A biogeographic comparison of two convergent bird families

Abstract

Convergence between species and entire clades can occur due to shared environmental conditions and shared resource use. Comparisons of biogeography between convergent clades and taxa may reveal some of these properties unique to each taxon. We sought to characterize and compare the global scale biogeography of hummingbirds (family Trochilidae), which possess unique adaptations for nectar feeding, with sunbirds (family Nectariniidae), which also feed on nectar but are more generalist in their feeding ecology. We collected the latitudinal and elevational range of all species in both clades to create species distributions along those gradients by way of empirical cumulative distribution functions. We compared those distributions to see 1) if they differed, by way of minimum difference estimation and 2) how they differed, by way of non-linear regression. Hummingbirds are shown to extend into higher elevations and latitudes compared to sunbirds, and better maintain their species number in these more extreme environments. We provide possible reasons for these patterns including dispersal limitation, land area, diversity of resources, and climatic conditions. In one particularly interesting hypothesis, we propose that hummingbirds' unique adaptations for nectar feeding allow them to exploit resources more efficiently, gain higher intrinsic fitness, and therefore speciate and spread into more extreme climates than less efficient nectar feeding sunbirds.

Fig 1. Geographic Map of Family Ranges.

A map of species number of hummingbirds (purple) and sunbirds (green), normalized to the maximum species number per area of each clade. Richer colors represent greater species number. Scales are chosen to reflect the difference in overall species number of each taxon. Hummingbirds not only have higher species number but also extend farther latitudinally. This figure was created with the ‘map’ function from the ‘maps’ package in R and the shapefiles from BirdLife International and NatureServe [27].

Fig 2. Figure of Elevational Gradients for the Families.

Changes in relative species number with elevation (normalized to each clade’s maximum number at a given elevation) for hummingbirds (purple) and sunbirds (green). Though both clades extend to similar altitudes, hummingbirds maintain species number at higher elevations as denoted by the richer color.

Fig 3. Normalized Species Richness of the Families.

A plot of the normalized species number $S_N$ of hummingbirds (Trochilidae) and sunbirds (Nectariinidae), along with the fitted line, for elevation (a, c) and “polewardness” (b, d). Triangles and purple lines denote hummingbirds, and circles and green lines denote sunbirds. Hummingbirds maintain species number at higher elevations and mid-latitudinal ranges, and extend farther latitudinally than sunbirds. Inflection (cross) and MMC points (asterisks) also are shown (c, d). Inflection points come later in hummingbirds than sunbirds. With regard to elevation, hummingbird $S_N$ and sunbird $S_N$ start their decline at a similar elevation but hummingbird $S_N$ declines more slowly. With latitude, sunbird $S_N$ declines earlier than hummingbird $S_N$.

Fig 4. ECDF Figures.

A plot of four select ECDFs used to compare hummingbird (Trochilidae) and sunbird (Nectariinidae) distributions. Purple lines indicate hummingbirds and green lines indicate sunbirds. Hummingbird ECDFs almost entirely lie below sunbird ECDFs, with the greatest difference occurring with respect to elevation measures.