Growth and foraging consequences of facultative paedomorphosis in the tiger salamander,Ambystoma tigrinum nebulosum

Summary Facultative paedomorphosis in salamanders occurs when larvae respond to varying environmental conditions by either metamorphosing into terrestrial metamorphic adults or retaining their larval morphology to become sexually mature paedomorphic adults. Several hypotheses have been proposed for the evolutionary maintenance of this environmentally induced dimorphism, but few data are available to assess them adequately. We studied a montane population of the tiger salamander,Ambystoma tigrinum nebulosum, and measured the adult growth rate and body condition across three growing seasons to assess the relative costs and benefits of each morph. Metamorphic adults grew more than paedomorphic adults in terms of snout—vent length across years and in weight within years. Dietary analyses and foraging experiments revealed some of the proximate factors that may underlie these differential growth patterns. Across all prey, metamorphs had significantly higher biomass and calories per stomach sample than paedomorphs. Metamorphic diets primarily consisted of the fairy shrimpBranchinecta coloradensis, whereas paedomorphic diets contained a variety of benthic and terrestrial invertebrates. Foraging experiments revealed that both morphs are more successful at capturing fairy shrimp relative to other prey types and both show high electivity toward this prey. However, fairy shrimp occurred only in non-permanent ponds and thus are inaccessible to paedomorphs, which can survive only in permanent ponds. Paedomorphs also experience higher levels of intraspecific competition with large larvae in permanent ponds than metamorphs do in non-permanent ponds. Thus, metamorphs obtain a growth advantage over paedomorphs by foraging in non-permanent ponds that contain fairy shrimp and have reduced intraspecific competition. These results suggest that paedomorphs should have decreased fitness relative to metamorphs, primarily because metamorphs can move into the best habitats for growth. The net fitness effect of morph-specific differences in dispersal depend on whether there are trade-offs with other life history traits. Nonetheless, because the relative benefit of metamorph dispersal will change with environmental conditions in permanent ponds and the surrounding habitat, the relative fitness payoff to each morph should track changes in the environment. Thus, facultative paedomorphosis may be maintained in part by variable, environmentally-specific fitness payoffs to each morph.     

Re‐epithelialization of large wound in paedomorphic and metamorphic axolotls

Axolotls (Ambystoma mexicanum ) may heal their skin wounds scar‐free in both paedomorphs and metamorphs. In previous studies on small punch skin wounds, rapid re‐epithelialisation was noted in these two axolotl morphs. However, large wound size in mammals may affect wound healing. In this study, large circumferential full thickness excision wounds on the hind limbs were created on juvenile paedomorphic and metamorphic axolotls. The results showed re‐epithelialisation was more quickly initiated in paedomorphs than in metamorphs after wounding. The migrating rate of epidermis on the wound bed was faster in paedomorphs than in metamorphs and thus completion of re‐epithelialisation was faster in paedomorphs than in metamorphs. Within these re‐epithelialisation periods, neither basement membrane nor dermis was reformed. Epidermal cell proliferation was detected by EdU‐labelling technique. In the normal unwounded skin, epidermal proliferation rate was higher in paedomorphs than in metamorphs. After wounding, the epidermal proliferation rate was significantly lower in the migrating front on the wound bed than in the normal skin in paedomorphs. The EdU‐labelling rate between normal skin and migration front was not different in metamorphs. Lacking of more proliferating epidermal cells on the wound bed indicated that the new epidermis here derived rather from migrating epidermal cells than from cell proliferation in situ. In conclusion, re‐epithelialisation in the large wound might be fully completed in both morphs despite it was initiated earlier and with faster rate in paedomorphs than in metamorphs. The new epidermis on the wound bed derived mainly from cell migration than by cell proliferation in the re‐epithelialisation period. J. Morphol. 278:228–235, 2017. © 2016 Wiley Periodicals,Inc.     

