The Island Syndrome in Coastal Wetland Ecosystems: Convergent Evolution of Large Bills in Mangrove Passerines

Abstract

Resumen.-Las aves paserinas que viven en islas tienden a tener picos más grandes que sus parientes continentales. Los cambios morfológicos podrían estar relacionados con la reducción de la competencia intra e interespecífica. Los gorriones emberízidos que habitan marismas de Norteamérica también presentan picos de tamaños consistentemente mayores. Pusimos a prueba la hipótesis de que los paserinos restringidos a bosques de manglares, otro sistema continental con baja diversidad de especies y con altas densidades, también tienen picos más grandes que los de sus parientes más cercanos no asociados con sistemas intermareales. Encontramos un patrón pantropical consistente de picos más largos y profundos en aves paserinas restringidas a manglares. Estos resultados indican lo que parece ser una regla ecológica general: las aves paserinas restringidas a hábitats costeros salinos presentan picos desproporcionadamente más largos en relación con su tamaño corporal, tal como se ha encontrado en islas. El patrón similar en la morfología del pico sugiere que los procesos ecológicos y evolutivos que se pensaba que ocurrían sólo en sistemas insulares pueden ocurrir también en algunos sistemas continentales.-201 - Abstract.-Passerine birds on islands tend to have larger bills than their mainland relatives. The morphological shift may be related to reduced interspecific and increased intraspecific competition. Emberizid sparrows in North American salt marshes also show consistently greater bill size. We tested the hypothesis that passerines restricted to mangrove forests, another continental system with low species diversity and high population densities, also have larger bills than their closest nontidal relatives. We found a consistent pantropical pattern of longer and deeper bills in passerine birds restricted to mangroves. These results indicate that disproportionately longer bills in relation to body size in passerines restricted to coastal saline habitats, just like those found on islands, seems to be a general ecological rule. The similar pattern in bill morphology suggests that ecological and evolutionary processes thought to occur only in island systems might also occur in some continental systems.

Morphological adaptation to coastal marshes in spite of limited genetic structure in the Neotropical passerineSpartonoica maluroides(Aves: Furnariidae)

“…The general pattern among passerine birds that inhabit islands versus continental habitats is a trend toward increasing bill size (Scott et al ., ). This pattern was also documented for bird species that inhabit saltmarshes (Greenberg & Olsen, ) and mangrove habitats (Luther & Greenberg, ), habitats that resemble island ecosystems. Greater bill size of island birds may be associated with a shift toward greater generalization in resource use, but specialization in diet (invertebrates) or foraging behaviour (probing in tidal mud) are legitimate alternatives (Greenberg & Olsen, ).…”

Habitat type and ambient temperature contribute to bill morphology

“…Previous research indicates that bill size is greater in male YEWA that are year‐round residents in mangrove forests compared with migratory YEWA that inhabit inland terrestrial habitats, even after correcting for body size (Luther and Greenberg ). Based on predictions from recent research on bill morphology of birds restricted to salt marshes (Greenberg and Olsen ; Greenberg et al.…”

Abundance and Distribution of Mangrove Landbirds in Florida

“…Although the short red mangroves that predominate in Hell's Bay may be structurally similar to the shrubby forest edges occupied elsewhere by this species, the presence of persistent standing water would seem to pose a challenge for a ground-foraging species such as eastern towhee. Additional observations on habitat use and behavior in this population would be of great interest, especially given recent evidence that selective pressures in mangroves may be substantially different than those in adjacent, upland environments (Luther and Greenberg 2011).…”

“…Red-bellied woodpecker, downy woodpecker, northern cardinal, common yellowthroat, red-shouldered hawk, Carolina wren, great crested flycatcher, blue jay, eastern towhee, northern flicker, and mourning dove all occurred at greater densities in the pine forests. Luther and Greenberg (2011) suggested that mangroves might function like islands, whereby low species richness would lead to density compensation (e.g., Crowell 1962;MacArthur et al 1972) and niche expansion. We found that the landbird assemblage in Florida's mangroves was species-poor, but we found no evidence of density compensation.…”