W. Alice Boyle

Storms drive altitudinal migration in a tropical bird

Although migration is a widespread and taxonomically diverse behaviour, the ecological factors shaping migratory behaviour are poorly understood. Like other montane taxa, many birds migrate along elevational gradients in the tropics. Forty years ago, Alexander Skutch postulated that severe storms could drive birds to migrate downhill. Here, we articulate a novel mechanism that could link storms to mortality risks via reductions in foraging time and provide, to our knowledge, the first tests of this hypothesis in the White-ruffed Manakin (Corapipo altera), a small partially migratory frugivore breeding on the Atlantic slope of Costa Rica. As predicted, variation in rainfall was associated with plasma corticosterone levels, fat stores, plasma metabolites and haematocrit. By collecting data at high and low elevation sites simultaneously, we also found that high-elevation residents were more adversely affected by storms than low elevation migrants. These results, together with striking temporal capture patterns of altitudinal migrants relative to storms, provide, to our knowledge, the first evidence that weather-related risks incurred by species requiring high food intake rates can explain altitudinal migrations of tropical animals. These findings resolve conflicting evidence for and against food limitation being important in the evolution of this behaviour, and highlight how endogenous and exogenous processes influence life-history trade-offs made by individuals in the wild. Because seasonal storms are a defining characteristic of most tropical ecosystems and rainfall patterns will probably change in ensuing decades, these results have important implications for understanding the ecology, evolution and conservation of tropical animals.

Evolutionary Divergence in Brain Size between Migratory and Resident Birds

Despite important recent progress in our understanding of brain evolution, controversy remains regarding the evolutionary forces that have driven its enormous diversification in size. Here, we report that in passerine birds, migratory species tend to have brains that are substantially smaller (relative to body size) than those of resident species, confirming and generalizing previous studies. Phylogenetic reconstructions based on Bayesian Markov chain methods suggest an evolutionary scenario in which some large brained tropical passerines that invaded more seasonal regions evolved migratory behavior and migration itself selected for smaller brain size. Selection for smaller brains in migratory birds may arise from the energetic and developmental costs associated with a highly mobile life cycle, a possibility that is supported by a path analysis. Nevertheless, an important fraction (over 68%) of the correlation between brain mass and migratory distance comes from a direct effect of migration on brain size, perhaps reflecting costs associated with cognitive functions that have become less necessary in migratory species. Overall, our results highlight the importance of retrospective analyses in identifying selective pressures that have shaped brain evolution, and indicate that when it comes to the brain, larger is not always better.

Altitudinal migration in bats: evidence, patterns, and drivers

Altitudinal migrations are common in all major vertebrate and some invertebrate lineages. Such migrations have important implications for the basic and applied ecology of animals making these movements. The idea that bats make altitudinal migrations has been suggested for nearly a century. However, studies documenting the existence and causes of altitudinal bat migrations are scarce, and are frequently published in the ‘grey’ literature. For the first time, we comprehensively review the evidence supporting the existence of altitudinal bat migrations worldwide, describe basic patterns of migration in temperate and tropical regions, and articulate and propose tests of hypotheses potentially explaining these migrations. We compiled a list of 50 studies indicative of altitudinal bat migration in 61 species (five families) from 21 countries (four continents). The temporal and spatial patterns of these migrations grouped biogeographically. Temperate bats generally exhibit sex‐biased migrations with females inhabiting lower elevations than males during reproductive periods. Although there is less information on tropical bat migration, few studies report sex‐biased migration. We compiled hypotheses proposed in the bat and (more extensive) avian literature to provide a list of hypotheses potentially explaining altitudinal bat migrations. These hypotheses rely upon temporal availability of (and competition for) food resources, spatial distribution of geomorphological features suitable for hibernation, sex‐related differences in the use of torpor, mating opportunities, and climatic factors that impose direct physiological challenges to survival or that restrict the ability to forage. A more thorough description of the migration patterns of most species will be required to distinguish effectively among these hypotheses. We identify research avenues that would broaden our understanding of bat migration patterns and provide critical information required for effective conservation.

