Joint effects of feeding and breeding behaviour on trophic dimorphism in hummingbirds

Abstract

A survey of 166 hummingbird species reveals novel associations of bill-length sexual dimorphism (BLSD) with plumage and breeding behaviours. Across all species, female bills become proportionately longer than male bills (higher female-to-male BLSD ratio) as sexual dichromatism increases. However, male bills are proportionately longer (lower female-to-male BLSD ratio) in both lekkers (traditional group display) and clustered breeders (female harems or colonial nests) compared with dispersed breeders. The overall positive association of plumage with BLSD suggests that social status determines priority of access to nectar-providing £owers. Furthermore, the distinctive BLSD associated with breeding aggregations may arise from behaviours that impose constraints on the usual male priority at £owers: female dominance over males around nest colonies and male residence on lek-mating territories. These various factors appear to alter plumage and bill characters of both sexes to produce the range of dimorphisms within the various dispersed and aggregated breeding system categories. Feedback loops caused by ecological consequences of breeding behaviour may alter the evolutionary dynamics of breeding systems, bird^plant interactions, and competing pollinators, as well as help explain the lek paradox.

Evolution of sexual dimorphism in bill size and shape of hermit hummingbirds (Phaethornithinae): a role for ecological causation

“…Likewise, 85 per cent of the hermits in our study had significant female-biased curvature dimorphism as determined by t-tests within species. Both Bleiweiss (1999) and Colwell (2000) also noted significant sexual dimorphism in bill length of many hummingbird species. Such pronounced sexual dimorphism in bill morphology of hermits raises two questions: (i) what mechanisms favour the evolution of bill dimorphism in hermits?…”

“…Sexual dimorphism in bill curvature was significantly related to both measurements of bill length for uncorrected and phylogenetically independent contrasts ( figure 3a,b). Because bill length dimorphism is female-biased in non-hermit species, which tend to have short bills, and male-biased in hermit species ( figure 1c,d ), which tend to have long bills (Bleiweiss 1999), and because bill curvature dimorphism tends to be absent or reduced in non-hermit species (figure 1a), these significant relationships could be driven by differences between hermit and non-hermit taxa, rather than by evolutionary forces within the hermit subfamily. When non-hermits were excluded from these analyses, sexual dimorphism in bill curvature was significantly related to bill length for both uncorrected and phylogenetically independent contrasts ( figure 3c,d ).…”

“…For each specimen, we measured (in millimetres) the length (chord) of the exposed culmen, total bill length (from bill tip to gape), arc length of the exposed culmen and wing chord. Previous studies of sexual dimorphism in hummingbird bills used either the length of the exposed culmen or total bill length as measures of bill length (Bleiweiss 1999;Colwell 2000). Such measures adequately quantify bill size for birds with straight bills, but they underestimate the amount of tissue devoted to the bill for birds with curved bills.…”

“…Hermit hummingbirds (Phaethornithinae) exhibit relatively modest sexual dimorphism in size and in bill length in comparison to many members of the other subfamily of hummingbirds, the Trochilinae, which includes E. jugularis (Bleiweiss 1999). Many hermits, however, are characterized by highly curved bills, and a preliminary examination of museum specimens by one of us suggested that sexual dimorphism in bill curvature was present in some species (Temeles et al 2009).…”

“…Although sexual dimorphism in bill length and body size of hummingbirds has been precisely studied (e.g. Bleiweiss 1999;Colwell 2000), the significance of sexual dimorphism in bill curvature was not previously considered.…”