Temporal and spatial variability of animal sound within a neotropical forest

Soundscape ecology aims to use biological, geophysical and anthropogenic sound to understand natural-human landscape dynamics. The analysis of natural soundscapes with no human noise is a prerequisite to understand and quantify the effects of human activity on animal ecology linked to sound. Preserved tropical forests are the location of unique, highly diverse, and animal sound. However, although the acoustic behavior of several tropical species has been examined, very few analyses have attempted tropical sounds at a spatial scale able to incorporate landscape characters. Here we analyze the acoustic structure of a neotropical forest landscape in French Guiana. We used a four dimensional synchronous acoustic sampling (three spatial dimensions and the temporal dimension) by deploying an array of 24 microphones in the understory and canopy of the Nouragues Nature Reserve during a 43 day period and we undertook a detailed signal analysis to detect spatial and temporal animal acoustic heterogeneity. We identified a clear pattern of acoustic activity with four distinct periods of activity that differed by their spectral characteristics indicating acoustic heterogeneity along the 24-hour cycle but periodicity at a longer time scale. We revealed acoustic divergences between the understory and the canopy layers in terms of amplitude level and frequency content. We highlighted vertical (understory/canopy) and horizontal acoustic heterogeneities with a more diverse (frequency) patch in the north of the study area sampled and a more active (intensity) patch in the southeast of the study area. Our results show that the soundscape of a tropical forest, in the absence of human disturbance, is subtly structured in time and is heterogeneous in space. This structure is probably linked to endogenous factors that rule out the acoustic time activity of animal species, to the vertical stratification of singing communities or guilds, to horizontal variations in the distributions of species and to vegetation spatial heterogeneity. Our study emphasizes that tropical soundscapes need to be recorded and analyzed in considerable spatial and temporal detail to understand their dynamics without the presence of human produced noise. Our analysis also suggests that tropical forests are unique places for acoustic diversity, supporting the need for preservation from all perturbations including anthropic noise.     

The effect of soundscape composition on bird vocalization classification in a citizen science biodiversity monitoring project

There is a need for monitoring biodiversity at multiple spatial and temporal scales to aid conservation efforts. Autonomous recording units (ARUs) can provide cost-effective, long-term and systematic species monitoring data for sound-producing wildlife, including birds, amphibians, insects and mammals over large areas. Modern deep learning can efficiently automate the detection of species occurrences in these sound data with high accuracy. Further, citizen science can be leveraged to scale up the deployment of ARUs and collect reference vocalizations needed for training and validating deep learning models. In this study we develop a convolutional neural network (CNN) acoustic classification pipeline for detecting 54 bird species in Sonoma County, California USA, with sound and reference vocalization data collected by citizen scientists within the Soundscapes to Landscapes project (www.soundscapes2landscapes.org). We trained three ImageNet-based CNN architectures (MobileNetv2, ResNet50v2, ResNet100v2), which function as a Mixture of Experts (MoE), to evaluate the usefulness of several methods to enhance model accuracy. Specifically, we: 1) quantify accuracy with fully-labeled 1-min soundscapes for an assessment of real-world conditions; 2) assess the effect on precision and recall of additional pre-training with an external sound archive (xeno-canto) prior to fine-tuning with vocalization data from our study domain; and, 3) assess how detections and errors are influenced by the presence of coincident biotic and non-biotic sounds (i.e., soundscape components). In evaluating accuracy with soundscape data (n = 37 species) across CNN probability thresholds and models, we found acoustic pre-training followed by fine-tuning improved average precision by 10.3% relative to no pre-training, although there was a small average 0.8% reduction in recall. In selecting an optimal CNN architecture for each species based on maximum F(β = 0.5), we found our MoE approach had total precision of 84.5% and average species precision of 85.1%. Our data exhibit multiple issues arising from applying citizen science and acoustic monitoring at the county scale, including deployment of ARUs with relatively low fidelity and recordings with background noise and overlapping vocalizations. In particular, human noise was significantly associated with more incorrect species detections (false positives, decreased precision), while physical interference (e.g., recorder hit by a branch) and geophony (e.g., wind) was associated with the classifier missing detections (false negatives, decreased recall). Our process surmounted these obstacles, and our final predictions allowed us to demonstrate how deep learning applied to acoustic data from low-cost ARUs paired with citizen science can provide valuable bird diversity data for monitoring and conservation efforts.     

