Handbook of the Birds of Europe, the Middle East and North Africa: the Birds of the Western Palaearctic. Volume 2. Hawks to Bustards

A growing body of work suggests that breeding birds have a significant capacity to assess and respond, over ecological time, to changes in the risk of predation to both themselves and their eggs or nestlings. This review investigates the nature of this flexibility in the face of predation from both behavioural and reproductive perspectives, and also explores several directions for future research. Most available work addresses different aspects of nest predation. A substantial change in breeding location is perhaps the best documented response to nest predation, but such changes are not always observed and not necessarily the best strategy. Changes in nesting microhabitat (to more concealed locations) following predation are known to occur. Surprisingly little work addresses the proactive avoidance of areas with many nest predators, but such avoidance is probably widespread. Individual birds could conceivably adopt anti-predator strategies based on the nest predators actually present in an area, but such effects have yet to be demonstrated. In fact, the ways in which birds assess the risk of nest predation is unclear. Nest defence in birds has historically received much attention, but little is known about how it interacts with other aspects of decision-making by parents. Other studies concentrate on predation risk to adults. Some findings suggest that risk to adults themselves influences territory location, especially relative to raptor nests. An almost completely unexplored area concerns the sorts of social protection from predators that might exist during the breeding season. Flocking typical of the non-breeding season appears unusual while breeding, but a mated pair may sometimes act as a "flock of two". Opportunistic heterospecific sociality may exist, with heterospecific protector species associations more prevalent than currently appreciated. The dynamics of singing during the breeding season may also respond to variation in predation risk, but empirical research on this subject is limited. Furthermore, a few theoretical and empirical studies suggest that changes in predation risk also influence the behaviour of lekking males. The major influence of predators on avian life histories is undoubtedly expressed at a broad phylogenetic scale, but several studies hint at much flexibility on an ecological time scale. Some species may forgo breeding completely if the risk of nest predation is too high, and a few studies document smaller clutch sizes in response to an increase in nest predation. Recent evidence suggests that a female may produce smaller eggs rather than smaller clutches following an increase in nest predation risk. Such an increase may also influence decisions about intraspecific brood parasitism. There are no clear examples of changes in clutch/egg size with changes in risk experienced by adults, but parental responses to predators have clear consequences for offspring fitness. Changes in risk to adults may also influence body mass changes across the breeding season, although research here is sparse. The topics highlighted herein are all in need more empirical attention, and more experimental field work whenever feasible.

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Predators and the breeding bird: behavioral and reproductive flexibility under the risk of predation

Alexander Skutch hypothesized that increased parental activity can increase the risk of nest predation. We tested this hypothesis using ten open-nesting bird species in Arizona, USA. Parental activity was greater during the nestling than incubation stage because parents visited the nest frequently to feed their young during the nestling stage. However, nest predation did not generally increase with parental activity between nesting stages across the ten study species. Previous investigators have found similar results. We tested whether nest site effects might yield higher predation during incubation because the most obvious sites are depredated most rapidly. We conducted experiments using nest sites from the previous year to remove parental activity. Our results showed that nest sites have highly repeatable effects on nest predation risk; poor nest sites incurred rapid predation and caused predation rates to be greater during the incubation than nestling stage. This pattern also was exhibited in a bird species with similar (i.e. controlled) parental activity between nesting stages. Once nest site effects are taken into account, nest predation shows a strong proximate increase with parental activity during the nestling stage within and across species. Parental activity and nest sites exert antagonistic influences on current estimates of nest predation between nesting stages and both must be considered in order to understand current patterns of nest predation, which is an important source of natural selection.

Nest predation increases with parental activity: separating nest site and parental activity effects.

Why are animal displays so complex? In contexts ranging from courtship and mating to parent-offspring communication to predator deterrence, biological signals often involve a number of different visual, auditory and/or olfactory components Previous models of communication have tended to ignore this complexity, assuming that only one kind of display is available Here, a new game-theoretical model of signalling is described, in which signallers may use more than one display to advertise their qualities Additional displays may serve to enhance the accuracy with which receivers assess a single quality (the \`backup signal' hypothesis), or to provide information about different qualities (the \`multiple message' hypothesis) Multiple signals are shown to be stable, even when multiple receiver preferences entail significant costs, provided that signalling costs are strongly accelerating In such cases, signallers bias their investment towards more efficient forms of signal, but not to the exclusion of other display types When costs are not strongly accelerating, by contrast, individual signallers employ only a single display at equilibrium If different signals provide information about different qualities, however, then the equilibrium may feature alternative signalling strategies, with signallers who excel in one quality employing one kind of display, and those who excel in another quality employing another kind

Multiple Displays in Animal Communication:`Backup Signals' and `Multiple Messages'

Throughout the animal kingdom, distinctive behaviour by offspring commonly precedes and accompanies their provisioning by parents. Here, we assess empirical support for the recent theory that begging advertises offspring need, that parents provision young in relation to begging intensity, and that the apparently costly nature of begging ensures the reliability of the signal. While there is some support for the predictions of honest signalling models, empirical work has also revealed a host of complexities (such as the use of multiple signals) that existing theoretical analyses have only begun to address.

Begging the question: are offspring solicitation behaviours signals of need?