Facultative paedomorphosis as a mechanism promoting intraspecific niche differentiation

Organisms with complex life cycles are characterized by a metamorphosis that allows for a major habitat shift and the exploitation of alternative resources. However, metamorphosis can be bypassed in some species through a process called paedomorphosis, resulting in the retention of larval traits at the adult stage and is considered important at both micro‐ and macroevolutionary scales. In facultatively paedomorphic populations of newts, some individuals retain gills and a fully aquatic life at the adult stage (paedomorphs), while others undergo complete metamorphosis (metamorphs), allowing for a terrestrial life‐stage. Because facultative paedomorphosis affects trophic structures and feeding mechanism of newts, one hypothesis is that it may be maintained as a trophic polymorphism, with the advantage to lessen intraspecific competition during the shared aquatic life‐stage. Here, we tested this hypothesis combining stomach content data with stable isotope techniques, using carbon and nitrogen stable isotopes, in facultatively paedomorphic alpine newts Ichthyosaura alpestris. Both stomach content and stable isotope analyses showed that paedomorphs had smaller trophic niches and were more reliant on pelagic resources, while metamorphs relied more on littoral resources, corresponding to a polyphenism along the littoral–pelagic axis and the extension of the population's trophic niche to otherwise ‘underused’ pelagic resources by paedomorphs. Interestingly, stable isotopes revealed that the trophic polyphenism was less marked in males than in females and potentially linked to sexual activity. Although paedomorphosis and metamorphosis are primarily seen as results of tradeoffs between the advantages of using aquatic versus terrestrial habitats, this study provides evidence that additional forces, such as intraspecific trophic niche differences between morphs and trophic niche expansion, may play an important role in the persistence of this dimorphism in heterogeneous environments. Moreover, the different patterns found in males and females show the importance of considering sex to understand the evolutionary ecology of trophic polymorphisms.     

Foraging tactics in alternative heterochronic salamander morphs: trophic quality of ponds matters more than water permanency

1. In lentic freshwater habitats, the composition of animal assemblages shifts along a gradient from temporary to permanent basins. When habitats with different degrees of permanence are at the scale of the home range of species, they constitute alternatives in terms of energy acquisition through feeding. 2. In this context, previous studies showed an advantage of metamorphic over paedomorphic tiger salamanders (Ambystoma tigrinum) in temporary ponds which are only available to metamorphs. The aim of this study was to establish whether salamanders obtain similar benefits in ponds that do not differ in water permanence and whether salamanders shifted from detrimental to advantageous ponds. To this end, we determined the feeding habits, body condition and movement patterns of the two morphs in a complex of four permanent and four temporary ponds. 3. Consistent with previous studies, metamorphs consumed higher‐quality diets than paedomorphs in term of energy intake. However, these differences occurred because metamorphs consumed fairy shrimp in a single temporary pond. Individual movement patterns confirmed that most of the metamorphs used different aquatic habitats both within and between years and that most of them moved from permanent ponds for breeding towards the most profitable temporary pond in terms of foraging. 4. These results indicate that habitat selection by salamanders is optimal in term of energy intake in metamorphs that use high quality ponds independently of hydroperiod. It seems that both spatial and temporal variation can influence the relative foraging success of each morph.     

Phenotype-specific feeding behaviour of two arctic charr Salvelinus alpinus morphs

The feeding performance of two morphs of a trophically polymorphic fish, the arctic charr Salvelinus alpinus, feeding on zooplankton, was studied in the laboratory. Limnetic morphs, with a fusiform body, terminal mouth, short pectoral fins, and relatively long and dense gill rakers, fed at significantly higher rates on Daphnia and Cyclops than did benthic morphs. The benthic morphs were characterized by a chunky body form, subterminal mouth, long pectoral fins, and relatively short and spaced-out gill rakers. The limnetic morphs attacked the prey at significantly higher rates and were more successful in capture. Benthic morphs were more reluctant to start feeding on zooplankton and reaction time was longer than in limnetic morphs. In the absence as well as presence of zooplankton food, the position of the two morphs in the aquaria differed markedly. The results agree with dietary differences found in field-collected fish. The limnetic morphs consume zooplankton, whereas benthic morphs specialize on Lymnaea gastropods. The results suggest that benthic morphs are phenotypically inferior at using the pelagic food niche. This implies that the current role of competitive or predatory interactions in segregating the morphs may be minor.     

