odiel Marshes, Southwest Spain

European populations of black-necked grebes (Podiceps nigricollis) congregate every year to moult at the salt ponds of the Odiel Marshes (SW Spain). However, the Odiel Marshes are part of one of the most metal-polluted coastal estuaries in the world, which may pose risks to wildlife. We assessed the exposure of grebes to metal pollution during the critical moulting period in the Odiel Marshes and its potential to cause adverse health effects. Levels of metals in red blood pellet (as a biomarker of exposure), plasma carotenoids, eye redness, and body condition (as biomarkers of effects) were studied. Metal content was also analyzed in the brine shrimp Artemia parthenogenetica, the most important food for grebes in this hypersaline ecosystem during the moulting period. Results showed that, in comparison to toxicity thresholds, grebes had relatively high blood levels of arsenic (As), mercury (Hg) and zinc (Zn). The high loads found in Artemia and the way blood levels vary during the moulting period indicate that shrimp consumption may be the main route of metal exposure for grebes. Plasma carotenoids and body condition showed a positive association with exposure to As, while the relationship of lutein-like carotenoids with Hg accumulation was negative at the beginning of the moulting period to become positive afterwards. Moreover, eye redness was negatively affected by As accumulation. Factors including food resource availability, seasonal fluctuations in physiological status, and interannual variations in the degree of environmental contamination should be considered in monitoring efforts when using moult migrant waterbirds as sentinel species.

Sexual dimorphism and sex determination in Black-necked Grebes Podiceps nigricollis: on the need to account for geographical variation in morphology and sex ratio

ABSTRACT Capsule: The sex of Black-necked Grebes Podiceps nigricollis in Spain can be determined successfully using a locally derived discriminant function analysis (DFA) but a function derived from a North American population was not successful. Aims: To determine the extent of sexual size dimorphism in the Black-necked Grebe and develop discriminant functions based on morphometric data. We also assessed the accuracy of visual sexing by fieldworkers with different levels of experience and compared the discriminant function for European birds with one for North American birds, in order to determine the geographical specificity of the function. Methods: We used a data set of 370 Black-necked Grebes from a European population throughout the non-breeding seasons of 2008–11 in the Odiel saltmarshes (Spain), as well as published data from North America, to compare the accuracy of DFAs developed for European and North American populations when applied to the same population. Results: Males were significantly larger than females for all body measurements. The DFA with the highest success rate included one single linear dimension (head-bill length), showing an efficiency of 88.5% in the same sample used to build the function and up to 91.5% in independent samples of birds. The more experienced ringer showed higher accuracy in visual sexing than the less experienced one (84 vs. 70%), although self-consistency in sex assignment of a given individual was similar. The function developed for European birds performed poorly for American birds (79.1% sexed correctly) and vice versa (18.4% sexed correctly). Conclusion: Morphological and sex ratio differences between European and American subspecies both contribute to the discrepancy in the success rate. This illustrates the need for caution when applying discriminant functions developed in one bird population to individuals elsewhere.

Seasonal and annual variations in waterbird populations of mountain and coastal wetlands of Morocco

We conducted counts of waterbirds (grebes, ducks and coots) during autumn and spring (1997, 1999 and 2002–2004) in mountain (Middle Atlas) and coastal wetlands of Morocco, to study annual and seasonal variations in abundance, as well as in the degree of spatial dispersion of species. A total of 18 species was recorded, which exhibited a higher abundance in autumn. Most species were found both in coastal and mountain wetlands, the exceptions being the Ferruginous Duck (Aythya nyroca; found only in coastal wetlands) and the Ruddy Shelduck (Tadorna ferruginea; found only in mountain wetlands). The most abundant species showed a higher spatial overdispersion in spring than in autumn. When water levels were low, the degree of spatial patchiness of some species was high, indicating that most individuals were found in very few wetlands. As most of the wetlands that we studied hold populations of threatened species, and suffer several conservation threats (see Green et al. 2002, Biological Conservation 104: 71–82), legal protection of all these wetlands is required to ensure the conservation of waterbirds. This may be especially critical for species that rely on few wetlands during drought periods.