Reduction In Intake Rate Research Articles

Morphological and digestive adjustments buffer performance: How staging shorebirds cope with severe food declines.

Organisms cope with environmental stressors by behavioral, morphological, and physiological adjustments. Documentation of such adjustments in the wild provides information on the response space in nature and the extent to which behavioral and bodily adjustments lead to appropriate performance effects. Here we studied the morphological and digestive adjustments in a staging population of migrating Great Knots Calidris tenuirostris in response to stark declines in food abundance and quality at the Yalu Jiang estuarine wetland (northern Yellow Sea, China). At Yalu Jiang, from 2011 to 2017 the densities of intertidal mollusks, the food of Great Knots, declined 15‐fold. The staple prey of Great Knots shifted from the relatively soft‐shelled bivalve Potamocorbula laevis in 2011–2012 to harder‐shelled mollusks such as the gastropod Umbonium thomasi in 2016–2017. The crushing of the mollusks in the gizzard would require a threefold to 11‐fold increase in break force. This was partially resolved by a 15% increase in gizzard mass which would yield a 32% increase in shell processing capacity. The consumption of harder‐shelled mollusks was also accompanied by reliance on regurgitates to excrete unbreakable parts of prey, rather than the usual intestinal voidance of shell fragments as feces. Despite the changes in digestive morphology and strategy, there was still an 85% reduction in intake rate in 2016–2017 compared with 2011–2012. With these morphological and digestive adjustments, the Great Knots remaining faithful to the staging site to a certain extent buffered the disadvantageous effects of dramatic food declines. However, compensation was not complete. Locally, birds will have had to extend foraging time and use a greater daily foraging range. This study offers a perspective on how individual animals may mitigate the effects of environmental change by morphological and digestive strategies and the limits to the response space of long‐distance migrating shorebirds in the wild.

Cryptic interference competition in swans foraging on cryptic prey

Interference can be defined as the reduction of intake rate caused by the presence of congeneric individuals. However, surrounding congeneric individuals may also accelerate food depletion. Therefore, it is difficult to quantify interference (contest) and exploitative (scramble) competition separately, particularly in the field. In an individual-based model, where food was immediately replenished in patches after a foraging bout, we isolated the effect of interference competition. In this way, we investigated how interference shapes the intake rates of flocking Bewick’s swans, Cygnus bewickii, consuming tubers of fennel pondweed, Potamogeton pectinatus. The model predicted that intake rate declines with increasing swan density, and that the rate of decline is greater in subordinate birds than in dominant birds. Subordinate birds suffered a large reduction in intake rate at high densities because they spent a large proportion of the time avoiding more dominant individuals. A major decline in intake rate of subordinates occurred close to the maximum swan densities observed in the field. We expected subordinates to leave high-density flocks and look for another foraging location: interference competition seemed to regulate the maximum swan flock density in the field. Hence, the mean population intake rate at realized densities was only slightly lower than in the absence of interference. As interference occurred mainly as avoidance behaviour, which is difficult to observe in the field, it might commonly remain cryptic for the observer. Our results may explain other field studies where interference competition seemed to be of lesser importance.

Experimental evidence for interference competition in oystercatchers, Haematopus ostralegus. II. Free-living birds

Field studies of interference competition in free-living animals have relied on natural fluctuations in forager density, which are often confounded with other factors. We therefore experimentally studied interference in the wild, capitalizing on 2 cockle beds in an isolated bay that were exploited by a population of individually marked oystercatchers. We successfully increased forager density by chasing birds from one cockle bed, leaving the other cockle bed as the only nearby alternative. The density increase was most pronounced on the eastern cockle bed where food stocks were poorer and initial feeding densities were lower compared with the western cockle bed. Oystercatchers residing on this eastern bed suffered a significant decline in intake rate when bird density was experimentally increased, providing evidence of interference. ‘‘Refugee’’ birds, that is, the birds that were displaced from their home bed, experienced an even stronger reduction in intake rate compared with the residents and compared with their intake rate on their ‘‘home bed,’’ probably partly due to the fact that the refugee birds were forced to feed on an unfamiliar cockle bed. The fact that disturbance at 1 site influenced both the refugees and the local resident birds indicates that human disturbance (an important conservation issue) has an effect that extends beyond the site where the disturbance takes place. The benefits and costs of site fidelity and interference competition are likely to play an important role in understanding animal distributions and how they change in response to environmental perturbations, including human disturbance of foraging sites. Key words: Cerastoderma edule, cockle, density dependence, despotic distribution, disturbance, dominance, experiment, Haematopus ostralegus, interference, intra-specific competition, oystercatcher. [Behav Ecol]

