Caledonian Crows Corvus Moneduloides Research Articles

Threat recognition and response in an avian brood-parasite host from New Caledonia

Abstract Nest predation and avian brood parasitism are the main sources of nest failure in many passerine birds. Large predators threaten both brood and parents, whereas brood parasites pose only a danger to eggs or nestlings. The fan-tailed gerygone Gerygone flavolateralis from New Caledonia is subjected to high rates of nest predation by the New Caledonian crow Corvus moneduloides (responsible for about 20–40% of predation) and moderate rates of brood parasitism by the shining bronze-cuckoo Chalcites lucidus (parasitizing about 18% of nests), which also depredates nests that are too advanced for parasitism (13% of nests). To test if fan-tailed gerygones are able to discriminate predators from brood parasites, we presented 3 bird models at active gerygone nests: a brood parasite/small nest predator (shining bronze-cuckoo), a large nest predator (crow), and a small non-native bird (common chaffinch Fringilla coelebs), which is unknown to the gerygone, as a control. We assessed the response of adult gerygones to the presentation of each model by measuring the minimum approach distance, number of alarm calls, number of attacks, and time to first nest visit after the presentation (latency). Adult gerygones often attacked the cuckoo, approached but never attacked the chaffinch and always avoided the crow. Latency was shorter after an attack response and during brooding, but similar among models. We did not find any link between the cuckoo model presentation and later ejection of cuckoo nestlings. We conclude that adult fan-tailed gerygones discriminate between different models and respond accordingly to the level of threat, but do not show awareness of parasitism risk and increase of nestling ejection rates following exposure to the cuckoo model.

Notes on the biology of the threatened Island ThrushTurdus poliocephalus xanthopusin New Caledonia

SummaryThree subspecies of Island ThrushTurdus poliocephaluswere historically found in New Caledonia. All these subspecies were considered extinct until 1978, whenT. p. xanthopuswas rediscovered on two small islands, Néba (∼ 3.5 km2) and Yandé (∼ 13 km2). On Néba, we estimated the population at 44–58 individuals. This Island Thrush population is dependent on the coastal forest, the richest habitat for invertebrates. However, the coastal forest habitat has been degraded in quality and extent. The Island Thrush forages in the litter by removing leaves with its bill. Fruits (diameter < 10 mm), picked from trees and swallowed whole, are also a significant component of its diet. Néba and Yandé are currently free of two nest predator species, the black ratRattus rattusand the New Caledonian CrowCorvus moneduloides. On Néba, a low breeding success rate of 15.4% was found. To ensure conservation of these relict thrush populations, two actions at least should be implemented: setting up a biosecurity programme to keep islands free of black rats and increasing the area of coastal forest. The latter could be done by cutting down coconut trees in abandoned groves and planting tree species used by the Island Thrush to provide shade, fruits, good leaf litter, and nest support.

Determinants of data deficiency in the impacts of alien bird species

To identify the factors that influence the availability of data on the negative impacts of alien bird species, in order to understand why more than 70% are currently classified as Data Deficient (DD) by the Environmental Impact Classification of Alien Taxa (EICAT) protocol.Information on factors hypothesised to influence the availability of impact data were collated for 344 alien bird species (107 with impact data and 237 DD). These data were analysed using mixed effects models accounting for phylogenetic non‐independence of species (MCMCglmm).Data deficiency in the negative impacts of alien birds is not randomly distributed. Residence time, relative brain size and alien range size were found to be strongly related to the availability of data on impacts.The availability of data on the negative impacts of alien birds is mainly influenced by the spatial and temporal extents of their alien ranges. The results of this study suggest that the impacts of some DD alien birds are likely to be minor (e.g. species with comparatively long residence times as aliens, such as the common waxbill Estrilda astrild and the Java sparrow Padda oryzivora). However, the results also suggest that some DD alien birds may have damaging impacts (e.g. species from orders of alien birds known for their impacts to biodiversity but with comparatively small alien ranges, such as the New Caledonian crow Corvus moneduloides). This implies that at least some DD alien birds may have impacts that are being overlooked. Studies examining the traits that influence the severity of alien bird impacts are needed to help to predict which DD species are more likely to impact upon biodiversity.

Strong between-site variation in New Caledonian crows' use of hook-tool-making materials.

Functional tool use requires the selection of appropriate raw materials. New Caledonian crows Corvus moneduloides are known for their extraordinary tool‐making behaviour, including the crafting of hooked stick tools from branched vegetation. We describe a surprisingly strong between‐site difference in the plant materials used by wild crows to manufacture these tools: crows at one study site use branches of the non‐native shrub Desmanthus virgatus, whereas only approximately 7 km away, birds apparently ignore this material in favour of the terminal twigs of an as‐yet‐unidentified tree species. Although it is likely that differences in local plant communities drive this striking pattern, it remains to be determined how and why crows develop such strong site‐specific preferences for certain raw materials.

