The widespread introduction of species has created novel communities in many areas of the world. Since introduced species tend to have generalized ecologies and often lack shared evolutionary history with other species in their communities, it would be expected that the relationship between form and function (i.e. ecomorphology) may change in novel communities. We tested this expectation in a subset of the novel bird community on O‘ahu, Hawai‘i. By relating foraging behavior observations to morphology obtained from live birds at four sites across the island, we found many relationships between species’ morphology and foraging ecology that mirrored relationships found in the literature for native dominated bird communities. Both movement and certain foraging behaviors were related to a species’ tarsus to wing ratio. Further, bill morphology was related to gleaning, frugivory, and flycatching behaviors. The commonness of significant ecomorphological relationships suggests that, within O‘ahu’s novel bird community, form is strongly related to function. We hypothesize that ecological fitting likely played a major role in the assembly of this novel community conserving the relationships between form and function found in many other bird communities. To further support this hypothesis, we used niche data from EltonTraits 1.0 to determine if the establishment of bird species introduced to O‘ahu was related to the distinctiveness of their ecological niche from the incumbent community. Introduced species were more likely to establish on O‘ahu if their diets were less similar to the bird species already present on the island. Our results support the idea that ecological fitting is an important mechanism in shaping ecological communities, especially in the Anthropocene, thereby influencing novel community assembly and functioning.

We used five morphological characters, including wing length, tarsus length, culmen length, and bill width and depth at the nares, as taken in Gleditsch and Sperry 2019 (doi:10.1111/evo.13744) for each species at each site. Mist netting was conducted at 7 sites on the Hawaiian island of Oahu from November 2014 through December 2017 and the five morphological measurements were taken on as many individuals as possible focusing on the five focal species. Mass was also collected for each bird species by weighing the birds using spring scales (PESOLA, Switzerland) in a bag and subtracting the mass of the bag. Morphological measurements of juvenile birds were not taken.

From the morphological data, we calculated morphological ratios (i.e. tarsus to wing ratio; and bill aspect ratios). The bill aspect ratios were calculated in two different ways: horizontal aspect ratio calculated by dividing the culmen length by the width of the bill at the proximal end of the nares, and the three-dimensional aspect ratio calculated by dividing the culmen length by the cross-sectional area of the bill at the proximal end of the nares (hereafter bill slenderness). For bill slenderness, a lower value means that the bill is more robust (i.e. shorter and fatter) and a higher value the bill is slenderer (i.e. longer and thinner). Due to a low rate of re-sighted birds (birds were color banded for another study), we averaged the morphological ratios for each species at each site.

We obtained diet resource and foraging strata use distributions for each terrestrial bird species that was reported by Pyle and Pyle 2017 (http://hbs.bishopmuseum.org/birds/rlp-monograph) to have been introduced to O‘ahu (126 species – 40 of which are considered established). Additionally, we obtained the same information for every native species documented in Pyle and Pyle (2017) for which the information was available (12 species). The resource use information for all species was obtained from EltonTraits 1.0 (Wilman et al. 2014 - doi:10.1890/13-1917.1). To obtain this information, Wilman and colleagues mined various texts (e.g. articles, accounts, guides, etc.) and scored each resource type according to the language used and order of description in the text. For each species they gave a certainty score for the distribution of resources used in the species’ diet. This ranged from high certainty (A), reasonably certain (B), unclear quality (C), and species lacking information so values represent typical values from genus (D1 or D2). For the 134 species that we were able to find data for, 115 were in certainty category A, 7 were in B, 3 were in C, and 9 were in D1. We could not find diet information for 4 native species that were considered extinct either before or shortly after European discovery of Hawai‘i. In addition to the diet resource distributions, EltonTraits also has foraging strata use distributions obtained in a similar way to diet, which we also used to determine if differences in foraging strata used influenced the probability of establishment. In order to determine the incumbent community at the time a species was introduced we needed each species’ introduction and, if applicable, extirpation dates. For a species’ introduction date, we used dates provided by Pyle and Pyle (2017) and used the first reported date if an introduction date was not explicitly provided. The extirpation dates were considered to be the last year the species was reported. When dates were not explicitly stated in Pyle and Pyle (2017), we used other sources (Moulton and Pimm 1983 - doi:10.1086/284094, Moulton 1985 - doi:10.2307/3544703, Simberloff and Boecklen 1991 - doi:10.1086/285219, Moulton 1993 - doi:10.1086/285463) including eBird (Sullivan et al. 2009 - http://www.ebird.org) for the last reported dates. 

Any missing data has a value of NA. There should be no blank cells.

Every data file has an associated metadata file that describes the data within each column.