Noise, predation risk compensation and vigilance in the chaffinch Fringilla coelebs

Background noise should in theory hinder detection of auditory cues associated with approaching danger. We tested whether foraging chaffinches Fringilla coelebs responded to background noise by increasing vigilance, and examined whether this was explained by predation risk compensation or by a novel stimulus hypothesis. The former predicts that only inter‐scan interval should be modified in the presence of background noise, not vigilance levels generally. This is because noise hampers auditory cue detection and increases perceived predation risk primarily when in the head‐down position, and also because previous tests have shown that only interscan interval is correlated with predator detection ability in this system. Chaffinches only modified interscan interval supporting this hypothesis. At the same time they made significantly fewer pecks when feeding during the background noise treatment and so the increased vigilance led to a reduction in intake rate, suggesting that compensating for the increased predation risk could indirectly lead to a fitness cost. Finally, the novel stimulus hypothesis predicts that chaffinches should habituate to the noise, which did not occur within a trial or over 5 subsequent trials. We conclude that auditory cues may be an important component of the trade‐off between vigilance and feeding, and discuss possible implications for anti‐predation theory and ecological processes.

Parasite-induced anorexia in a free-ranging mammalian herbivore: an experimental test using Soay sheep

Theory suggests that gastrointestinal parasites can influence herbivore population dynamics by increasing host mortality rates. In addition, parasites may have a non-lethal range of both physiological and behavioural effects on their hosts. Two potential behavioural effects are parasite-induced anorexia and the alteration of diet selection patterns — both of which may influence plant communities without necessarily causing herbivore mortality. We report here the results of an experiment carried out in August–September 2001 to examine herbivore response to parasitism using feral Soay sheep ( Ovis aries L., 1758) living on Hirta, St. Kilda (Scotland, UK), as a model system. The aim of the experiment was to determine whether the sheep exhibit significant parasite-induced anorexia and whether diet selectivity was also altered in response to parasitism. The n-alkane technique was used to estimate forage intake rates and diet composition. Overall, intake rate increased with body mass and the mean (±SE) over both sexes was 737 ± 40 g DM / day (range 458–1241 g DM / day). However, we found no effect of parasite burden upon intake rate or selection patterns. The results of a power analysis imply that any reduction in intake rate was less than 30%, which is less than the 30%–60% reduction in intake rate recorded for domestic sheep under moderate parasite burdens. This suggests that Soay sheep are more resilient to parasitism than domestic breeds. Despite evidence to the contrary from simple farm-based studies, the lack of any effect on selection patterns in this case suggests that parasites do not cause significant alterations to the selection patterns of herbivores in complex non-agricultural environments.

The effect of physiological state (lactating or dry) and sward surface height on grazing behaviour and intake by dairy cows

The effect of physiological state lactating vs. non-lactating (dry) on grazing behaviour and herbage intake by Holstein-Friesian cows was examined on grass pastures maintained at 5, 7 or 9 cm sward surface height (SSH), typical of those provided under continuous variable stocking management. Intake rates were estimated over periods of 1 h by weighing the animals before and after grazing, retaining the faeces and urine excreted, and applying a correction for insensible weight loss. Grazing behaviour during these periods and over 24 h was recorded automatically using sensors to measure jaw movements. Bite mass (BM) did not differ significantly between lactating and dry cows but decreased ( P<0.001) from 0.42 to 0.30 g organic matter (OM) bite −1 as overall mean SSH decreased from 9 to 5 cm. An increase ( P=0.040) in grazing jaw movement (GJM) rate, from 75.3 to 80.3 GJM min −1, as SSH decreased, did not compensate for reductions in bite mass, and intake rate declined linearly ( P=0.006) from 24.6 to 18.9 g OM min −1. Lactating and dry cows compensated for the reduction in intake rate, by increasing total grazing time and total number of bites per day. As SSH decreased from 9 to 5 cm, lactating and dry cows increased total eating time (528 to 607 and 419 to 510 min), total GJM (40 400 to 49 300 and 31 300 to 40 600 GJM) and total bites (31 100 to 37 900 and 24 600 to 31 200 bites, respectively). As a result, there was no significant effect of SSH on daily intake of OM, although lactating cows had greater intakes than dry cows; 12.9 vs. 9.3 kg day −1, ( P<0.001). The increased time spent grazing as SSH decreased was associated with a reduction in the time spent ruminating ( P<0.001), despite similar levels of daily intake being achieved across SSH treatments. Although dry cows had much lower daily intakes, they spent only about 30 min less each day ruminating than the lactating cows ( P=0.060), allowing them 120 to 160 min more idling (i.e., non-grazing, non-ruminating) behaviour ( P=0.001).