Patterns and drivers of intraspecific variation in avian life history along elevational gradients: a meta-analysis

Elevational gradients provide powerful natural systems for testing hypotheses regarding the role of environmental variation in the evolution of life‐history strategies. Case studies have revealed shifts towards slower life histories in organisms living at high elevations yet no synthetic analyses exist of elevational variation in life‐history traits for major vertebrate clades. We examined (i) how life‐history traits change with elevation in paired populations of bird species worldwide, and (ii) which biotic and abiotic factors drive elevational shifts in life history. Using three analytical methods, we found that fecundity declined at higher elevations due to smaller clutches and fewer reproductive attempts per year. By contrast, elevational differences in traits associated with parental investment or survival varied among studies. High‐elevation populations had shorter and later breeding seasons, but longer developmental periods implying that temporal constraints contribute to reduced fecundity. Analyses of clutch size data, the trait for which we had the largest number of population comparisons, indicated no evidence that phylogenetic history constrained species‐level plasticity in trait variation associated with elevational gradients. The magnitude of elevational shifts in life‐history traits were largely unrelated to geographic (altitude, latitude), intrinsic (body mass, migratory status), or habitat covariates. Meta‐population structure, methodological issues associated with estimating survival, or processes shaping range boundaries could potentially explain the nature of elevational shifts in life‐history traits evident in this data set. We identify a new risk factor for montane populations in changing climates: low fecundity will result in lower reproductive potential to recover from perturbations, especially as fewer than half of the species experienced higher survival at higher elevations.

Lekking birds in a tropical forest forego sex for migration

Facultative, partially migratory animals provide a contemporary window into the evolution of migration, offering rare opportunities to examine the life-history trade-offs associated with migration. For the first time, to our knowledge, we describe the nature of these trade-offs, using a lek-breeding tropical bird, the white-ruffed manakin (Corapipo altera). Previous evidence indicated that weather drives post-breeding migration to lower elevations bringing condition-related benefits. Using elevation-sensitive stable isotope measurements and more than 1200 h of behavioural observations, we show that male manakins which migrate incur costs of diminished social status and matings with females the following breeding season. Because migratory tendency depends on inter-annual variation in weather, physical costs of displays and breeding prospects the following year, migratory decisions are subject to both natural and sexual selection, with the outcome of such decisions linked to changing climatic regimes.

Altitudinal bird migration in North America

ABSTRACT Altitudinal bird migration involves annual seasonal movements up and down elevational gradients. Despite the fact that species from montane avifaunas worldwide engage in altitudinal migration, the patterns, causes, and prevalence of these movements are poorly understood. This is particularly true in North America where the overwhelming majority of avian migration research has focused on obligate, long-distance, temperate–tropical movements. Elsewhere in the world, most altitudinal migrants are partial migrants, making downhill movements to nonbreeding areas. However, spatial and temporal patterns, the prevalence and predictability of migration at individual and population levels, and the ultimate ecological factors selecting for movement behavior vary considerably among taxa and regions. I conducted a systematic survey of the evidence for altitudinal migration to fill gaps in our understanding of this behavior among the landbirds of North America and Hawaii. Altitudinal migration was as prevalent as in other avifaunas, occurring in >20% of continental North American and nearly 30% of Hawaiian species. Of the species wintering within the USA and Canada, ∼30% engage in altitudinal migrations. Altitudinal migrants are far more common in the West, are taxonomically and ecologically diverse, and North American species exhibit patterns similar to altitudinal migrants elsewhere in the world. Because altitudinal migration systems are relatively tractable, they present excellent opportunities for testing hypotheses regarding migration generally. Altitudinal migration has likely been overlooked in North America due to contingency in the history of ornithological research. Our need to understand the patterns and causes of altitudinal migrations has never been greater due to emerging environmental threats to montane systems.

North American ornithology in transition.

There are a large number of ornithological societies in North America, some dating back to the 1880s. Although they differ in part by history, region, scientific approach and taxonomic focus, many of the memberships overlap and the societies are increasingly holding joint meetings. The largest of

Patterns and correlates of within-season breeding dispersal: A common strategy in a declining grassland songbird

