John R. Krebs

Optimal prey selection in the great tit (Parus major)

We tested the predictions of an optimal foraging model using five captive great tits as predators. The birds were presented with two prey types, profitable and unprofitable, on a moving belt. Both prey types were made out of mealworms. When the encounter rate with both prey types was low, the birds were non-selective, but at a higher encounter rate with profitable prey, the birds selectively ignored the less profitable type and did so irrespective of the encounter rate with them. These results are as predicted by the model, but the birds did not as predicted change from no selection in a single step. We suggest that this is because the birds invest time in sampling to determine the availability and profitability of different prey types.

Repertoires and Geographical Variation in Bird Song

Publisher Summary This chapter deals with repertoires and geographical variation in bird song. Three methods have been used to investigate the function of a behavior pattern: (1) interspecific comparisons in which differences between species are related to ecological and life history variables; (2) intraspecific comparisons relating variations between individuals to variations in survival, reproductive success, or success in acquiring resources; and (3) direct experimental tests of the effect of a behavior. All three methods have been applied to the study of how selection favors repertoires. The chapter emphasizes micro-geographical variation, and examines the two factors that most significantly influence local spatial variation in bird song: (1) the extent and accuracy of imitation, and (2) the site of song imitation with respect to the site of breeding. It discusses the relationship between genetic separation of populations and song variations, and the influence of song repertoire size on the ability to establish or maintain sharp discontinuities in song traditions between neighboring populations. The usefulness and definition of the term “dialect” and the functional significance of vocal imitation and its influence on song dialects are also discussed.

Memory for spatial and object-specific cues in food-storing and non-storing birds

Two storer/non-storer pairs of species, marsh tit (Parus palustris)/blue tit (P. caeruleus) and jay (Garrulus glandarius)/jackdaw (Corvus monedula) were compared on a one-trial associative memory task. In phase I of a trial birds searched for a reward in one of four feeders which differed in their trial-unique spatial location and object-specific cues. Following a retention interval, the birds had to return to the same feeder to obtain a further reward. In control trials the array of feeders was unaltered, whilst in dissociation tests it was transformed to separate spatial location and object-specific cues.In control trials there was no difference in performance between species. In dissociation tests, the two storing species went first to the correct spatial location and second to the correct object-specific cues, whereas the two non-storing species went first with equal probability to the correct spatial and local object cues.Monocular occlusion was used to investigate differences between the two eye-systems. In control trials there was no effect of occlusion. In dissociation trials, all 4 species preferentially returned to the feeder with the correct object-specific cue when the left eye had been covered in phase I and to the feeder in the correct spatial position when the right eye had been covered in phase I.These results suggest that (a) food-storing birds differ from non-storers in responding preferentially to spatial information and (b) in storers and non-storers the right eye system shows a preference for object-specific cues and the left eye system for spatial cues.

The significance of song repertoires: The Beau Geste hypothesis

Abstract I briefly discuss the major current hypotheses which have been put forward to explain the apparently redundant song repertoires of many oscine birds: individual recognition; sexual selection; matched countersinging; and habituation. I then propose an alternative idea, that song repertoires in some species have evolved in the context of density assessment. In the great tit, non-territorial birds use the song of residents as a cue in assessing density, and I suggest that repertoires are used by resident birds to increase the apparent density of singing residents, and hence decrease the apparent suitability of the area to new birds. I explore some implications of the idea and list some predictions.

Song Repertoires and Lifetime Reproductive Success in the Great Tit (Parus major)

We recorded the song repertoires of breeding male great tits during six successive seasons and tested for correlations between repertoire size and reproductive success. Males with larger repertoires are more likely to survive to breed in a second season. They are also more likely to father offspring which survive to breed. Lifetime reproductive success appears to be highest for males with intermediate-sized repertoires. Males with larger repertoires produce heavier fledglings, and this may be related to territory quality.

The evolution of alarm calls: Altruism or manipulation?