High Bird Species Diversity in Structurally Heterogeneous Farmland in Western Kenya

Tropical ecosystems are globally important for bird diversity. In many tropical regions, land‐use intensification has caused conversion of natural forests into human‐modified habitats, such as secondary forests and heterogeneous agricultural landscapes. Despite previous research, the distribution of bird communities in these forest‐farmland mosaics is not well understood. To achieve a comprehensive understanding of bird diversity and community turnover in a human‐modified Kenyan landscape, we recorded bird communities at 20 sites covering the complete habitat gradient from forest (near natural forest, secondary forest) to farmland (subsistence farmland, sugarcane plantation) using point counts and distance sampling. Bird density and species richness were on average higher in farmland than in forest habitats. Within forest and farmland, bird density and species richness increased with vegetation structural diversity, i.e., were higher in near natural than in secondary forest and in subsistence farmland than in sugarcane plantations. Bird communities in forest and farmland habitats were very distinct and very few forest specialists occurred in farmland habitats. Moreover, insectivorous bird species declined in farmland habitats whereas carnivores and herbivores increased. Our study confirms that tropical farmlands can hardly accommodate forest specialist species. Contrary to most previous studies, our findings show that structurally rich tropical farmlands hold a surprisingly rich and distinct bird community that is threatened by conversion of subsistence farmland into sugarcane plantations. We conclude that conservation strategies in the tropics must go beyond rain forest protection and should integrate structurally heterogeneous agroecosystems into conservation plans that aim at maintaining the diverse bird communities of tropical forest‐farmland mosaics.     

Are Centrally Displaying Males Always the Centre of Female Attention? Acoustic Display Position and Female Choice in a Lek Mating Subterranean Insect

Several conceptual models seek to explain patterns of male display and factors that influence female mate choice in lek mating systems. The central advantage model predicts that males displaying at or near the lek centre should be more attractive to females than are males positioned along the lek periphery. Females may exhibit biases toward these centrally displaying males based on either spatial or display‐related cues. We tested the prediction of the central advantage model in investigating the importance of male display position in the subterranean and lek mating prairie mole cricket (Gryllotalpa major). Gryllotalpa major males form mating aggregations in the early spring and produce an acoustic advertisement signal from a constructed calling chamber at the soil surface. Pair formation occurs in the calling chamber, and males typically maintain these structures for the duration of the reproductive season. To assess whether G. major females exhibit a preference for males calling from centrally located acoustic burrows, we documented the spatial position and number of female attractions for all advertising males across the focal lek. Six spatial attributes related to display position were reduced using principal component analysis and examined for an association with male attractiveness. We found that in general, female attractions were distributed randomly across the lek; male attractiveness was not related to proximity to the lek centre nor to any factor associated with display position. The most highly attractive males, however, were located further from the lek centre and from nearest calling neighbours than other attractive males. Advertising males that segregate themselves within the aggregation and locate nearer the lek margin may gain a geometric advantage resulting in the increased probability of attracting a searching female.     

Temporal Changes in Randomness of Bird Communities across Central Europe

Many studies have examined whether communities are structured by random or deterministic processes, and both are likely to play a role, but relatively few studies have attempted to quantify the degree of randomness in species composition. We quantified, for the first time, the degree of randomness in forest bird communities based on an analysis of spatial autocorrelation in three regions of Germany. The compositional dissimilarity between pairs of forest patches was regressed against the distance between them. We then calculated the y-intercept of the curve, i.e. the ‘nugget’, which represents the compositional dissimilarity at zero spatial distance. We therefore assume, following similar work on plant communities, that this represents the degree of randomness in species composition. We then analysed how the degree of randomness in community composition varied over time and with forest management intensity, which we expected to reduce the importance of random processes by increasing the strength of environmental drivers. We found that a high portion of the bird community composition could be explained by chance (overall mean of 0.63), implying that most of the variation in local bird community composition is driven by stochastic processes. Forest management intensity did not consistently affect the mean degree of randomness in community composition, perhaps because the bird communities were relatively insensitive to management intensity. We found a high temporal variation in the degree of randomness, which may indicate temporal variation in assembly processes and in the importance of key environmental drivers. We conclude that the degree of randomness in community composition should be considered in bird community studies, and the high values we find may indicate that bird community composition is relatively hard to predict at the regional scale.     