Genetic structure of lake and stream populations in a Pyrenean amphibian (Calotriton asper) reveals evolutionary significant units associated with paedomorphosis

Differences in environmental conditions such as those between lakes and streams can produce phenotypic variation and ultimately promote evolutionary diversification. Some species of newts and salamanders can occupy these habitats and express alternative phenotypes: metamorphs that lose gills at metamorphosis and paedomorphs that retain them at the adult stage. Whereas this process is facultative in some species, it is obligatory in others, thus suggesting that isolation and environmental pressures may have canalized developmental pathways. In this study, we focused our research on the Pyrenean brook newt, Calotriton asper, which is present in both lakes and streams, but whose fully aquatic paedomorphic individuals are only present in lakes. We aimed to determine the genetic structure and differentiation of two paedomorphic populations, including their surrounding stream and lake metamorphic populations, to test whether populations of paedomorphs can constitute evolutionary significant units. Although gene flow was identified between lakes and nearby stream populations, there was a low percentage of dispersers, and the paedomorphic populations were genetically differentiated from the populations of metamorphs. It is likely that the studied lakes have offered peculiar conditions that have allowed the development of a paedomorphic phenotype. These populations and phenotypes therefore constitute good models to understand local adaptations. As each of these populations of paedomorphs can be considered evolutionary significant units that cannot be replaced by other nearby populations in case of a population crash, conservation actions should be focused directly on them.     

Filling ephemeral ponds affects development and phenotypic expression in Ambystoma talpoideum

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The role of gill raker number variability in adaptive radiation of coregonid fish

Gill raker divergence is a general pattern in adaptive radiations of postglacial fish, but few studies have addressed the adaptive significance of this morphological trait in foraging and eco-evolutionary interactions among predator and prey. Here, a set of subarctic lakes along a diversifying gradient of coregonids was used as the natural setting to explore correlations between gill raker numbers and planktivory as well as the impact of coregonid radiation on zooplankton communities. Results from 19 populations covering most of the total gill raker number gradient of the genus Coregonus, confirm that the number of gill rakers has a central role in determining the foraging ability towards zooplankton prey. Both at the individual and population levels, gill raker number was correlated with pelagic niche use and the size of utilized zooplankton prey. Furthermore, the average body size and the abundance and diversity of the zooplankton community decreased with the increasing diversity of coregonids. We argue that zooplankton feeding leads to an eco-evolutionary feedback loop that may further shape the gill raker morphology since natural selection intensifies under resource competition for depleted prey communities. Eco-evolutionary interactions may thus have a central role creating and maintaining the divergence of coregonid morphs in postglacial lakes.     

Diet reveals links between morphology and foraging in a cryptic temperate reef fish

Predators select prey so as to maximize energy and minimize manipulation time. In order to reduce prey detection and handling time, individuals must actively select their foraging space (microhabitat) and populations exhibit morphologies that are best suited for capturing locally available prey. We explored how variation in diet correlates with habitat type, and how these factors influence key morphological structures (mouth gape, eye diameter, fin length, fin area, and pectoral fin ratio) in a common microcarnivorous cryptic reef fish species, the triplefin Helcogrammoides cunninghami. In a mensurative experiment carried out at six kelp‐dominated sites, we observed considerable differences in diet along 400 km of the Chilean coast coincident with variation in habitat availability and prey distributions. Triplefins preferred a single prey type (bivalves or barnacles) at northern sites, coincident with a low diversity of foraging habitats. In contrast, southern sites presented varied and heterogeneous habitats, where triplefin diets were more diverse and included amphipods, decapods, and cumaceans. Allometry‐corrected results indicated that some morphological structures were consistently correlated with different prey items. Specifically, large mouth gape was associated with the capture of highly mobile prey such as decapods, while small mouth gape was more associated with cumaceans and copepods. In contrast, triplefins that capture sessile prey such as hydroids tend to have larger eyes. Therefore, morphological structures co‐vary with habitat selection and prey usage in this species. Our study shows how an abundant generalist reef fish exhibits variable feeding morphologies in response to the distribution of potential habitats and prey throughout its range.     