So we may conclude that certain insects cross hard-surface roads by the shortest possible route in a tropistic response to the symmetry of radiant energy coming from up and down the road. The response is made by both flying and crawling insects, but it is not made by all flying insects (e.g., dragonflies). Such behavior has survival value for crawling insects, for it causes them to spend the least possible time on hot and heavily traveled highways. Similarly, this response leads insects to cross bare spots of the earth in the shortest possible time.

Food storing by marsh tits

Wild marsh tits (Parus palustris) were allowed to hoard radioactively labelled sunflower seeds, which were subsequently found using a portable scintillation counter. Seeds were stored singly, in various sites close to the ground. Different birds favoured different types of site, although this preference was changeable. Seed density decreased with increasing distance from the feeder, and there was a negative correlation between seed sequence number and the distance it was carried. Hoarded seeds disappeared more rapidly than control seeds in identical sites 100 cm away, suggesting that the birds remember their exact location. Seeds also tended to be stored and recovered in the same sequence. The adaptive significance of these results is discussed.

Food Storing and the Hippocampus in Paridae

Food storing passerines have a larger hippocampus, relative to the rest of the telencephalon and/or body mass, than do non-storing species. This study looked at the relationship between relative size of the hippocampus and degree of food storing in six species of Paridae (blue tit, Parus caeruleus, great tit, P major, marsh tit, P palustris, coal tit, P ater, black-capped chickadee, P. atricapillus, and willow tit, P montanus). The degree of storing by these species varies from little or none to thousands of food items. The period over which food is stored also varies from a few hours to several months. The results showed that hippocampal volume, relative to the rest of the telencephalon, is larger in those species that store more food, store for longer, or both. In an analysis of intraspecific variation within two of the species, the food storing marsh tit and the non-storing blue tit, there was a significant positive relationship between hippocampal volume relative to body mass, and telencephalic volume relative to body mass, in the marsh tit but no relationship between these variables in the blue tit.

Spatial learning induces neurogenesis in the avian brain

It is known from previous work that neurones are born continuously in the ventricular zone of the bird brain. In this study, we show that the amount of cell proliferation in the ventricular zone of the hippocampus (HP) and the hyperstriatum ventrale (HV) is influenced by behavioural experience. Two groups of birds (marsh tits) were compared: those allowed to store and retrieve food once every 3 days between days 35 and 56, and age-matched controls treated in an identical way, except that they were not allowed to store and retrieve food. After three trials of storing and retrieval, between days 35 and 41 posthatch, experienced birds showed a significantly higher rate of cell proliferation than did controls. The experienced birds also showed a significant increase in total cell and neuronal number by day 56 posthatch, after eight trials of storing and retrieval. There were no significant differences in the amount of programmed cell death in the hippocampus in this study. In a novel analysis of the data we demonstrate that the effect of experience between days 35 and 41 was to increase the daily rate of neurogenesis in the ventricular zone from 3.9 to 10%, and that this change could account for the increase in total hippocampal neuronal number by day 56 in the experienced birds. Thus, the observed increase in hippocampal volume and neuronal number as a result of food storing and retrieval, may be caused by an increase in neurogenesis in the first few trials of food storing experience.

Habituation and song repertoires in the great tit

SummaryThis paper is concerned with the idea that the song repertoires of passerine birds are an evolutionary adaptation to reduce habituation in listeners. In an experiment involving 16 territorial males I played either a single song or a repertoire of songs for 15 two-min trials through a single loudspeaker near the edge of the territory. In a second experiment with 10 birds I played the songs through one of two loudspeakers in different parts of the territory, alternating between loudspeakers on successive trials. The birds tended to habituate more rapidly to single song playback than to repertoires. In the second experiment the overall level of habituation was lower and the difference between the two treatments was less marked.Two features of song repertoire organisation are consistent with the habituation hypothesis (1) the avoidance of low recurrence intervals in switches between song types, and (2) the fact that within repertoire variability is as great or greater than between repertoire varability.The main problem with the habituation hypothesis is that habituation by listeners does not seem to be adaptive, so it is not clear why they should habituate. I suggest a hypothesis. Intruders may assess the density of birds in an area by listening to songs, so that habituation may be a mechanism by which this density assessment is achieved. Repertoires could be a mechanism by which resident birds cheat, through increasing the apparent density of singing birds.