Acoustic niche partitioning among seven cuckoo species in a forest in eastern China

Acoustic communication among birds plays an important role in attracting mates and defending territories. For the successful transmission of songs, individuals of different species often avoid singing at the same time to reduce acoustic interference from background noise and overlapping signals from heterospecifics. Such behavioural acoustic niche partitioning may occur especially among closely related species due to their ecological similarities. In this study, we recorded bird sounds in a subtropical forest in China in May–June 2019 and detected seven cuckoo species. Extracting characteristics of the cuckoo calls, we found that only four of the 21 pairs of species overlapped in frequency range, and 19 pairs were classified accurately using a linear discriminant analysis classifier based on their features. The remaining two species pairs could be separated based on temporal or spatial distribution patterns. We also analysed the temporal distribution patterns and overlap time of the calls, finding that the seven species exhibit partitioning in at least one of three acoustic dimensions (site, frequency, activity time). We conclude that the seven sympatric cuckoo species were strongly partitioned in acoustic signal space and minimally masked each other's signals.     

A test of the Acoustic Adaptation Hypothesis in three types of tropical forest: degradation of male and female Rufous-and-white Wren songs

Many animals produce complex vocalizations that show pronounced variation between populations. The Acoustic Adaptation Hypothesis helps to explain this variation, suggesting that acoustic signals are optimized for transmission through different environments. Little is known about the transmission properties of female vocalizations because most studies of the Acoustic Adaptation Hypothesis have focused on male vocalizations of organisms living at temperate latitudes. We explored the relationship between environmental variation and the transmission properties of songs of Rufous-and-white Wrens, resident Neotropical songbirds where both sexes sing. Using playback, we broadcast and re-recorded elements of male and female songs from three populations of wrens living in three different forest habitats in Costa Rica. We measured four variables of the re-recorded sounds: signal-to-noise ratio, excess attenuation, tail-to-signal ratio and blur ratio. Our results show a significant difference between transmission characteristics of both male and female song elements across the three habitats, indicating that sounds transmit differently through different types of tropical forest. The population from which the broadcast sounds were recorded (source population) had little effect on sound transmission, however, suggesting that acoustic differences between these populations may not arise through acoustic adaptation to these habitats. Male and female elements showed similar transmission properties overall, although signal-to-noise ratio of male elements was influenced by source population, whereas blur ratio and excess attenuation of female elements were influenced by source population. Our study highlights the differences in transmission characteristics of animal sounds through different habitats, and reveals some sex differences in transmission properties.     

First automatic passive acoustic tool for monitoring two species of procellarides (Pterodroma baraui and Puffinus bailloni) on Reunion Island,Indian Ocean

Here are proposed two automatic detectors of Barau's petrel (Pterodroma baraui) and tropical shearwater (Puffinus bailloni) vocalisations in noisy audio recordings (1) trained with a low number of positive training instances, and (2) whose performances would be the highest possible. To do so, acoustic recordings were performed in one Barau's petrel colony between February and May 2014 (85 h) and in two tropical shearwater colonies in March and April (21 h). Manual and automatic methods of segmentation were combined. Manual segmentation allowed (1) to miss a very few number of positive segments and (2) to avoid introducing false positive instances. Automatic segmentation provided quickly a diversified set of negative instances. Manual labelling must be regarded as an investment, for current and future works. A random forest classifier and classical methods of acoustic signal characterisation (cepstral coefficients, spectral moments, etc.) were tested. Best models were able to discriminate each target species calls from other sounds of its colony with F1 scores of 88% (Barau's petrel, 1015 samples) and 85% (tropical shearwater, 1217 samples). The acoustic monitoring of nocturnal burrow-nesting seabirds based on (1) data collected by autonomous recording units in harsh, windy and wet environments and (2) automatic analysis tools is feasible. The size of our database was limited. Consequently further works will be necessary to study robustness of models on long time-series data.     

