Ontogeny of food-caching site preferences in young Florida Scrub-Jays: evidence of learning or changing social status?

Florida Scrub-Jays (Aphelocoma coerulescens) are cooperative breeders endemic to Florida’s oak scrub. In autumn, Florida Scrub-Jays cache thousands of acorns and exhibit behaviors that appear to balance cache site selection against food degradation or cache robbery. However, both experience and position within a social dominance hierarchy could affect individual cache preferences. We examined the cache site preferences of birds with differing levels of caching experience and at different strata within a complex social dominance hierarchy. Our objective was to determine how experience, social position, and social context when caching influenced microhabitat preferences, and if these change as jays age, gain experience, and their social position changes. Naïve first-year birds preferred to cache in well-hidden, densely vegetated sites with relatively high soil moisture content. Naïve birds also cached farther from provisioning points if observed by a socially dominant bird than when they were alone or in the presence of birds of equal social status. Experienced adults preferred to cache in open, dry sandy sites and social context at the time of caching did not influence their preferences. As naïve birds aged, they gained experience and their social position changed. Experienced second-year birds shifted their preference to more open, drier sites, and did so more often when they remained subordinate within their group rather than becoming dominant breeders. Second-year birds that remained subordinate did not alter their caching behavior if observed by dominant birds. These patterns suggest that after gaining experience, jays learned which sites were more appropriate for caching and shifted their preference, regardless of their changing social status. We suggest that the risk of cache loss to food degradation might be greater than the risk of pilfering for Florida Scrub-Jays, especially for birds in any social strata except the most subordinate, but this requires additional study.     

Cost of reproduction and allocation of food between parent and young in the swift (Apus apus)

We manipulated brood sizes to promote different levels of parentaleffort in the common swift (Apus apus). This provided a powerfulmethod for testing hypotheses regarding parental investmentdecisions concerning optimal allocation strategies between parentsand young. Data were analyzed on a visit-by-visit basis regardingchanges in parental and chick body mass, the mass of prey delivered,and the estimated mass of parental self-feeding. Our resultswere consistent with current theory in that food delivery increasedwith brood size, whereas the food received per chick, and hencemean chick body mass, decreased with brood size. Parental bodymass decreased with brood size and increasing parental effortbut recovered quickly during lower levels of chick feeding immediatelybefore fledging, suggesting some short-term cost of reproduction.Parents feeding at the highest level experienced criticallylow body mass and responded by a temporary cessation of chickfeeding. On any one foraging trip, total mass of prey captureddid not differ between brood sizes, but load mass deliveredto the young was negatively related to the amount of estimatedparental self-feeding. Allocation decisions of parents feedingthemselves and their young matched differential allocation theories,but estimated provisioning efficiency of parents at differentbody masses did not suggest any adaptive advantage from parentalmass loss.     

Benefits of parental care: Do juvenile lizards obtain better‐quality habitat by remaining with their parents?

Abstract Evolutionary theory suggests that parental care is favoured by natural selection when the benefits to offspring fitness outweigh the costs of parental expenditure. The nature of such benefits may differ among species, however, especially in species reflecting independent evolutionary origins of parental care. Black rock skinks (Egernia saxatilis, Scincidae) of south‐eastern Australia are viviparous rock‐dwelling lizards with prolonged parent–offspring association; adult pairs vigorously defend their home range – and, when present, their offspring – against conspecifics. We addressed the hypothesis that, by remaining within their parents' (vigorously defended) home range, juveniles thereby obtain access to better‐quality habitats. Measurements of biologically significant variables (crevice size, sun exposure, vegetation cover) revealed little difference between shelter‐rocks used by solitary (‘orphan’) juveniles and those within family groups containing adults. Indeed, the only consistent differences involved larger (and therefore, less predator‐proof) crevices for juveniles within family groups than for solitary conspecifics. Our data thus falsify the hypothesis that parental care evolves because of benefits associated with habitat quality; instead, it appears that parental protection of juveniles against infanticidal conspecifics may be the most plausible benefit to parent–offspring association in this system.     

Habitat distribution of the mallard in relation to vegetation structure,food, and population density

The mallard (Anas platyrhynchos Linnaeus) is a generalist feeder, breeding in a wide range of habitats, yet showing considerable between site differences in density. Variations in density and habitat use may result from inter- and intea-specific competition, habitat structure or food.We studied habitat selection of the mallard in four regions of Finland and Sweden. In each region, ten lakes were chosen ranging from oligotrophic to eutrophic. Habitat distribution of the mallard did not differ between regions despite variation in the density of the species and congenerics. Mallard density did not correlate with vegetation structure, but increased with food abundance and the number and density of congenerics although there were regional differences in mallard response.     

Feeding and Growth Response of Roach, Rutilus rutilus, to Alarm Substance

Feeding and growth responses of roach from three size classes to alarm substance (Schreckstoff) were quantified in laboratory experiments. Larger fish (60.0–80.0 mm in length) reacted stronger to treatment than two smaller sized groups (35.0–45.0 and 46.0–55.0 mm) lowering feeding rate by 80 and 40 and 50%, respectively. The reduction in feeding rate of larger fish caused decrease in growth rate in length and weight, while the lowered consumption of smaller fish caused only reduction in growth rate in weight. Condition factor of exposed to alarm substance small sized roach was lower than that of the control individuals and roach from other two size classes, both, treated and untreated. The difference in growth response to a danger of predation has its roots probably in different metabolism and growth rates of small and large fish. Small fish have higher metabolic rate and less lipid reserves than larger ones, therefore they are probably forced to feed to be able to grow. Also, small sized roach is more vulnerable to predation than large sized fish, thus growing fast seem to be crucial for survival in a risky environment. Study shows that small roach trade off their safety against food, feeding in risky environment to sustain fast growth. This ability of fast outgrowing of a dangerous, vulnerable to predators, size increases survival of juveniles in dangerous environment.     

Dominance in demand-feeding behaviour in Arctic charr and rainbow trout: the effect of stocking density

The effects of stocking density on the demand feeding behaviour of Arctic charr Salvelinus alpinus and rainbow trout Oncorhynchus mykiss were evaluated using demand feeders in combination with a monitoring system based on automatic passive integrated transponders. The proportion of total bites accounted for by top-ranked charr and trout decreased from 87 to 15% and from 66 to 15%, respectively, when moving from the lowest to the highest densities, indicating that the ability of the highest-ranked fish to monopolize the demand feeders was reduced at high densities. However, when the individuals were grouped into quartiles, based on their individual rank, both charr and trout ranked within the upper quartile accounted for the majority of bites independent of stocking density (87 and 72% of the total, respectively). Instead of there being a single dominant individual, as was the case under low density conditions, a small group of individuals dominated the actuation of the trigger at high densities. The two species responded differently to crowding, with the total daily bite activity of charr increasing linearly with increasing density, whereas the corresponding relationship was quadratic for rainbow trout, with a peak value at 24 to 36kg m⁻³.