Final Report Summary - AVIAN COMPETITION (The role of ecological competition in the evolution of animal signals)
When related species come into contact their interactions may include competition for resources, territory, even mates. Such interactions can play an important role in community ecology by influencing species range limits and driving phenotypic evolution. This project examined the intensity and extent of interactions between related bird species in the Amazon rainforest. The aim was to identify the extent to which competition amongst related species can lead to displacement of characters that can facilitate their coexistence. Classical ecological theory suggests that character divergence can facilitate coexistence between related species because it reduces ecological competition, reproductive incompatibilities, and direct interference between species. In Hypocnemis antbirds however, we investigated a relationship where the reverse process of convergent character displacement might result from interactions between related species.
The study involved work at the population scale to determine the extent of competition between Hypocnemis species, and at the landscape scale for an assessment of patterns of widespread phenotypic variation that could result from these interactions. The first part involved a thorough analysis of datasets on various aspects of the species ecology and behaviour from CICRA, a site where the species H. peruviana and H. subflava coexist along the Los Amigos River in Madre de Dios, Peru. Specifically, we examined differences between the species in their habitat, which was assessed in the field with vegetation surveys within the territories of each species, and then at a more extensive scale with remotely sensed light detection and ranging data (LIDAR) covering entire territories. It is the first time in our knowledge that LIDAR has been used to determine differences in bird species territories in tropical forests. We also examined differences in morphology, based on measurements collected from ringed individuals in the field, foraging strata, based on field observations, and the extent of territory overlap, based on geographical information system (GIS) analyses of observation points of study pairs at the field site. We also examined overlap in diet with stable isotope analysis of blood samples.
The landscape scale study involved extensive fieldwork recording Hypocnemis songs within and outside the contact zone of the related species. This involved recordings in Ecuador, Peru, Brazil and Bolivia, the contact zone extending across the borders of the last three. Songs recorded in the field were supplemented by those recorded previously and others obtained from sound libraries. Songs were geo-referenced and mapped into GIS software for analyses with remotely sensed data to determine possible environmental influences on song characters and consequently potential impacts of species interactions on song.
We found conclusive evidence of song convergence towards the contact zone of the two Hypocnemis species. The two species are more similar in specific song characters where they coexist, than they are with subspecies of their own species away from the contact zone. This represents clear evidence of song convergence and this is illustrated in figures prepared in a manuscript about to be submitted for publication. The patterns of convergence are supported by evidence of strong competition at the population level. There is considerable overlap in the two species habitat, despite a preference for Guadua bamboo in H. subflava. The two species are interspecifically territorial and previous work has shown that they respond as strongly to one anothers songs as they do to their own. They forage at the same strata and body size and bill size shows are also no different between the two species. There is evidence of some variation between their respective diets. We attribute this variation to differences in the arthropod assemblages between Guadua bamboo habitat and Terra firma forest, where most H. peruviana are found. Stable isotopes reveal a likely greater percentage of carnivorous arthropods in the diet of H. peruviana, which presumably means more spiders. We believe spiders may occur in greater abundances in Terra firma than in bamboo, rather than a particular specialisation of H. peruviana on spiders, but this is something that needs further investigation.
So why would strong competitors converge in their territorial signals? Classical theory would suggest this would lead to more competition, and even hybridisation. In these species there is no evidence of hybridisation. Previous work has shown females recognise the differences between male songs of each species. The context for song convergence is that it helps maintain interspecific territoriality. Just like within a socially monogamous territorial species, it is adaptive to maintain interspecific territories, so it is within such strong ecological competitors. It is less costly to maintain interspecific territories than to overlap in territories and compete for resources therein.
Such patterns of character convergence may be more widespread than currently acknowledged by ecologists and evolutionary biologists, and may play an important role in shaping broad patterns of evolutionary dynamics and species distribution.
The study involved work at the population scale to determine the extent of competition between Hypocnemis species, and at the landscape scale for an assessment of patterns of widespread phenotypic variation that could result from these interactions. The first part involved a thorough analysis of datasets on various aspects of the species ecology and behaviour from CICRA, a site where the species H. peruviana and H. subflava coexist along the Los Amigos River in Madre de Dios, Peru. Specifically, we examined differences between the species in their habitat, which was assessed in the field with vegetation surveys within the territories of each species, and then at a more extensive scale with remotely sensed light detection and ranging data (LIDAR) covering entire territories. It is the first time in our knowledge that LIDAR has been used to determine differences in bird species territories in tropical forests. We also examined differences in morphology, based on measurements collected from ringed individuals in the field, foraging strata, based on field observations, and the extent of territory overlap, based on geographical information system (GIS) analyses of observation points of study pairs at the field site. We also examined overlap in diet with stable isotope analysis of blood samples.
The landscape scale study involved extensive fieldwork recording Hypocnemis songs within and outside the contact zone of the related species. This involved recordings in Ecuador, Peru, Brazil and Bolivia, the contact zone extending across the borders of the last three. Songs recorded in the field were supplemented by those recorded previously and others obtained from sound libraries. Songs were geo-referenced and mapped into GIS software for analyses with remotely sensed data to determine possible environmental influences on song characters and consequently potential impacts of species interactions on song.
We found conclusive evidence of song convergence towards the contact zone of the two Hypocnemis species. The two species are more similar in specific song characters where they coexist, than they are with subspecies of their own species away from the contact zone. This represents clear evidence of song convergence and this is illustrated in figures prepared in a manuscript about to be submitted for publication. The patterns of convergence are supported by evidence of strong competition at the population level. There is considerable overlap in the two species habitat, despite a preference for Guadua bamboo in H. subflava. The two species are interspecifically territorial and previous work has shown that they respond as strongly to one anothers songs as they do to their own. They forage at the same strata and body size and bill size shows are also no different between the two species. There is evidence of some variation between their respective diets. We attribute this variation to differences in the arthropod assemblages between Guadua bamboo habitat and Terra firma forest, where most H. peruviana are found. Stable isotopes reveal a likely greater percentage of carnivorous arthropods in the diet of H. peruviana, which presumably means more spiders. We believe spiders may occur in greater abundances in Terra firma than in bamboo, rather than a particular specialisation of H. peruviana on spiders, but this is something that needs further investigation.
So why would strong competitors converge in their territorial signals? Classical theory would suggest this would lead to more competition, and even hybridisation. In these species there is no evidence of hybridisation. Previous work has shown females recognise the differences between male songs of each species. The context for song convergence is that it helps maintain interspecific territoriality. Just like within a socially monogamous territorial species, it is adaptive to maintain interspecific territories, so it is within such strong ecological competitors. It is less costly to maintain interspecific territories than to overlap in territories and compete for resources therein.
Such patterns of character convergence may be more widespread than currently acknowledged by ecologists and evolutionary biologists, and may play an important role in shaping broad patterns of evolutionary dynamics and species distribution.