基于语谱图特征信息分割提取的声景观中鸟类生物多样性分析

声学手段是监测和研究生态系统生物活动规律、评价生态系统健康状况的一种新方法,声景观生态学也是景观生态学的一个新兴研究领域。声景指数是描述复杂的音频数据生态学特征的有效方法,但是,单一的声景指数并不能有效的指示物种的真实丰度。在充分挖掘音频文件时频结构特征的基础上,将遥感领域常用的面向对象图像分割技术引入语谱图分割,并提出了适合于自然界鸟类生物多样性提取的知识规则和斑块统计分析方法。研究实验在杭州植物园的不同区域布点采集音频数据,研究结果表明:鸟类的多样性与地物景观类型和人类活动影响程度密切相关,鸟类叫声的中心频率集中分布在2.5—4.5k Hz之间,最低频率分布在0.67—2.1k Hz之间,最高频率分布在7.6—8.9k Hz之间,人类活动较多的区域,鸟类活动较少且叫声更短促。此外,该方法提取的斑块面积周长比参数,可以定量的反映鸟类叫声的婉转程度。     

Distinct carbon sources indicate strong differentiation between tropical forest and farmland bird communities

The conversion of forest into farmland has resulted in mosaic landscapes in many parts of the tropics. From a conservation perspective, it is important to know whether tropical farmlands can buffer species loss caused by deforestation and how different functional groups of birds respond to land-use intensification. To test the degree of differentiation between farmland and forest bird communities across feeding guilds, we analyzed stable C and N isotopes in blood and claws of 101 bird species comprising four feeding guilds along a tropical forest-farmland gradient in Kenya. We additionally assessed the importance of farmland insectivores for pest control in C₄ crops by using allometric relationships, C stable isotope ratios and estimates of bird species abundance. Species composition differed strongly between forest and farmland bird communities. Across seasons, forest birds primarily relied on C₃ carbon sources, whereas many farmland birds also assimilated C₄ carbon. While C sources of frugivores and omnivores did not differ between forest and farmland communities, insectivores used more C₄ carbon in the farmland than in the forest. Granivores assimilated more C₄ carbon than all other guilds in the farmland. We estimated that insectivorous farmland birds consumed at least 1,000 kg pest invertebrates km^?2 year^?1. We conclude that tropical forest and farmland understory bird communities are strongly separated and that tropical farmlands cannot compensate forest loss for insectivorous forest understory birds. In tropical farmlands, insectivorous bird species provide a quantitatively important contribution to pest control.     

Avian Habitat Preference in Tropical Forest Restoration in Southern Costa Rica

An important question for tropical forest restoration is whether degraded lands can be actively managed to attract birds. We censused birds and measured vegetation structure at 27 stations in young (6–9‐yr old) actively and passively restored pasture and old growth forest at Las Cruces Biological Station in southern Costa Rica. During 481 10‐min point counts, we detected a high diversity—186 species—of birds using the restoration area. Surprisingly, species richness and detection frequency did not differ among habitats, and proportional similarity of bird assemblages to old growth forest did not differ between restoration treatments. Bird detection frequency was instead explained by exotic grass cover and understory stem density—vegetation structures that were not strongly impacted by active restoration. The similarity of bird assemblages in actively and passively restored forest may be attributed to differential habitat preferences within and among feeding guilds, low structural contrast between treatments, or the effect of nucleation from actively restored plots into passively restored areas. Rapid recovery of vegetation in this recently restored site is likely due to its proximity to old growth forest and the lack of barriers to effective seed dispersal. Previous restoration studies in highly binary environments (i.e., open pasture vs. tree plantation) have found strong differences in bird abundance and richness. Our data contradict this trend, and suggest that tropical restoration ecologists should carefully consider: (1) when the benefits of active restoration outweigh the cost of implementation; and (2) which avian guilds should be used to measure restoration success given differential responses to habitat structure.     

