Some forest-dwelling tropical bird species are unable or unwilling to fly even a few hundred meters across habitat discontinuities, restricting connectivity of isolated populations in fragmented landscapes. Experimental evidence for flight limitation has reliably predicted occurrence across archipelagoes of habitat fragments varying in their distance from potential source populations. Mechanistic explanations for large differences in flight capacity of tropical birds, even over remarkably short distances, have not been tested. We evaluated myoglobin concentration in pectoralis muscles and hearts of eight Neotropical species for which experimental evidence revealing a wide range of flight abilities exists. We found a strong positive relationship between myoglobin concentration in the pectoralis muscles, but not hearts, and average over-water flight distance during dispersal-challenge experiments. The approximately 2.5-fold difference in pectoralis myoglobin concentration is directly associated with flight capacity and allows predictions of species or species groups most likely to be impeded by habitat discontinuities and therefore at conservation risk in fragmented landscapes.

Study Species and Tissue Samples

We obtained heart and pectoralis major tissue samples from 8 non-migratory, resident, forest-dwelling bird species studied in overwater dispersal challenge experiments in Panama ((Moore et al., 2008); supplemental Data File). The species varied in body mass (6-39 g) and represented five taxonomic families and five foraging guilds (Table 1). Fresh samples were obtained from birds collected during expeditions (1982-2007) of the Louisiana State Museum of Natural Sciences. Samples were frozen immediately in dry ice or liquid nitrogen. Samples were collected primarily in Panama; a few were taken in nearby Costa Rica and Honduras (supplemental Data File). Thus, we assume that measurements are representative of each species as they did not originate from the same individuals used in the flight experiments.

Myoglobin Assays

Myoglobin quantification methods followed (Reynafarje, 1963; Egginton, 1986). Tissues were homogenized in ice cold buffer, at a ratio of 30uL buffer per mg tissue. Buffer consisted of 2 mM DTT, 50 mM phosphate buffer, pH 7.4 at 15° C. Homogenates were centrifuged at 10,000 g, and the supernatant was used for analysis. Absorbance spectra of homogenates were scanned at 1 nm intervals between 490 and 620 nm (BioTek plate reader), which included oxymyoglobin peaks at 542 and 581 nm. The [Mb] specific absorbance was calculated by subtracting baseline absorbance using a millimolar extinction coefficient of 12.8. The amount of [Mb] in a sample is sensitive to dilution; variability in sample preparation was accounted for by standardizing each sample using a total protein assay, measured in triplicate. A path length correction was applied (KC Junior software) to account for variation in pipetting volumes.

Average Flight Distance

Mean flight distance in the overwater dispersal-challenge experiments was used as an index of flight ability because it correlated strongly with bird distributions across islands varying in distance of isolation (50-2500 m) in Lake Gatun, Panama (Moore et al., 2008); Table 2). Briefly, to measure flight distance, birds were captured in mist nests on islands in Lake Gatun or the surrounding mainland forests, quickly transported from capture sites by boat to 100, 200, or 300 m, then released over water. Distances flown back toward forest were measured with a handheld GPS. Average flight distance was the mean calculated from all challenges (100, 200, and 300m) per species. Experiments were conducted primarily in the months of June - September in 2003 and 2004. For further details see Moore et al. (2008). The hummingbird P. longirostris did not fail at any distance, so we arbitrarily set its mean flight distance at 315 m, slightly farther than the maximum challenge distance.

Scientific names of species follow the American Ornithological Society North American Checklist Committee, 2020.

Flyers categorized as good or poor depending on results from Moore et al. 2008. Ecology Letters 11:960

Specimen numbers from Lousiana State University Museum of Natural Sciences.

Specimens from muscle tissues from heart or pectoralis.

Myoglobin concentrations in mg Mb/g