Insectivorous birds reach their highest diversity in the tropics and represent a striking variety of morphological and behavioral specializations for foraging, yet explanations for these patterns are inadequate because of both our limited understanding of the drivers of ecological diversification within and among clades and of coexistence mechanisms in particular. Here we synthesize recent information on Neotropical insectivorous birds, including their diversity, evolutionary ages and locations of origin, phylogenies, and both competitive and predator–prey species interactions. We propose a novel evolutionary hypothesis for the origin and coexistence of the phenotypic diversity of insectivore foraging morphologies in species-rich communities, based on their extraordinary food-resource specializations. Specifically, we develop the Biotic Challenge Hypothesis to explain the evolution of these specializations, and we provide preliminary evidence in support of this hypothesis based on a synopsis of both Neotropical insectivore specializations by family and arthropod antipredator adaptations by category. We argue that, from the perspective of tropical insectivorous birds, and particularly in the most species-rich, mainland Neotropical communities, the environment is an arthropod desert. Coexistence with all of the other insectivores requires feeding specialization to compete exploitatively and diffusely against evolutionarily diverse species and far less frequently against sister species. The arthropod desert arises primarily because of (1) the tactical diversity of arthropod predators as insectivore competitors and (2) the evolutionary arms races involving arthropod predators with their prey, which render many arthropods inaccessible to most insectivorous predators. Our idea provides an explicit mechanism for pervasive, diffuse tropical interspecific competition, for evolutionary specialization, and for positive feedback on speciation rates at low latitudes, thereby generating new predictions and insights into tropical life histories and the Latitudinal Diversity Gradient. Other recent ideas concerning the coexistence of Neotropical insectivores, including positive species interactions within mixed species flocks, are recognized and evaluated. We discuss ways to test predictions resulting from the new view of communities developed here, including a case study of diet specialization by Costa Rican tyrannid flycatchers. Our synthesis of the origin and nature of Neotropical insectivore communities injects new life into the “zombie” idea that evolution works differently in the species-rich tropics.

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Songbirds have been traditionally classified into close-ended or open-ended learning species according to the length of the sensitive period during which birds are able to memorize new vocalizations. Closed-ended learners are generally not capable of changing their song after the first year of life, while open-ended learners show song plasticity as adults. A few Turdus species have been suggested to be open-ended learners, but no long-term study has been conducted to investigate their song plasticity over time. We analyzed the songs of clay-colored thrushes, T. grayi, over four successive breeding seasons to assess song plasticity in their syllable repertoires within and between breeding seasons. A total of 16,262 syllables were classified through visual inspection of spectrograms and multidimensional scaling analysis based on spectrogram correlations. On average, 563 ± 153 (SD) syllables per male per breeding season were analyzed. Male repertoire size was 9-20 syllable types. Males were capable of modifying their syllable repertoire between the initial and final periods of the breeding season. Song plasticity within breeding seasons may be associated with imitation between neighboring males, suggesting song learning in males that were ≥2 years old. This short-term plasticity is not enough, however, to explain the high proportion of change (mean = 65 % syllable types) in repertoire composition between breeding seasons in adult males. Song plasticity resulting from annual changes in repertoire composition could be explained by open-ended learning, but another mechanism, extended memory and re-expression, could also explain long-term plasticity. Experimental studies controlling the acoustic environment are needed to determine which mechanism is responsible for such a high level of song plasticity.

Abstract:The neotropical terrestrial insectivoreHenicorhina leucosticta(Troglodytidae) maintains long-term territories through vocalizations and forages among leaf litter trapped in the understorey vegetation and ground litter. The relationship between forest structure andH. leucostictaterritory size was studied in La Selva Biological Station, Costa Rica, during the non-breeding season in 2009. Forest structure was measured by assessing canopy openness and leaf area index (LAI) using hemispherical photography, while territory size was estimated with the playback technique using local conspecific vocalizations. Mean territory area was 3.8 ± 2.8 ha (mean ± SD, n = 10). Territory radius length was similar in old-growth forest and abandoned agro-forest plantations. We found thatH. leucostictaterritory size decreased as median LAI increased. We propose that LAI is related to territory size through the amount of leaf fall and subsequent leaf litter accumulation over the understorey plants, which constitutes an important reservoir of arthropod prey and nest materials forH. leucosticta. The long-term supply of food resources is likely to affect territory size in this species, as well as other insectivorous birds with similar foraging behaviour. These results are congruent with the structural cues hypothesis.

ABSTRACT. Some understory insectivorous birds manage to persist in tropical forest fragments despite significant habitat loss and forest fragmentation. Their persistence has been related to arthropod biomass. In addition, forest structure has been used as a proxy to estimate prey availability for understory birds and for calculating prey abundance. We used arthropod biomass and forest structural variables (leaf area index [LAI] and aerial leaf litter biomass) to explain the abundance of White-breasted Wood-Wrens (Henicorhina leucosticta), tropical understory insectivorous birds, in six forests in the Caribbean lowlands of Costa Rica. To estimate bird abundance, we performed point counts (100-m radius) in two old-growth forests, two second-growth forests, and two selectively logged forests. Arthropod abundance was the best predictor of wood-wren abundance (wi = 0.75). Wood-wren abundance increased as the number of arthropods increased, and the estimated range of bird abundance obtained from the model varied from 0.51 (0.28 -0.93 [95%CI]) to 3.70 (1.68 -5.20 [95%CI]) within sites. LAI was positively correlated to prey abundance (P = 0.01), and explained part of the variation in wood-wren abundance. In forests with high LAI, arthropods have more aerial leaf litter as potential habitat so more potential prey are available for wood-wrens. Forests with a greater abundance of aerial leaf litter arthropods were more likely to sustain higher densities of wood-wrens in a fragmented tropical landscape. RESUMEN. Efecto de la disponibilidad de presas sobre la abundancia de Henicorhina leucosticta, un ave insectívora de los bosques tropicales de tierras bajas Algunas aves insectívoras de sotobosque tienen la capacidad de persistir en fragmentos de bosques tropicales a pesar de la perdida significativa de hábitat y fragmentación del bosque. Su persistencia se ha relacionado con la biomasa de artrópodos. Adicionalmente, la estructura del bosque se ha utilizado como una variable para estimar la abundancia de presas disponibles para las aves del sotobosque y para calcular la abundancia de presas. Usamos la biomasa de artrópodos y variables de la estructura del bosque (Índice delÁrea Foliar [IAF] y biomasa de la hojarasca aérea) para explicar la abundancia de un ave tropical insectívora de sotobosque, Henicorhina leucosticta, en seis bosques en la región de tierras bajas del caribe de Costa Rica. Para estimar la abundancia de aves, realizamos puntos de conteo (radio de 100-m) en dos bosques maduros, dos de crecimiento secundario y dos con tala selectiva. La abundancia de artrópodos fue el mejor predictor de la abundancia de H. leucosticta (wi = 0.75). La abundancia de H. leucosticta incremento a medida que incrementaba el numero de artrópodos, el rango de estimación de la abundancia de aves obtenido a partir del modelo varió entre 0.51 (0.28 -0.93 [95%CI]) y 3.70 (1. 68 -5.20 [95%CI]) dentro de los sitios. IAF se correlacionó positivamente con la abundancia de presas (P = 0.01), y explicó parte de la variación en la abundancia d...