The relationship between begging behaviour, chick nutritional state, and parental distribution of food within broods was studied in 4- and 5-chick magpie Pica pica broods under natural conditions. Three components of the begging display (duration, latency, and posture) were highly correlated with each other and also with the emission and duration of begging calls. Begging performance was strongly influenced by the food intake of nestlings during the preceding 1-h interval, indicating that begging may reliably reflect the nutritional need of nestlings. Daily growth during the preceding day, as well as average cumulative food intake by the brood during the preceding 24 h, seemed not to affect begging in a similar way. Begging signals employed by hungrier nestlings involved a higher degree of muscular activity, thus supporting the prediction that nestlings in greater need should employ more costly signals. Overall, those nestlings who begged more tended to obtain more food, but the relationship between feeding success and begging behaviour was weak due to a high variation between broods in the way that parents seemed to respond to variations in begging behaviour. Possible causes for this variation, and its implications for the evolution of reliable begging displays, are discussed.

Signalling of Nutritional Need by Magpie Nestlings

Functional explanations for variations in levels of avian nest defence have been often based upon assumptions concerning optimal allocation of reproductive effort. Such a theoretical framework has led to the formulation of mathematical models aimed at predicting patterns of nest defence in the field. Here, a model is proposed that, starting from a general theoretical standpoint about optimal parental care, integrates previous models and generates new predictions concerning variables not previously included, namely predatory risk for both parents and offspring and age-dependent defensive tactics of nestlings. Optimal levels of parental defence are then expected to increase with (1) parent-offspring relatedness, (2) brood size, (3) offspring quality, (4) potential risk for the offspring, (5) offspring age, and (6) enhancement of offspring's self-defensive tactics while (7) parental residual reproductive value and (8) potential risk for parents are expected to exert a negative effect upon optimal levels of defence. Empirical data are reviewed for testing the model's predictions. Available data provide strong support for predictions concerning parameters 4, 5, 6, and 8. Failed tests concerning parameters 2 and 3 are shown to be flawed in the sense that we should expect defence levels to reflect intra-, instead of inter-individual variations in size and quality of the brood. A more detailed analysis is made concerning variations in residual reproductive value-correlated traits, such as brood number or time in the breeding season. It is argued that, when dealing with such traits, severe violations of basic assumptions (i.e. coeteris paribus statements or absence of an actual correlation between such traits and residual reproductive value) can occur, leading to unexpected results. It is concluded that life-history theory can account for most of the variability observed in nest defence patterns.

Avian Nest Defence: Theoretical Models and Evidence

Territoriality as a mating strategy in red deer

Magpie (Pica pica) brood defense against a human at the nest was studied in a Mediterranean population with low renesting potential. Variations in two defense measures recorded during 106 trials at 41 different nests were positively correlated with brood age. Ineremental effects due to the number of successive visits to nests by us, brood size, and the time in the breeding season were not significant. Partial correlation analyses showed that visit rate was not an important determinant of nest defense, which thus favors an adaptive explanation of nest defense patterns. Two functional hypotheses to account for the increase in defense intensity with brood age were tested: whether (1) increased parental defense serves to compensate the higher predation risk of older nests or (2) increased parental defense reflects the increasing reproductive value of nestlings as they grow older. Daily mortality and incidende of predation (estimated from contribution of whole-brood losses to total mortality) was higher early in the nestling period, hence providing weak evidence for the assumption on which hypothesis (1) is based. The timing of parental defense intensity did not mirror variations in predation risk for the nest but variations in reproductive value of the brood, as can be estimated from daily mortality, thus supporting hypothesis (2). Magpie parents increased defense intensity in response to premature escaping by almost fully-developed nestlings. Since such a response lowers predation risk for the offspring and increases their probability of survival, this finding supports hypothesis (2), but runs contrary to hypothesis (1). Parents also increased defense in response to play-backs of alarm calls uttered by nestlings during escaping episodes. It is argued that parents should continuously monitor the degree of offspring development in order to assess their reproductive value and that, by alarm calling, chicks honestly make their parents aware of the gain in reproductive value that results from enhancement in locomotory abilities that occur at the end of the nestling period.

Offspring reproductive value and nest defense in the magpie (Pica pica)

We investigated whether an increase in begging levels delays growth of chicks. In experiment 1, we hand-reared nine pairs of ring dove squabs, divided into a control and a begging group. All squabs received similar amounts of food, but those in the begging group had to beg for a prolonged period in order to be fed, while squabs in the control group received food without begging. Squabs stopped responding to the treatment after 10 days and, at that time, there was no effect of induced begging on their body mass. In experiment 2, we hand-reared 27 pairs of magpie chicks for 3 days. The design of experiment 2 was similar to that of experiment 1. Daily food intake and begging affected growth rates. On average, chicks in the begging group grew 0.8 g/day less than control chicks, which represents a decrease of 8.15% in growth rate. Because growth is usually positively associated with expected fitness, this demonstrates that begging is a costly behavior, an assumption routinely made in models of begging behavior. Key words: cost of signaling, handicap principle, magpies, Pica pica, ring doves, signaling of need, Streptopelia risoria. [Behav Ecol 12:269–274 (2001)]

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Tomas Redondo

Papers

Signalling of Nutritional Need by Magpie Nestlings

Avian Nest Defence: Theoretical Models and Evidence

Territoriality as a mating strategy in red deer

Offspring reproductive value and nest defense in the magpie (Pica pica)

Effects of begging on growth rates of nestling chicks