Proponents of optimal foraging theory attempt to predict the behavior of animals while they are foraging; this theory is based on a number of assump­ tions ( 133 , 155 , 2 10, 23 1 ) . First, an individual's contribution to the next generation (i.e. its "fitness") depends on its behavior while foraging. This contribution may be measured genetically or culturally as the proportion of an individual's genes or "ideas", respectively, in the next generation. In the former case, the theory is simply an extension of Darwin's theory of evolution. Second, it is assumed that there should be a heritable component of foraging behavior, i.e. an animal that forages in a particular manner should be likely to have offspring that tend to forage in the same manner. This heritable compo­ nent can be either the actual foraging responses made by an animal or the rules by which an animal learns to make such responses. In other words, optimal foraging theory may apply regardless of whether the foraging behavior is learned or innate. Given these first two assumptions, it follows that the proportion of individuals in a population foraging in ways that enhance their fitness will tend to increase over time. Unless countervailed by sufficiently strong group selection (see 287, 242), foraging behavior will therefore evolve, and the average foraging behavior will increasingly come to be characterized by those characteristics that enhance individual fitness. The third assumption is that the relationship between foraging behavior and fitness is known. This relationship is usually referred to as the currency of fitness (23 1 ) . In general, any such currency will include a time scale, although in some cases it may be assumed that fitness is a function of some rate.

Optimal Foraging Theory: A Critical Review

Predators and food are the keys to understanding fish shoals; synchronised co-operation defeats predators, and optimal food gathering in shoals reflects a shifting balance between joining, competing in, or leaving the group In the wild, predators may arrive while shoaling fish are feeding, and so vigilance is a crucial behaviour Once detected, predator defence takes precedence over feeding, since an animal’s life is worth more than today’s dinner

Functions of Shoaling Behaviour in Teleosts

One commonly cited benefit to animals that forage in groups is an increase in the probability of detecting a predator, and a decrease in the time spent in predator detection. A mathematical model (Pulliam 1973) predicts a negative relationship between group size and vigilance rates. Over fifty studies of birds and mammals report that the relationship at least partly explains why individuals forage in groups. This review evaluates the strength of these conclusions based on their evidence. Those variables that may confound the relationship between vigilance and group size are outlined, and their control is assessed for each study. The variables I consider to be important include the density and type of food; competition between individuals; the proximity to both a safe place and the observer; the presence of predators; the visibility within the habitat; the composition of the group; the ambient temperature and the time of day. Based on these assessments, most of the studies fail to adequately demonstrate an unambiguous relationship between vigilance behavior and group size. Nevertheless, many studies reveal interesting features of the relationship between vigilance and group size that should provide fruitful avenues for future research.

Predator vigilance and group size in mammals and birds: a critical review of the empirical evidence

Oddity and the ‘confusion effect’ in predation

Why do birds participate in mixed-species foraging flocks? A large-scale synthesis

1. In mixed species flocks of lapwings and golden plovers, lapwing net rate of energy intake was positively related to the number of conspecifics and the amount of time individuals spent crouching during feeding, but was unaffected by the presence of golden plovers. Plover net rate of energy intake, however, was positively related to crouching tendency, negatively related to the number of lapwings present but unaffected by the number of conspecifics. 2. Golden plovers appeared to join lapwing flocks because they could then crouch more and scan less. The independent negative effect of lapwing number may have been due to feeding interference when there were large numbers of lapwings relative to plovers. 3. When black-headed gulls joined a flock, both lapwings and plovers tended to crouch less and take smaller worms. However, in lapwings these changes appeared to reduce feeding efficiency but plover net rate of energy intake increased as a function of the gull: plover ratio. 4. When gulls were present, the effect of conspecifics on lapwing feeding efficiency disappeared and the presence of plovers had a negative effect. Conversely, in plovers the presence of conspecifics enhanced feeding efficiency and the negative effect of lapwing number disappeared. 5. The arrival of gulls resulted in a decrease in equilibrium lapwing number but no consistent, directional change in plover number. When gulls were present, lapwing number tended to decrease when large numbers of plovers arrived, but increase when small numbers arrived. Increased lapwing number had no significant directional effect on plover number. Changes in lapwing and plover energy intake and species numbers are interpreted in terms of feeding interference and 'dilution' of gull kleptoparasitism. 6. In the absence of gulls, there was no significant directional relationship between increased plover number and lapwing number, but the presence of plovers generally correlated with a reduction in lapwing number. Plover number, however, decreased when large numbers of additional lapwings arrived but increased with small numbers of arrivals. Changes in species energy intake and numbers here are interpreted in terms of feeding interference and time budget changes.