Frugivoría por Aves en un Paisaje Fragmentado: Consecuencias en la Dispersión de Semillas1

Few data exist on seed dispersal by frugivorous birds in fragmented landscapes, originating from tropical dry forests, in contrast to more abundant data from tropical rain forests. In this study, we assessed the effect of frugivorous birds in a fragmented landscape of Veracruz, Mexico, now occupied by remnant fragments of tropical semi‐deciduous forest and dry deciduous forest, grassland, and shrubby patches on sand dunes. We determined four characteristics related to seed dispersal by birds: the interacting species of plants and birds, the characteristics of these species, spatio‐temporal variation in the dispersal system, and the outcome of the process. During one year, we recorded 54 frugivorous bird species and 33 ornithochorous plant species, which engaged in 176 different bird‐plant species interactions. Similarity (Sorensen index) of frugivorous bird communities using different vegetation types was high (>70%), suggesting that many bird species used all of the vegetation types. In contrast, the similarity of ornithochorous plant communities among vegetation types commonly was low (<37%), suggesting that most plant species were restricted to particular sites in this landscape. At the landscape level, as well as for tropical deciduous forest, we detected a significant positive relationship (Spearman's correlation of rank coefficient >0.65, P <0.05) among richness per month of frugivorous birds and plant species bearing fleshy fruits. Seeds of many plant species previously detected in studies of seed rain at the site were eaten by birds during this study. Most seeds of zoochorous species, which are deposited in the dry and decidous tropical forests patches, are produced within these vegetation types (i.e., they are autochthonous species), whereas bird‐dispersed seeds arriving in grassland and shrubby patches are produced outside (i.e., allochthonous) and are mostly woody species. Birds are important seed dispersers among vegetation types in this landscape but they have different effects in each one. The four characteristics studied, as well as the landscape approach of this research, allowed us to detect spatial and temporal patterns that otherwise would have remained undetected.     

Temporal bird community dynamics are strongly affected by landscape fragmentation in a Central American tropical forest region

Habitat loss and fragmentation are considered the main causes of species extinctions, particularly in tropical ecosystems. The objective of this work was to evaluate the temporal dynamics of tropical bird communities in landscapes with different levels of fragmentation in eastern Guatemala. We evaluated five bird community dynamic parameters for forest specialists and generalists: (1) species extinction, (2) species turnover, (3) number of colonizing species, (4) relative species richness, and (5) a homogeneity index. For each of 24 landscapes, community dynamic parameters were estimated from bird point count data, for the 1998–1999 and 2008–2009 periods, accounting for species’ detection probability. Forest specialists had higher extinction rates and a smaller number of colonizing species in landscapes with higher fragmentation, thus having lower species richness in both time periods. Alternatively, forest generalists elicited a completely different pattern, showing a curvilinear association to forest fragmentation for most parameters. Thus, greater community dynamism for forest generalists was shown in landscapes with intermediate levels of fragmentation. Our study supports general theory regarding the expected negative effects of habitat loss and fragmentation on the temporal dynamics of biotic communities, particularly for forest specialists, providing strong evidence from understudied tropical bird communities.     

Acoustic region workflow for efficient comparison of soundscapes under different invasive mammals' management regimes

One quarter of all terrestrial native bird species have become extinct since human arrival in New Zealand, leading to a pervasive silence in many natural environments due to the decrease in native bird song. Passive acoustic techniques are a potential tool for environmental monitoring, especially for testing whether the control of mammals can reverse the ‘silent forest’ effect. Here we compare soundscapes from two nearby sites within the Waitakere Ranges Regional Park, New Zealand, that have contrasting predator control levels: one with high-level pest mammal control, and the other with low-level pest control. Measurements of twelve acoustic indices extracted from two seasons of passive acoustic recordings are split into 20 acoustic regions to identify which regions best discriminate between the two management regimes. We define the acoustic regions as units of analysis bounded by a specific time period and frequency range chosen to capture the main groups of biologically relevant acoustic events within a soundscape. Analysis of variance and pairwise comparisons indicated the acoustic region bounded from 9 pm to 11:59 pm and a range of 0.988–3.609 kHz in autumn presented the greatest differences between sites. The sounds responsible for these acoustic differences were generated by invasive mammals in the site with no pest control. Results also supports spring season as the most important for bird monitoring in New Zealand. Acoustic indices analysis did not detect a reversal of the “silence forest” effect in the site with high-level predator control.     