Calibrating animal-borne proximity loggers.

Summary Growing interest in the structure and dynamics of animal social networks has stimulated efforts to develop automated tracking technologies that can reliably record encounters in free‐ranging subjects. A particularly promising approach is the use of animal‐attached ‘proximity loggers’, which collect data on the incidence, duration and proximity of spatial associations through inter‐logger radio communication. While proximity logging is based on a straightforward physical principle – the attenuation of propagating radio waves with distance – calibrating systems for field deployment is challenging, since most study species roam across complex, heterogeneous environments.In this study, we calibrated a recently developed digital proximity‐logging system (‘Encounternet’) for deployment on a wild population of New Caledonian crows Corvus moneduloides. Our principal objective was to establish a quantitative model that enables robust post hoc estimation of logger‐to‐logger (and, hence, crow‐to‐crow) distances from logger‐recorded signal‐strength values. To achieve an accurate description of the radio communication between crow‐borne loggers, we conducted a calibration exercise that combines theoretical analyses, field experiments, statistical modelling, behavioural observations, and computer simulations.We show that, using signal‐strength information only, it is possible to assign crow encounters reliably to predefined distance classes, enabling powerful analyses of social dynamics. For example, raw data sets from field‐deployed loggers can be filtered at the analysis stage to include predominantly encounters where crows would have come to within a few metres of each other, and could therefore have socially learned new behaviours through direct observation. One of the main challenges for improving data classification further is the fact that crows – like most other study species – associate across a wide variety of habitats and behavioural contexts, with different signal‐attenuation properties.Our study demonstrates that well‐calibrated proximity‐logging systems can be used to chart social associations of free‐ranging animals over a range of biologically meaningful distances. At the same time, however, it highlights that considerable efforts are required to conduct study‐specific system calibrations that adequately account for the biological and technological complexities of field deployments. Although we report results from a particular case study, the basic rationale of our multi‐step calibration exercise applies to many other tracking systems and study species.

Programmable, miniature video‐loggers for deployment on wild birds and other wildlife

Summary Animal‐borne miniature video cameras hold the potential to revolutionise field ornithological research, with their ability to collect detailed behavioural data from a bird's eye view, in places and contexts where conventional observation techniques would fail. Here, we describe the development of a new generation of solid‐state video‐loggers that are cheap, light‐weight, programmable and easy to use and that overcome many problems associated with earlier transmission‐based technologies. Our loggers weigh c. 12·3–13·6 g fully packaged for deployment and record up to 94 min of video footage, at 640 × 480 pixels and 19·7 frames per second, on a 4‐GB micro‐SD card. Loggers are fitted with a custom‐designed, microprocessor‐controlled timer that enables flexible duty‐cycling, switching the unit on and off following a preprogrammed schedule. Packaged loggers contain a miniature very high frequency (VHF) radio‐tag (battery life c. 5–9 weeks) for positional tracking of the bird before, during and after scheduled video shoots, and to enable logger recovery for data download. To make our loggers suitable for deployment on wild birds, we developed novel techniques: (i) for the light‐weight packaging of electronics (thin sheets produced from thermoplastic) and (ii) for the attachment of units to, and their controlled release from, subjects (UV‐sensitive, rapidly degrading rubber tubing). Loggers can be manufactured at comparatively low cost (components are c. 94 GBP, plus c. 145 GBP for the VHF radio‐tag) and are easily refurbished after recovery, making the technology suitable for large‐scale deployments and projects on modest research budgets. A study in 2009/2010, with logger deployments on 19 wild New Caledonian crows Corvus moneduloides, demonstrated that our new technology is field‐worthy and that it can generate rich data sets on the foraging behaviour, habitat use and social interactions of an elusive bird species. The young field of wildlife video‐tracking is maturing quickly, and in only a few years, technology has advanced sufficiently to enable cost‐effective, hypothesis‐driven field studies of terrestrial birds, mammals and reptiles. Large‐scale deployment of video‐loggers can generate data sets of unprecedented information content, promising major quantitative insights in basic and applied ecology.