Behavior and aquatic plants as factors affecting predation by three species of larval predaceous diving beetles (Coleoptera: Dytiscidae)

Predation among aquatic invertebrate predators can have important effects on patterns of exclusion and coexistence in aquatic habitats, especially if these predators also act as intraguild predators. Such patterns may be explained by variation in predator foraging mode and in the extent and overlap of habitat use. Predaceous diving beetles (Coleoptera: Dytiscidae) are abundant in isolated bodies of water and are effective predators on many aquatic organisms, including other dytiscids. The under-investigated role of hunting behavior and habitat use in altering outcomes of predation under different plant densities may offer insights into patterns of coexistence among larval dytiscids. I performed experiments that quantified behavior of larvae of three common genera of dytiscids that share common prey and then measured predation among genera in the presence or absence of aquatic plants. Behavioral analyses concluded that there were significant differences in foraging modes, with Dytiscus primarily exhibiting sit-and-wait tactics, Graphoderus engaging in active, open water searching, and Rhantus displaying combinations of these behaviors. Predation among larvae was common and occurred when predators were larger than the prey, with no indication of prey preference. Incidence of predation among generic combinations depended on the presence of plants and appeared to be related to behavioral differences among genera. The presence or absence of plants and differences in larval behavior may help to mitigate predation by reducing negative interactions in natural aquatic systems. These results have implications for IGP interactions and may be one of the explanations for the observed richness of this group of predators within aquatic habitats.     

Mouth dimorphism in scale‐eating cichlid fish from Lake Tanganyika advances individual fitness

Random asymmetry, that is the coexistence of left‐ and right‐sided (or ‐handed) individuals within a population, is a particular case of natural variation; what triggers and maintains such dimorphisms remains unknown in most cases. Here, we report a field‐based cage experiment in the scale‐eating Tanganyikan cichlid Perissodus microlepis, which occurs in two morphs in nature: left‐skewed and right‐skewed individuals with respect to mouth orientation. Using underwater cages stocked with scale‐eaters and natural prey fish, we first confirm that, under semi‐natural conditions, left‐skewed scale‐eaters preferentially attack the right flank of their prey, whereas right‐skewed individuals feed predominantly from the left side. We then demonstrate that scale‐eaters have a higher probability for successful attacks when kept in dimorphic experimental populations (left‐ and right‐skewed morphs together) as compared to monomorphic populations (left‐ or right‐skewed morphs), most likely because prey fishes fail to accustom to strikes from both sides. The significantly increased probability for attacks appears to be the selective agent responsible for the evolution and maintenance of mouth dimorphism in P. microlepis, lending further support to the hypothesis that negative frequency‐dependent selection is the stabilizing force balancing the mouth dimorphism at quasi‐equal ratios in scale‐eating cichlids.     

Short- and long-term advantages of an alternative ontogenetic pathway

We documented hypotheses on the evolution of developmental heterochronies by estimating short- and long-term advantages of alternative morphs. In this respect, we compared food energy intakes and body condition between paedomorphic and metamorphic Alpine newts ( Triturus alpestris , Caudata, Amphibia) in four populations. Because we found a strong correlation between fat reserves and body condition, we used this last parameter as an indicator of long-term gains. In all studied sites, paedomorphic females showed higher body condition than metamorphic ones. Paedomorphic males were also at advantage in two populations. Paedomorphs exhibited higher energy intakes than metamorphs in two populations and similar gains in two others. Our results support unifying theories that predict the occurrence of facultative paedomorphosis in varied habitats as paedomorphs exhibited better energy balance than metamorphs. This work shows the need for considering integrative parameters such as body condition in the comparative study of performances when studying developmental heterochronies. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 105–112     

