Selection by wild birds on artificial dimorphic prey on varied backgrounds

Our aim was to test the effects of prey frequency and background composition on selection by free-ranging birds. We did three series of experiments with populations of grey and orange pastry prey scattered among coloured stones that made the prey either conspicuous or inconspicuous. Series 1 tested whether the predicted equilibrium frequency of the two prey types was influenced by the frequency of matching grey and orange stones. Birds at a single site were given a random sequence of combinations of prey frequency and stone frequency. Selection was dependent on background and the effect of prey frequency also varied with background. In series 2, we explored the frequency-independent effect of background: birds at five sites were given equal numbers of the two prey in three frequencies of matching stones and two of non-matching. There was a higher risk of predation for prey that matched rarer stones. In series 3 we attempted to measure, at a single site, the actual equilibrium prey frequencies in three different backgrounds: two extreme stone frequencies and one intermediate. Each experiment started with a population of equal numbers of grey and orange prey. After half the prey had been eaten we calculated the frequencies of the survivors and presented a new population of the original size but with the new prey frequencies; each experiment ran for 25 such 'generations'. The results suggested that at equilibrium the commoner 'morph' was the one that resembled the commoner colour of stone. Overall, our findings support the idea that visual selection can result in morph frequencies becoming related to the proportions of their matching background components and that this equilibrium will 'track' temporal or spatial changes in the background.     

Within-colony feeding selectivity by a corallivorous reef fish: foraging to maximize reward?

Foraging theory predicts that individuals should choose a prey that maximizes energy rewards relative to the energy expended to access, capture, and consume the prey. However, the relative roles of differences in the nutritive value of foods and costs associated with differences in prey accessibility are not always clear. Coral‐feeding fishes are known to be highly selective feeders on particular coral genera or species and even different parts of individual coral colonies. The absence of strong correlations between the nutritional value of corals and prey preferences suggests other factors such as polyp accessibility may be important. Here, we investigated within‐colony feeding selectivity by the corallivorous filefish, Oxymonacanthus longirostris, and if prey accessibility determines foraging patterns. After confirming that this fish primarily feeds on coral polyps, we examined whether fish show a preference for different parts of a common branching coral, Acropora nobilis, both in the field and in the laboratory experiments with simulated corals. We then experimentally tested whether nonuniform patterns of feeding on preferred coral species reflect structural differences between polyps. We found that O. longirostris exhibits nonuniform patterns of foraging in the field, selectively feeding midway along branches. On simulated corals, fish replicated this pattern when food accessibility was equal along the branch. However, when food access varied, fish consistently modified their foraging behavior, preferring to feed where food was most accessible. When foraging patterns were compared with coral morphology, fish preferred larger polyps and less skeletal protection. Our results highlight that patterns of interspecific and intraspecific selectivity can reflect coral morphology, with fish preferring corals or parts of coral colonies with structural characteristics that increase prey accessibility.     

Crypsis in a heterogeneous environment: relationships between changeable polymorphic colour patterns and behaviour in a galaxiid fish

1. The ability to achieve optimal camouflage varies between microhabitats in heterogeneous environments, potentially restricting individuals to a single habitat or imposing a compromise on crypsis to match several habitats. However, animals may exhibit morphological and behavioural attributes that enhance crypsis in different habitats. 2. We used an undescribed fish species, Galaxias‘nebula’, to investigate two objectives. First, we examined two potential methods of enhancing crypsis: change in colour pattern and selection of a suitable background. Second, we characterised the colour pattern of this unstudied fish and assessed its capacity for crypsis. 3. No background selection was apparent but the area of dark pigment expressed varied between backgrounds, which may negate the requirement to be choosy about habitats. The capacity to change colour and selection of a background that maximises crypsis are most likely separate, non‐mutually exclusive strategies. 4. Galaxias‘nebula’ exhibits polymorphic, non‐interchangeable colour patterns that have elements of both background pattern matching and disruptive colouration. This, coupled with habitat characteristics, suggests a combination of generalist and specialist strategies of habitat use. The fish’s camouflage strategy and air‐breathing ability may be key to survival under increasing pressure from habitat degradation and invasive predators.     

Is coarse woody debris in lakes a refuge or a trap for benthic invertebrates exposed to fish predation?

1. Coarse woody debris (CWD) in the littoral zone of lakes constitutes a preferred habitat for macroinvertebrates and fish. CWD differs in the surface complexity depending on its decay status. Therefore, CWD may provide distinct types of shelters and thus modify the structure of the macroinvertebrate community as well as its susceptibility to fish predation.
2. We ran an enclosure experiment in a lake littoral zone to test the effect of surface complexity of CWD on the interactions between the predator, Eurasian perch (Perca fluviatilis) and its potential macroinvertebrate prey. We deployed 10 enclosures containing fresh wood with a smooth surface and 10 enclosures containing decayed wood with a more complex, rough surface and allowed colonisation by macroinvertebrates. Five enclosures of each type were then stocked by perch and exposed to fish predation.
3. The abundance and biomass of macroinvertebrates were significantly higher on decayed wood with greater surface complexity than on fresh wood; however, the type of CWD did not strongly influence the taxonomic composition and diversity of invertebrates.
4. The direct effect of perch predation on the macroinvertebrate community was weak. Perch reduced only the abundance of adult Dikerogammarus villosus, while other potential prey, such as chironomids, was more abundant in the presence of the fish. The impact of perch consumption of these larvae was probably obscured by interspecific interactions among chironomids and D. villosus, which were impaired in the fish enclosures.
5. We found no clear evidence that the influence of perch on macroinvertebrates was mediated by the complexity of the wood surface. However, fish diet analysis showed that on decayed wood, perch preferentially consumed chironomids, and consumption of D. villosus was much lower, while on fresh wood, the preferential consumption of chironomids decreased with increasing consumption of gammarids. This suggests that such differences in fish diet could be an effect of complex interactions between wood microstructure, prey density and its ability to find refuge in CWD.
6. The effect of CWD microstructure on predator–prey interactions was visible with respect to interspecific relationships between chironomids and gammarids, which on more complex decayed wood were moderated in the absence of perch.