Study system and sampling

The study included 10 sites along an elevational gradient from 0-2,700m in eastern Taiwan. The distances between sites are 2-24 km. All 10 sites are composed of mosaics of forests and open vegetations such as bushes, grasslands, abandoned/fallow farmlands. The lower elevation sites (DON, CHO, SAZ, ZHA) are in rural setting, whereas the higher elevation sites (XIB, LIA, LUS, CIE, GUA, HEH) are within the recreational area of a national park (Taroko National Park). Therefore, all sites receive low to intermediate anthropogenic influences. However, none of them are in urban area or pristine forests. 

Bird netting was performed between 2009 and 2017 to capture the study species Cyanoderma ruficeps, Sinosuthora webbiana, and Zosterops simplex. Ten to 15 mist-nets were set up at a site on the day of sampling and checked every 15 minutes. All three species can be found in forests. However, due to logistic difficulties in setting up mist-nets in dense vegetation, we conducted the netting in tall grasses or bushes along forest edges. We observed that the netting sites were used as foraging habitats for all three species. Each captured bird was banded for individual identification, and recaptured individuals were excluded from this study. Upon first capture, morphological traits of the birds were measured, and a few feathers from their chest were collected for stable isotope analysis. Approximately 20 µL of blood was obtained by venipuncture from the brachial vein and stored in 100% ethanol for molecular sexing. All individuals were released on site immediately after sampling. 

The netting, handling and sampling procedures were approved by the Institutional Animal Care and Use Committee of National Dong Hwa University, Taroko National Park, Hualien county government and Taiwan’s Council of Agriculture. 

Morphological measurement 

A digital caliper (Mitutoyo, Kanagawa, Japan) and a ruler with a zero-stop were used to measure six morphological traits: bill length, bill width, bill depth (bill traits), head length, tarsus length and wing length (body-size traits). The measurements were made to the nearest 0.01 mm except for wing length which was made to the nearest 0.1 mm.      

Stable isotope analysis

The feather samples were lipid-extracted in 2:1 chloroform:methanol solution for 24 hours, rinsed with distilled water, and oven-dried at 55°C for 48-72 hours. Dried feather samples were carefully removed of shafts with surgical forceps and scissors, and approximately 1 mg of the feather tissues were loaded into tin capsules for isotope analysis. In order to provide site-specific baselines to correct feather isotope values, we collected 8-14 foliar samples of the common plant species at each site (one foliar sample from each plant species; 8-14 plant species across sites) opportunistically in 2009, 2010, 2018 and 2019. The plant foliar samples were rinsed with distilled water, oven-dried at 55°C for 48-72 hours, and grounded into find powder. Approximately 3 mg of plant foliar samples were loaded into tin capsules for isotope analysis. Because lipids tend to have more negative carbon isotope values than proteins and carbohydrates, lipid-extraction for consumer tissues could help reduce the influence of different lipid contents among organisms on their carbon isotope values. On the other hand, all components of plant foliar tissues, including lipids, proteins and carbohydrates, are eaten by consumers. Therefore, it is not necessary to lipid-extract plant samples. Stable carbon and nitrogen isotope analysis was performed at UC Davis Stable Isotope Facility (ThermoFinnigan Delta Plus, Bremen, Germany). 

This data contains two sheets: 1) birds 2) plants. The 'birds' data has 18 columns: the 1st-4th columns identity the site (siteCD, site, latitude, longitude), the 5th column (birdnumer) is the tag number that uniquely identifies an individual bird, the 6th-7th columns (speciesCD, species) identify the species of the bird, the 8th column is the sex of the bird, the 9th column (date) is the date the bird was sampled, the 10th-15th colums are the bird' morphological measurements, the 16th column (techCD) identifies the technician who made the measurement, and the 17th-18th columns (d13C, d15N) are the stable carbon and nitrogen isotope values of the bird's feather tissue (unadjusted isotope value). The 'plants' data has 7 columns: the 1st-4th columns identity the site (siteCD, site, latitude, longitude), the 5th column (species) identifies the species of the plant, and the 6th-7th columns (d13C, d15N) are the stable carbon and nitrogen isotope values of the plant's foliar tissue.