Genetically-based variation between two spider populations in foraging behavior

Summary Optimal foraging theory is based on the assumption that at least some aspects of foraging behavior are genetically determined (Pyke et al. 1977; Kamil and Sargent 1980; Pyke 1984). Nonetheless, very few studies have examined the role of genetics in foraging behavior. Here, we report on geographical differences in the foraging behavior of a spider (Agelenopsis aperta) and investigate whether these differences are genetically determined. Field studies were conducted on two different populations of A. aperta: one residing in a desert riparian habitat, and the other in a desert grassland habitat. Data from the spiders' natural encounters with prey demonstrated that grassland spiders exhibited a higher frequency of attack than riparian spiders towards 13 of 15 prey types, including crickets and ants. Grassland spiders also had shorter latencies to attack 12 of 15 prey types, including crickets and ants, than riparian spiders. Subsequently, we reared grassland and riparian spiders under controlled conditions in the laboratory and observed their interactions with prey to determine whether the populational differences we found in the field could be genetic. Again, grassland spiders showed a shorter latency to attack prey (crickets, ants) than riparian spiders. These latencies were not significantly affected by the hunger state or age of the spiders. Finally, we reared a second generation (F2) of grassland and riparian spiders in the laboratory and observed their interactions with prey to determine whether the populational differences in the previous generation were due to genetic effects or maternal effects. As before, grassland spiders exhibited a shorter latency to attack prey (crickets) than riparian spiders. We conclude that the foraging differences we observed between these two populations of A. aperta are genetically determined. These differences probably have resulted from either natural selection acting directly on attack frequency and the latency to attack prey, or natural selection acting on traits which are genetically correlated with these aspects of foraging behavior.     

Strategy-switching in the gaffing bat

Foraging in Daubenton's bats Myotis daubentonii , at two altitudinal locations along a river gradient in North Wales was investigated in relation to aerial insect density and to the density of prey on the water surface. Prey capture in Daubenton's bats consisted of aerial hawking, where prey was taken in the air, and trawling, where bats gaffed invertebrates from the water surface. Aerial hawking accounted for 86% of all prey capture attempts, despite aerial insect availability falling close to zero for much of the night. Conversely, prey density on the water surface was an order of magnitude higher than aerial prey density and increased through the night due to aquatic invertebrate drift. At the higher altitude site, M. daubentonii switched prey capture strategy to gaffing, possibly to reflect this change in prey availability on the water's surface, but at the lower altitude site, they maintained aerial hawking as the preferred strategy. The switch to gaffing may be inhibited by the significant downstream accumulation of large numbers of inedible exuviae of caddis flies, Trichoptera, at the low-altitude site, which form both acoustic clutter and increase the probability of capturing inedible prey, making foraging less efficient. These small altitudinal differences in foraging strategy should be factored into the design of future altitudinal bat foraging studies and if found to be a widespread strategy, taken into consideration by conservation planners when reviewing the habitat requirements of Daubenton's bats in river valleys within the United Kingdom.     

Paedomorphosis in <Emphasis Type="Italic">Ambystoma talpoideum</Emphasis>: effects of initial body size variation and density