Time Budgets, Feeding Efficiency and Flock Dynamics in Mixed Species Flocks of Lapwings, Golden Plovers and Gulls

1. Flock size in lapwings correlated positively with the mean length of worm taken and birds in large flocks tended to take a more profitable mixture of worm sizes than those in small flocks. They also did better than expected on the basis of worm size mixtures in the ground. 2. Worm size taken was influenced by whether or not birds crouched before pecking. Crouching was associated with birds finding larger worms and taking predominantly the most profitable worm size classes. Crouching may have helped birds avoid broken fragments of worm common in feeding areas and small worms near the surface of the ground. 3. The proportion of pecks preceded by crouching increased significantly with flock size but decreased with the numbers of kleptoparasitic gulls in the flock. Perhaps as a consequence, lapwings tended to take smaller and, overall, less profitable mixtures of, worms in the presence of gulls. 4. The positive relationship between crouching and flock size appeared to be due to the relationship between flock size and worm density, birds feeding more selectively when searching costs were low. The negative relationship between crouching and the number of gulls, however, may have been due to lapwings reducing their risk of attack.

Prey Size Selection By Lapwings in Lapwing/Gull Associations

1. Single species flocks of fieldfares and redwings tended to occur on different types of pasture: fieldfare flocks on new pasture ( < 4 years old) and redwing flocks on old pasture (25-400 + years old). Worms tended to be less abundant but were apparently easier to detect on new pasture. In single species flocks, fieldfares had a significantly higher capture rate and slightly high net rate of energy intake on new pasture than on old, but, on average, individuals of the two species had similar net rates of energy intake per gram body weight. 2. Fieldfare net rate of energy intake in single species flocks appeared to be a consequence of flocks size effects on time budgeting. Birds scanned less and crouched more in larger flocks. In redwings, however, there was no direct effect of flock size on individual feeding efficiency. 3. Mixed flocks almost always formed by fieldfares joining redwings on old pasture. One possible reason was the apparent drop in worm abundance on new pasture as the winter progressed. 4. Birds of both species had higher net rates of energy intake in mixed flocks than in single species flocks on old pasture. Fieldfares appeared to benefit directly from the presence of redwings and time budget analysis suggested that they used information from redwings about the whereabouts of prey, although the mode of interaction involved is not yet clear. The increased net rate of energy intake for redwings in mixed flocks, however, may have been due simply to increased worm abundance at the times mixed flocks formed.Zusammenfassung 1. Getrennte Fluge von Wacholder- und Rotdrosseln waren gew6hnlich auf verschie- denen Weidetypen zu finden: Wacholderdrosselfluge auf neuen Weiden (bis zu 4 Jahren alt), und Rotdrosselfluge auf alten Weiden (25-400+ Jahren alt). Auf neuen Weiden wa- ren im ganzen weniger Wurmen, aber scheinbar war es einf?cher, sie zu entdecken. Auf diesen Weiden gelangten die Wacholderdrosseln zu signifikant h6heren Fangziffern und einer etwas h6heren Reinziffer von Energieaufnahme als auf alten Weiden. Im Durch- schnitt aber hatten Einzelv6gel der zwei Arten dhnliche Reinziffern von Energieaufnahme per Gramm K6rpergewicht. 2. Es schien, dass die Gr6sse der Wacholderdrosselfluge die effektive Futterzeit und so- mit die netto Energieaufnahme beeinflusste. In gr6sseren Flugen suchten die V6gel weni- ger und hockten sie mehr. Nicht aber bei den Rotdrosseln, bei denen kein Einfluss der Fluggr6sse auf die wirksame Futterzeit gefunden wurde. 3. Gemisschte Flilge wurden fast immer durch Wacholderdrosseln geformt, die zich mit Rotdrosseln auf alten Weiden vereinigten. Eine m6gliche Ursache war eine Abnahme der Wurzelzahl auf neuen Weiden, die wahrscheinlich im Laufe des Winters auftrat. 4. V6geln von beiden Arten hatten in gemischten F16gen h6here Reinziffern von Ener- gieaufnahme als Vogel in Einzelffugen, die auf alten Weiden waren. Wacholderdrosseln schienen Vorteil von der Rotdrosselgegenwart zu haben, und eine Zeitrechnungsanalyse deutete an dass sie Information von den Rotdrosseln benutzen Cber die Stellung der Beu- te, obgleich die Mitteilungsweise noch nicht klar ist. Der h6heren Reinziffer der Energie- aufnahme von Rotdrosseln in gemischten Flugen k6nnte einfach eine gr6ssere Anzahl von Wurmern zugrunde liegen, zu dem Zeitpunkt, an dem die F16ge gebildet wurden.

Hilary Stephens

Papers

Time Budgets, Feeding Efficiency and Flock Dynamics in Mixed Species Flocks of Lapwings, Golden Plovers and Gulls

Prey Size Selection By Lapwings in Lapwing/Gull Associations

Costs and Benefits of Single and Mixed Species Flocking in Fieldfares (Turdus Pilaris) and Redwings (T. Iliacus)