Low-cost open-source recorders and ready-to-use machine learning approaches provide effective monitoring of threatened species

Passive acoustic monitoring is a powerful tool for monitoring vocally active taxa. Automated signal recognition software reduces the expert time needed for recording analyses and allows researchers and managers to manage large acoustic datasets. The application of state-of-the-art techniques for automated identification, such as Convolutional Neural Networks, may be challenging for ecologists and managers without informatics or engineering expertise. Here, we evaluated the use of AudioMoth — a low-cost and open-source sound recorder — to monitor a threatened and patchily distributed species, the Eurasian bittern (Botaurus stellaris). Passive acoustic monitoring was carried out across 17 potential wetlands in north Spain. We also assessed the performance of BirdNET — an automated and freely available classifier able to identify over 3000 bird species — and Kaleidoscope Pro — a user-friendly recognition software — to detect the vocalizations and the presence of the target species. The percentage of presences and vocalizations of the Eurasian bittern automatically detected by BirdNET and Kaleidoscope Pro software was compared to manual annotations of 205 recordings. The species was effectively recorded up to distances of 801–900 m, with at least 50% of the vocalizations uttered within that distance being manually detected; this distance was reduced to 601–700 m when considering the analyses carried out using Kaleidoscope Pro. BirdNET detected the species in 59 of the 63 (93.7%) recordings with known presence of the species, while Kaleidoscope detected the bittern in 62 recordings (98.4%). At the vocalization level, BirdNet and Kaleidoscope Pro were able to detect between 76 and 78%, respectively, of the vocalizations detected by a human observer. Our study highlights the ability of AudioMoth for detecting the bittern at large distances, which increases the potential of that technique for monitoring the species at large spatial scales. According to our results, a single AudioMoth could be useful for monitoring the species' presence in wetlands of up to 150 ha. Our study proves the utility of passive acoustic monitoring, coupled with BirdNET or Kaleidoscope Pro, as an accurate, repeatable, and cost-efficient method for monitoring the Eurasian bittern at large spatial and temporal scales. Nonetheless, further research should evaluate the performance of BirdNET on a larger number of species, and under different recording conditions (e.g., more closed habitats), to improve our knowledge about BirdNET's ability to perform bird monitoring. Future studies should also aim to develop an adequate protocol to perform effective passive acoustic monitoring of the Eurasian bittern.     

Spatio‐temporal scaling of biodiversity in acoustic tropical bird communities

Automated analysis of acoustic communities is a rapidly emerging approach for the characterization and monitoring of biodiversity. To evaluate its utility, we should verify that such ‘bioacoustics’ can accurately detect ecological signal in spatiotemporal acoustic data. Targeting the ‘Biological Dynamics of Forest Fragments Project’ sites in Brazil, we ask: What is the relative contribution of the spatial, temporal and habitat dimension to variation in bird acoustic communities in a previously fragmented tropical rainforest? Does the functional diversity of bird communities scale similarly to space and time as does species diversity, when both are recorded by bioacoustics means? Overall, is the imprint of landscape fragmentation 30 years ago still audible in the present‐day soundscape? We sampled forty‐four sites in secondary forest and 107 sites in old‐growth forest, resulting in 11 000 h of audio recordings. We detected 60 bird species with satisfactory precision and recovered a linear log–log relation between sampling time and species diversity. Sites in primary forest host more species than sites in secondary forest, but the difference decreased with sampling time, as the slope was slightly higher in secondary than primary forests. Functional diversity, as exposed by vocalizing birds, accumulates faster than does species diversity. The similarity among local communities decreases with distance in both time and space, but stability in time is remarkably high: two acoustic samples from the same site one year (or more) apart prove more similar than two samples taken at the same time but from sites situated just a few hundred meters apart. These findings suggest that habitat modification can be heard as a long‐lasting imprint on the soundscape of regenerating habitats and identify soundscape–area and soundscape–time relations as a promising tool for biodiversity research, applied biomonitoring and restoration ecology.     

