David B. Green

Drivers of Time-Activity Budget Variability during Breeding in a Pelagic Seabird

During breeding, animal behaviour is particularly sensitive to environmental and food resource availability. Additionally, factors such as sex, body condition, and offspring developmental stage can influence behaviour. Amongst seabirds, behaviour is generally predictably affected by local foraging conditions and has therefore been suggested as a potentially useful proxy to indicate prey state. However, besides prey availability and distribution, a range of other variables also influence seabird behavior, and these need to be accounted for to increase the signal-to-noise ratio when assessing specific characteristics of the environment based on behavioural attributes. The aim of this study was to use continuous, fine-scale time-activity budget data from a pelagic seabird (Cape gannet, Morus capensis) to determine the influence of intrinsic (sex and body condition) and extrinsic (offspring and time) variables on parent behaviour during breeding. Foraging trip duration and chick provisioning rates were clearly sex-specific and associated with chick developmental stage. Females made fewer, longer foraging trips and spent less time at the nest during chick provisioning. These sex-specific differences became increasingly apparent with chick development. Additionally, parents in better body condition spent longer periods at their nests and those which returned later in the day had longer overall nest attendance bouts. Using recent technological advances, this study provides new insights into the foraging behaviour of breeding seabirds, particularly during the post-guarding phase. The biparental strategy of chick provisioning revealed in this study appears to be an example where the costs of egg development to the female are balanced by paternal-dominated chick provisioning particularly as the chick nears fledging.

Sex Differentiation Based on the Gular Stripe in the Apparently Monomorphic Cape Gannet

The Cape gannet, Morus capensis, has long been considered a monomorphic species despite attempts at phenotypically separating the sexes. In a further effort we here recorded morphometric measurements from genetically sexed Cape gannets. Discriminant function analysis was used to classify sexes according to these morphometric measurements. Culmen and gular stripe lengths were selected as the best discriminators, correctly classifying 65% of birds. Male Cape gannets had significantly longer culmens and gular stripes than females. We thereby provide the first evidence suggesting that the Cape gannet demonstrates some level of dimorphism. Nonetheless, morphometric criteria used to separate sexes holds limited value due to overlap in measurements between sexes.

An automated approach towards measuring time‐activity budgets in colonial seabirds

Summary 1. Seabird proxies have the potential to act as useful and cost-effective indicators of the state of the marine environment. Seabird time-activity budgets, in particular, reflect short-term changes in prey conditions. 2. We tested an automated technique for long-term continuous recording of Cape gannet, Morus capensis ,t imeactivity budgets using coded very high frequency (VHF) transmitters allowing for simultaneous monitoring of a large sample of study birds. 3. Radiotransmitters attached to leg-rings had no impact on adult foraging trip and nest attendance durations, breeding success or chick growth. Furthermore, frequencies of nest attendance and foraging trip durations estimated by the VHF logging system were no different to those estimated from hourly direct observations. 4. Using time-depth recorders, the relationship between the time that birds rested on the sea surface in relation to foraging trip duration was assessed. Trip duration during chick rearing was clearly an accurate proxy for foraging effort. 5. The VHF monitoring system provides a simple method of accurately assessing the time-activity budgets of colonial seabirds, which can be expanded to a range of other colonially breeding taxa. In the case of seabirds, this approach can potentially provide sensitive, real-time indicators of prey abundance for fisheries management.

Living on the bare edge: fitness consequences for Cape Gannets Morus capensis at Bird Island, Algoa Bay

Avian breeding colonies often suffer high levels of predation along their margins. Nest predation is a major fitness concern for parents and therefore nests inside of the colony edge might be at a premium. This was tested through studying the colonial breeding seabird, the Cape Gannet Morus capensis, at Bird Island, Algoa Bay, South Africa. We found that breeding success is much lower for nests on the edge. Consequently, we tested whether birds nesting inside the colony had shorter foraging trips and higher nest attendance than birds along the colony edge. Although nest attendance averaged slightly less along the colony edge, the difference was not significant. Nest predation was found to be the likely cause of the higher nest failure along the colony edge. Nest positioning in Cape Gannets clearly has a profound influence on fitness, especially as the Cape Gannet appears to display a high level of nest-site fidelity.

Communication calls produced by electrical stimulation of four structures in the guinea pig brain

One of the main central processes affecting the cortical representation of conspecific vocalizations is the collateral output from the extended motor system for call generation. Before starting to study this interaction we sought to compare the characteristics of calls produced by stimulating four different parts of the brain in guinea pigs (Cavia porcellus). By using anaesthetised animals we were able to reposition electrodes without distressing the animals. Trains of 100 electrical pulses were used to stimulate the midbrain periaqueductal grey (PAG), hypothalamus, amygdala, and anterior cingulate cortex (ACC). Each structure produced a similar range of calls, but in significantly different proportions. Two of the spontaneous calls (chirrup and purr) were never produced by electrical stimulation and although we identified versions of chutter, durr and tooth chatter, they differed significantly from our natural call templates. However, we were routinely able to elicit seven other identifiable calls. All seven calls were produced both during the 1.6 s period of stimulation and subsequently in a period which could last for more than a minute. A single stimulation site could produce four or five different calls, but the amygdala was much less likely to produce a scream, whistle or rising whistle than any of the other structures. These three high-frequency calls were more likely to be produced by females than males. There were also differences in the timing of the call production with the amygdala primarily producing calls during the electrical stimulation and the hypothalamus mainly producing calls after the electrical stimulation. For all four structures a significantly higher stimulation current was required in males than females. We conclude that all four structures can be stimulated to produce fictive vocalizations that should be useful in studying the relationship between the vocal motor system and cortical sensory representation.