Facultative paedomorphosis is the ability of a salamander to either metamorphose into a terrestrial, metamorphic adult or retain a larval morphology to become a sexually mature paedomorphic adult. It has been hypothesized that density and initial body size variation within populations are instrumental in cueing metamorphosis or paedomorphosis in salamanders, yet few studies have adequately tested these hypotheses in long-term experiments. Beginning in the spring of 2004, 36 experimental ponds were used to manipulate three body size variation levels (low, medium, high) and two density levels (low, high) of Ambystoma talpoideum larvae. Larvae were individually marked using visible implant elastomers and collected every 2 weeks in order to measure snout–vent length and mass. Bi-nightly sampling was used to collect new metamorphs as they appeared. Analysis revealed significant effects of density, size variation and morph on body size of individuals during the summer. Individuals that metamorphosed during the fall and following spring were significantly larger as larvae than those becoming paedomorphic across all treatments. These results support the Best-of-a-Bad-Lot hypothesis, which proposes that the largest larvae metamorphose in order to escape unfavorable aquatic habitats. Large larvae may metamorphose to leave aquatic habitats, regardless of treatment, due to the colder climate and lower productivity found in Kentucky, which is in the northern-most part of A. talpoideum’s range. By maintaining a long-term experiment, we have provided evidence for the transition of both larvae and paedomorphs into metamorphs during fall and spring metamorphosis events. Furthermore, the production of similar morphs under different environmental conditions observed in this research suggests that the ecological mechanisms maintaining polyphenisms may be more diverse that first suspected. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Cost of morphological specialization: feeding performance of the two morphs in the trophically polymorphic cichlid fish,Cichlasoma citrinellum

Summary The feeding performance on soft and hard prey of two morphs of the trophically polymorphic Neotropical cichlid fish, Cichlasoma citrinellum, was investigated in the laboratory. The molariform morphs, specialized to feed on hard prey, are able to crack snail shells that are twice as hard as those cracked by the papilliform morphs. During ecological bottlenecks in food resources this ability should allow molariform morphs to exploit alternate, less preferred prey sources that are not available to papilliform morphs. Analysis of stomach contents revealed that molariform morphs feed significantly more often on hard snails than do papilliform morphs (Meyer 1989a). The performance advantage of the trophically specialized morphs when feeding on hard prey is countered by their less efficient performance on soft diets. The morphologically generalized papilliform morph feeds more efficiently on soft prey. The abundance of preferred soft prey, seasonal fluctuations in prey availability and the frequency of ecological bottlenecks may determine the relative abundance of these two morphs in natural populations in Nicaraguan lakes.     

Opportunistic predator prefers habitat complexity that exposes prey while reducing cannibalism and intraguild encounters

Structural features of habitat are known to affect the density of predators and prey, and it is generally accepted that complexity provides some protection from the environment and predators but may also reduce foraging success. A next step in understanding these interactions is to decouple the impacts of both spatial and trophic ingredients of complexity to explicitly explore the trade-offs between the habitat, its effects on foraging success, and the competition that ensues as predator densities increase. We quantified the accumulation of spiders and their prey in habitat islands with different habitat complexities created in the field using natural plants, plant debris and plastic plant mimics. Spiders were observed at higher densities in the complex habitat structure composed of both live plants and thatch. However, the numerically dominant predator in the system, the wolf spider Pardosa milvina, was observed at high densities in habitat islands containing plastic mimics of plants and thatch. In a laboratory experiment, we examined the interactive effects of conspecific density and habitat on the prey capture of P. milvina. Thatch, with or without vertical plant structure, reduced prey capture, but the plastic fiber did not. Pairwise interactions among spiders reduced prey capture, but this effect was moderated by thatch. Taken together, these experiments highlight the flexibility of one important predator in the food web, where multiple environmental cues intersect to explain the role of habitat complexity in determining generalist predator accumulation.     

Expression of sexual ornaments in a polymorphic species: phenotypic variation in response to environmental risk