Cavity-nesting birds are limited by nesting habitat in Neotropical agricultural landscapes

Most studies comparing biodiversity between natural and human-modified landscapes focus on patterns in species occurrence or abundance, but do not consider how different habitat types meet species' breeding requirements. Organisms that use or nest in tree cavities may be especially threatened by habitat conversion due to the loss of their nesting sites. Although cavity-nesting bird diversity is highest in the tropics, little is known about how tropical birds use cavities, how agriculture affects their reproductive biology, and how effective nest boxes could be as a conservation strategy in tropical agriculture. Here, we explored how habitat conversion from tropical forests to pasture affects the abundance, nesting habitat availability, and nest success of cavity-nesting birds in Northwest Ecuador. We conducted bird surveys and measured natural cavity availability and use in forest and agriculture. We also added artificial nest boxes to forest and agriculture to see whether cavity limitation in agriculture would elicit higher use of artificial nest boxes. We found evidence of cavity limitation in agriculture—there were many more natural cavities in forest than in agriculture, as well as more avian use of nest boxes placed in agriculture as compared to forest. Our results suggest that it is important to retain remnant trees in tropical agriculture to provide critical nesting habitat for birds. In addition, adding nest boxes to tropical agricultural systems could be a good conservation strategy for certain species, including insectivores that could provide pest-control services to farmers. Abstract in Spanish is available with online material.     

LEK BEHAVIOUR AND BREEDING OF GUY'S HERMIT HUMMINGBIRD PHAETHORNIS GUY

Guy's Hermit Phaethornis guy, a large forest hummingbird, was studied for 2¹/₂ years in the Northern Range of Trinidad. Regular observations made at a lek of 16 males showed that leks are occupied from November until the beginning of July, and that individual birds retain their lek territories from year to year and use the same perches. The usual song is a monosyllabic note regularly repeated, but the quality of the note varied between different leks and between different parts of a lek, and a few males had a disyllabic song. Young males join the lek between April and June and learn the song of their immediate neighbours. Their plumage, when they first join, is similar to a female's, with distinct pale super- and subciliary stripes. The acquisition of the dark head of the adult male probably takes four years. Before they land on their lek perches, males perform an elaborate aerial display in which the red gape is flashed open and a loud tock is uttered. When males visit each other they rapidly change places on the lek perch, the airborne bird performing the tock-display over the perched bird. A similar tock-displaying with changing of place occurs when females visit males; twice these were seen to culminate in mounting. Periodically all males at the lek ‘false-mate’ with a leaf or other piece of vegetation near their lek perches, after tock-displaying above it. Young males false-mate much more frequently than older males. The breeding season lasts for eight months, the peak of breeding coinciding closely with the period when the two most important sources of nectar (Heliconia bihai and Pachystachys coccinea) are available. Males occasionally visit nests, and possibly assist in defending them. Undefended nests are quickly dismantled by other birds in search of nest-material. Of 19 nests found before the clutch was complete, five (26%) were successful. The incubation period is 17–18 days, and the fledging period 21–23 days.     

Attaining the canopy in dry and moist tropical forests: strong differences in tree growth trajectories reflect variation in growing conditions

Availability of light and water differs between tropical moist and dry forests, with typically higher understorey light levels and lower water availability in the latter. Therefore, growth trajectories of juvenile trees—those that have not attained the canopy—are likely governed by temporal fluctuations in light availability in moist forests (suppressions and releases), and by spatial heterogeneity in water availability in dry forests. In this study, we compared juvenile growth trajectories of Cedrela odorata in a dry (Mexico) and a moist forest (Bolivia) using tree rings. We tested the following specific hypotheses: (1) moist forest juveniles show more and longer suppressions, and more and stronger releases; (2) moist forest juveniles exhibit wider variation in canopy accession pattern, i.e. the typical growth trajectory to the canopy; (3) growth variation among dry forest juveniles persists over longer time due to spatial heterogeneity in water availability. As expected, the proportion of suppressed juveniles was higher in moist than in dry forest (72 vs. 17%). Moist forest suppressions also lasted longer (9 vs. 5 years). The proportion of juveniles that experienced releases in moist forest (76%) was higher than in dry forest (41%), and releases in moist forests were much stronger. Trees in the moist forest also had a wider variation in canopy accession patterns compared to the dry forest. Our results also showed that growth variation among juvenile trees persisted over substantially longer periods of time in dry forest (>64 years) compared to moist forest (12 years), most probably because of larger persistent spatial variation in water availability. Our results suggest that periodic increases in light availability are more important for attaining the canopy in moist forests, and that spatial heterogeneity in water availability governs long-term tree growth in dry forests.