Effect of sward surface height on intake and grazing behaviour by lactating Holstein Friesian cows

The effect of sward surface height (SSH) on grazing behaviour and intake by lactating Holstein Friesian cows on continuously stocked grass pastures maintained at mean heights of 5, 7 and 9 cm was studied during the growing season. Intake rate was estimated over periods of 1 h by weighing animals before and after grazing, with a correction made for insensible weight loss. Grazing behaviour during that hour and over 24 h was recorded automatically using sensors to measure jaw movements.Although maintained at the overall mean SSH, swards had a patchy appearance with short, frequently grazed areas interspersed with taller, infrequently grazed areas, which is typical of pastures continuously stocked with cattle. Daily organic matter (OM) intake, calculated as the product of daily grazing time and intake rate, was greater at a SSH of 7 cm than at 5 or 9 cm (14·1 vs. 10·5 and 12·1 kg respectively). On the 5‐cm sward, OM intake per grazing jaw movement (GJM) was reduced compared with that on the 7‐cm sward (0·182 vs. 0·264 g respectively), and because cows were unable either to increase significantly GJM rate (95·8 vs. 90·1 GJM min−1) or the proportion of GJM that were bites (0·80 vs. 0·81) OM intake rate was reduced (16·9 vs. 23·5 g min−1). Cows were unable to increase their grazing time significantly (628 vs. 604 min d−1) to compensate for the reduction in intake rate, and as a result daily intakes were lower. Cows grazing the 9‐cm sward also incurred a reduction in OM intake GJM−1 compared with those on the 7‐cm sward (0·237 vs. 0·264 g respectively) and therefore there was a reduction in OM intake rate (21·6 vs. 23·5 g min−1). These animals did not compensate by increasing the time spent grazing (581 min d−1), probably owing to an increased ruminating requirement per kg of herbage ingested compared with those on 7 cm SSH (2264 vs. 1780 ruminating jaw movements respectively). The results show that SSH can significantly influence intake rate, but, while the cow's only effective strategy to compensate for any reduction in intake rate is to increase grazing time, this may be limited by the requirement for ruminating and non‐grazing, non‐ruminating activities, which is influenced by qualitative and quantitative aspects of the herbage ingested, whereas the cows' only effective strategy to compensate for any reduction in intake rate is to increase grazing time.

Deriving Population Parameters from Individual Variations in Foraging Behaviour. I. Empirical Game Theory Distribution Model of Oystercatchers Haematopus ostralegus Feeding on Mussels Mytilus edulis

1. The basic structure is described of a game theory model of the distribution of a wintering shorebird, the oystercatcher Haematopus ostralegus, foraging on 12 mussel Mytilus edulis beds whose quality as feeding areas for the birds differed. Model parameters were derived from field studies made over 15 years. A companion paper tests model predictions and illustrates how the model can be used to derive survival curves and predict estuary carrying capacity. 2. Mussel-bed quality was measured as the intake rate achieved by an oystercatcher of average competance foraging at random over the bed, and varied threefold across beds. Each individual in the model was given its own foraging efficiency, drawn at random from a normal distribution whose mean and variance depended on the bird's age and feeding method. Its susceptibility to interference from other foraging birds was calculated from its local dominance score, the percentage of wins it had over the other individuals currently on the same bed. A bird's local dominance score on a particular mussel bed was calculated from the proportion of birds on that bed having a lower global dominance rank, a measure of a bird's competitive ability relative to those of all other birds on the estuary. Across individuals, foraging efficiency and susceptibility to interference were unrelated. 3. In every iteration, the model calculated a bird's putative intake rate on each bed by subtracting the reduction in intake rate due to interference from the intake rate the bird could achieve there in the absence of interference. Each individual could then move to the mussel bed which, at that time, gave it the highest intake rate, provided the differences were sufficiently large (3%) for it to discriminate. 4. The main model limitations were the absence of: (i) a real-time base, so time-lags due to learning were not included; (ii) the energy costs associated with moving between beds; (iii) the opportunity for birds to increase their dominance score or foraging efficiency through familiarity with a mussel bed; (iv) the opportunity for individuals to control the amount of competition to which they are exposed by selecting particular times in the tidal cycle at which to feed; and (v) factors known to affect bed attractiveness to oystercatchers, such as the consistency of the substrate. None the less, the model was regarded as a suitable starting point for exploring how the proportion failing to acquire enough food, and either emigrating or starving, is affected by population size