ABSTRACT Dispersal is a ubiquitous behavior with important consequences for gene flow, demography, and conservation. Some birds engage in between-year breeding dispersal, but the factors shaping variation in this behavior are not well understood. In mid-continental grasslands, preliminary evidence suggested that Grasshopper Sparrows (Ammodramus savannarum) dispersed not only between seasons, but also within breeding seasons—an apparently uncommon avian behavior. We studied a population of Grasshopper Sparrows breeding in northeastern Kansas, USA, to document the spatial and temporal patterns of within-season breeding dispersal in an experimentally managed tallgrass prairie from 2013 to 2015. We combined color-band resighting, territory mapping, and radio telemetry to quantify changes in territory density, turnover of territorial males, and dispersal distances. Density of Grasshopper Sparrows varied seasonally in management-specific ways, simultaneously increasing and decreasing in watersheds that differed in management regime. Turnover was unexpectedly high, with over half of territorial males being replaced each month. We documented over a third of males dispersing up to ∼9 km between breeding attempts. Our study provides the first comprehensive description of the patterns of within-season breeding dispersal in a grassland songbird. Our results reveal the remarkable prevalence of within-season movement in this system and the relatively large distances over which birds disperse. Such mobility has important implications for survey design and habitat management, as birds select habitat at much larger spatial scales than is generally appreciated. These results also provide foundational information for tests of alternative hypotheses explaining the ecological and evolutionary basis for such movements.

Pallid bands in feathers and associated stable isotope signatures reveal effects of severe weather stressors on fledgling sparrows.

In August 2013, we observed a high incidence (44%) of synchronous bands of reduced melanin (a type of fault bar we have termed “pallid bands”) across the rectrices of juvenile Grasshopper Sparrows (Ammodrammus savannarum) captured near El Reno, Oklahoma. Earlier that year, on May 31, the site was struck by a severe storm which rained hailstones exceeding 5.5 cm diameter and spawned an historic 4.2 km-wide tornado <8 km to the south of the site. We hypothesized that this stressor had induced the pallid bands. An assessment of Grasshopper Sparrow nesting phenology indicated that a large number of nestlings were likely growing tail feathers when the storm hit. The pallid bands were restricted to the distal half of feathers and their widths significantly increased as a function of distance from the tip (i.e., age at formation). We predicted that if stress had caused these pallid bands, then a spike in circulating δ15N originating from tissue catabolism during the stress response would have been incorporated into the developing feather. From 18 juveniles captured at the site in August we measured δ15N and δ13C stable isotope ratios within four to five 0.25–0.40 mg feather sections taken from the distal end of a tail feather; the pallid band, if present, was contained within only one section. After accounting for individual and across-section variation, we found support for our prediction that feather sections containing or located immediately proximal to pallid bands (i.e., the pallid band region) would show significantly higher δ15N than sections outside this region. In contrast, the feathers of juveniles with pallid bands compared to normal appearing juveniles showed significantly lower δ15N. A likely explanation is that the latter individuals hatched after the May 31 storm and had consumed a trophically-shifted diet relative to juveniles with pallid bands. Considering this, the juveniles of normal appearance were significantly less abundant within our sample relative to expectations from past cohorts (z = − 2.03; p = 0.042) and, in as much, suggested widespread nest losses during the storm. Severe weather events may represent major stressors to ground-nesting birds, especially for recent fledglings. We call for others to exploit opportunities to study the effects of severe weather when these rare but devastating stressors impact established field research sites.

Individual and temporal variability in the courtship behavior of White-ruffed Manakins (Corapipo altera), a species with facultative cooperative displays

ABSTRACT Investigation of the ecological and evolutionary basis for the often-intriguing courtship behavior of animals requires that we understand the patterns of variation inherent in such behaviors. The courtship displays of the White-ruffed Manakin (Corapipo altera) are not well-known, and previously published descriptions and interpretations of displays conflict with one another. We studied the reproductive behavior of C. altera during 6 breeding seasons, observing 72 display courts (mean 29 ± 2.5 courts annually) for a total of 2688 hr. We updated the behavioral characterization of C. altera by reconciling 8 previous ethologies and describing 2 new behavioral elements, vouchering all with audio and video recordings. We evaluated evidence for the occurrence of male–male cooperation and characterized the physical attributes and temporal dynamics of displays and display courts. We found strong evidence of cooperation among males; 32% of displays for females were highly coordinated displays performed by 2 males, and 8% of those ended in copulation. Males of the highest social status (alphas) retained that status for an average of 1.7 yr (range 1.5 mo to ≥5 yr). Most alphas remained at a single court during their alpha tenure and rarely declined in social status. Only 23% of second-ranked (beta) males transitioned to alpha status, and of those 70% became alphas at a new display court. Display courts did not seem to be limited because few measured physical attributes differed between active display logs and random logs. Several elements of C. altera display behavior and social organization were more variable than in other manakin species, including high turnover of the display courts. This work provides key information for comparative studies investigating the evolution of cooperation in Pipridae.