Extreme binocular vision and a straight bill facilitate tool use in New Caledonian crows

Humans are expert tool users, who manipulate objects with dextrous hands and precise visual control. Surprisingly, morphological predispositions, or adaptations, for tool use have rarely been examined in non-human animals. New Caledonian crows Corvus moneduloides use their bills to craft complex tools from sticks, leaves and other materials, before inserting them into deadwood or vegetation to extract prey. Here we show that tool use in these birds is facilitated by an unusual visual-field topography and bill shape. Their visual field has substantially greater binocular overlap than that of any other bird species investigated to date, including six non-tool-using corvids. Furthermore, their unusually straight bill enables a stable grip on tools, and raises the tool tip into their visual field's binocular sector. These features enable a degree of tool control that would be impossible in other corvids, despite their comparable cognitive abilities. To our knowledge, this is the first evidence for tool-use-related morphological features outside the hominin lineage.

Vocal culture in New Caledonian crows Corvus moneduloides

While recent research suggests that some animal species may possess ‘cultural’ traditions, much of the current evidence for wild populations remains contentious. This is largely due to the difficulty of demonstrating a fundamental prerequisite for the existence of culture: social learning. As the only case where social learning has been demonstrated conclusively, and subsequently linked to spatial or temporal trait variation, avian vocal dialects are the best studied, and most widely accepted, form of animal culture. Here, we investigate the potential for vocal culture in one of the few animals for which material culture has been suggested: the New Caledonian crow Corvus moneduloides. We show that this species: (1) possesses the capacity for social learning of vocalizations (experimental evidence in the form of a captive subject that reproduces human speech and other anthropogenic noises); and (2) exhibits significant large-scale, population-level variation in its vocalizations (cross-island playback experiments, with analyses controlling for a substantial set of potentially confounding variables). In combination, this provides strong evidence for the existence of ‘culture’ in these birds. More specifically, our findings reveal that the species exhibits sufficient social learning mechanisms, and within-population structuring, to generate and perpetuate cultural variation in at least one behavioural domain. This is a critical first step towards demonstrating cultural transmission in other behaviours, including tool manufacture and tool use, opening the door for the simultaneous investigation of vocal and material culture in a nonhuman species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101, 767–776.

Grass-Stem Tool use in New Caledonian CrowsCorvus moneduloides

We report an observation of a wild New Caledonian Crow Corvus moneduloides manufacturing two tools from dry grass stems, and using them to extract lizards from the crevices of a fencepost. We recovered one of the tools and were able to examine the raw material from which tools had been manufactured. This confirms our earlier observations with animal-borne video cameras, where we inferred use of this hitherto undocumented tool material from frame-by-frame analysis of video footage.

The right tool for the job: what strategies do wild New Caledonian crows use?

New Caledonian crows Corvus moneduloides (NC crows) display sophisticated tool manufacture in the wild, but the cognitive strategy underlying these skills is poorly understood. Here, we investigate what strategy two free-living NC crows used in response to a tool-length task. The crows manufactured tools to extract food from vertical holes of different depths. The first tools they made in visits were of a similar length regardless of the hole depth. The typical length was usually too short to extract food from the deep holes, which ruled out a strategy of immediate causal inference on the first attempt in a trial. When the first tool failed, the crows made second tools significantly longer than the unsuccessful first tools. There was no evidence that the crows made the lengths of first tools to directly match hole depth. We argue that NC crows may generally use a two-stage heuristic strategy to solve tool problems and that performance on the first attempt in a trial is not necessarily the 'gold standard' for assessing folk physics.

Morphology and sexual dimorphism of the New Caledonian Crow Corvus moneduloides, with notes on its behaviour and ecology

New Caledonian Crows Corvus moneduloides are known to be extraordinary tool makers and users, but little is known of other aspects of their biology. Here, we report recent field observations of their behaviour and ecology, along with measurements of 19 morphological traits and two flight performance parameters taken from 22 captured Crows. These measurements showed that the Crows were sexually dimorphic in size (the males were larger) but not in shape. We also found that the crows lived in mixed‐sex groups, and we observed juvenile‐type begging behaviour and feeding by regurgitation, which supports the hypothesis that these may be family groups.

Tool Use by the New Caledonian Crow Corvus moneduloides to Obtain Cerambycidae from Dead Wood

SummaryFrom a successful effort in late 1997 to film crow tool-use with the BBC Natural History Unit, I present the first detailed description of New Caledonian Crows Corvus moneduloides using tools to extract larvae of an endemic Cerambycidae:Prioninae from dead wood. I observed birds over a six-week period at Sarraméa where fallen Aleurites moluccana logs were ‘salted’ with the larvae. Crows habitually used mostly A. moluccana leaf-stems to extract larvae from holes and manufacture was also common when birds removed leaves from leaf-stems before using tools. The techniques of adult crows varied but were more proficient than those of a juvenile crow that frequented the site. Tool-use to extract Cerambycidae from dead wood was probably an important part of the foraging behaviour of the crows year-round. My observations here and elsewhere show that crows in different localities can have distinctly different traditions in their tool behaviour.