Secondary sexual traits may evolve under the antagonistic context of sexual and natural selection. In some polymorphic species, these traits are only expressed during the breeding period and are differently expressed in alternative phenotypes. However, it is unknown whether such phenotypes exhibit phenotypic plasticity of seasonal ornamentations in response to environmental pressures such as in the presence of fish (predation risk). This is an important question to understand the evolution of polyphenisms. We used facultative paedomorphosis in newts as a model system because it involves the coexistence of paedomorphs that retain gills in the adult stage with metamorphs that have undergone metamorphosis, but also because newts exhibit seasonal sexual traits. Our aim was therefore to determine the influence of fish on the development of seasonal ornamentation in the two phenotypes of the palmate newt (Lissotriton helveticus). During the entire newt breeding period, we assessed the importance of phenotype and fish presence with an information‐theoretic approach. Our results showed that paedomorphs presented much less developed ornamentation than metamorphs and those ornamentations varied over time. Fish inhibited the development of sexual traits but differently between phenotypes: in contrast to metamorphs, paedomorphs lack the phenotypic plasticity of sexual traits to environmental risk. This study points out that internal and external parameters act in complex ways in the expression of seasonal sexual ornamentations and that similar environmental pressure can induce a contrasted evolution in alternative phenotypes.     

Covariation between plumage colour polymorphism and diet in the Barn Owl Tyto alba

Several hypotheses might explain the evolution and maintenance of colour morphs within animal populations. The 'alternative foraging strategy' hypothesis states that alternative colour morphs exploit different ecological niches. This hypothesis predicts that morphs differ in diet, either because foraging success on alternative prey species is morph-dependent or because differently coloured individuals exploit alternative habitats. I examined this prediction in the Barn Owl Tyto alba , a bird that varies in plumage coloration continuously from dark reddish-brown to white. On the European continent, Owls are light-coloured (subspecies T. a. alba ) in the south and reddish-brown ( T. a. guttata ) in the north; in central Europe the two subspecies interbreed, generating many colour variants. If plumage coloration indicates alternative foraging strategies, in sympatry dark- and light-coloured owls should consume prey species that are typical of the diets of T. a. guttata and T. a. alba in allopatry, respectively. In line with this prediction, both in allopatry and in sympatry in Switzerland T. a. guttata fed primarily upon Common Voles Microtus arvalis and T. a. alba upon Wood Mice Apodemus spp. Statistical analyses suggest that morph-dependent diet did not arise from a non-random habitat distribution of owls with respect to plumage coloration. This suggests that foraging success upon alternative prey is morph-dependent.     

Spatio-temporal shifts in gradients of habitat quality for an opportunistic avian predator

We used the conceptual framework of the theory of natural selection to study breeding habitat preferences by an opportunistic avian predator, the black kite Milvus migrans. In Europe, black kite populations are mostly found near large networks of aquatic habitats, usually considered optimal for foraging and breeding. We hypothesized that proximity to wetlands could vary among individuals and affect their fitness, and thus be subject to natural selection. We tested the hypothesis first on a population on Lake Lugano (Italian pre‐Alps) which has been monitored for nine years, and then on seven other populations, each studied for four–five years, located along a continuum of habitat from large water bodies to scarce aquatic habitat of any kind. In the Lake Lugano population, black kite abundance was negatively related to distance to the lake in all the nine years of study, consistent with long‐term natural selection. There was evidence of ongoing directional selection on strategic nest location in three of the years, and evidence of stabilizing selection in two years. In eight of the nine years the trend was for a linear increase in fitness with increasing proximity to the lake. At the population level, results were consistent with adaptive habitat choice in relation to the previous year's spatial variation in fitness: higher associations between fitness and distance to the lake (i.e. higher selection gradients) resulted in higher density variations in the following year, in turn related to the availability of fish, the main local prey. The progressive decline of inland pairs and increase in the density of lakeshore pairs caused a directional long‐term trend of declining mean distance to the lake. Breeding near aquatic habitats was associated with higher foraging success, and higher frequency and biomass of prey deliveries to offspring. There was weak evidence of selection in other populations. The inland‐wetland gradient of habitat quality may have been affected by predation risk, as estimated by density of a major predator of adults and nestlings, the eagle owl Bubo bubo. Behavioral decisions at the level of the individual probably translated into population effects on density and distribution at various spatial scales. Populations in optimal habitats showed higher density and produced six times as many young per unit space as those in sub‐optimal habitats.