Mark C. Mainwaring

The design of artificial nestboxes for the study of secondary hole-nesting birds: a review of methodological inconsistencies and potential biases

Abstract. The widespread use of artificial nestboxes has led to significant advances in our knowledge of the ecology, behaviour and physiology of cavity nesting birds, especially small passerines. Nestboxes have made it easier to perform routine monitoring and experimental manipulation of eggs or nestlings, and also repeatedly to capture, identify and manipulate the parents. However, when comparing results across study sites the use of nestboxes may also introduce a potentially significant confounding variable in the form of differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. However, the use of nestboxes may also introduce an unconsidered and potentially significant confounding variable due to differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. Here we review to what extent the characteristics of artificial nestboxes (e.g. size, shape, construction material, colour) are documented in the ‘methods’ sections of publications involving hole-nesting passerine birds using natural or excavated cavities or artificial nestboxes for reproduction and roosting. Despite explicit previous recommendations that authors describe in detail the characteristics of the nestboxes used, we found that the description of nestbox characteristics in most recent publications remains poor and insufficient. We therefore list the types of descriptive data that should be included in the methods sections of relevant manuscripts and justify this by discussing how variation in nestbox characteristics can affect or confound conclusions from nestbox studies. We also propose several recommendations to improve the reliability and usefulness of research based on long-term studies of any secondary hole-nesting species using artificial nestboxes for breeding or roosting.

The energetic costs of nest building in birds

Parental investment in reproduction is generally limited by the availability of food and so avian life-history research has generally focused on the brood rearing phase, when food requirements are greatest. Only relatively recently has the focus extended to the incubation phase and, even more recently, to the nest-building phase where there is now observational, comparative and experimental evidence that avian nest building is an energetically and temporally expensive activity. This review emphasises that nest construction is energetically expensive and we suggest that such costs should be incorporated into avian life-history research.

Local temperature and not latitude determines the design of Blue Tit and Great Tit nests

Recent studies are documenting the extent to which the mass and construction of bird nests varies between individuals and locations. In the Blue Tit (Cyanistes caeruleus) and Great Tit (Parus major), temperatures experienced by females during nest construction are inversely related to nest mass. Moreover, Mainwaring et al. (Journal of Biogeography, 2012) showed that nests constructed at high latitude are heavier and better insulated than nests built by conspecifics in the south. Although mean spring temperature was used as a proxy for latitude in the Mainwaring et al. study, it remains untested whether individual birds build nests in response to a narrower range of temperatures experienced at the start of the breeding season. Our study showed that irrespective of latitude nest mass, and in particular nest cup mass, of Blue Tits and Great Tits was significantly affected by the temperature experienced by the birds for the seven days preceding clutch initiation. Similar results were seen with the insulatory properties of nests. The potential impact of variation in nest construction and insulation on subsequent incubation and chick-rearing behaviour is discussed.

Experimental evidence for state-dependent nest weight in the blue tit, Cyanistes caeruleus.

Parental investment in reproduction is generally limited by food availability, and so avian life-history research has traditionally focused on the brood rearing phase, when food requirements are greatest. Only relatively recently has the focus extended to the incubation phase, and even more recently to the nest-building phase, where observational and comparative evidence suggest that avian nest building is an energetically expensive and time-consuming activity. We aimed to experimentally test the limitations on this cost in a hole-breeding passerine, the blue tit (Cyanistes caeruleus), by providing supplementary food to experimental pairs during the nest-building period. In comparison with control females, that did not receive supplementary food, experimental females constructed heavier nests, with greater amounts of moss base but similar amounts of cup lining, despite there being no differences in the time taken to build the nest. This study provides empirical support for the hypothesis that avian nest building is a costly behaviour, limited by food availability.

Zebra finches are bolder in an asocial, rather than social, context

Despite an expanding interest in animal personalities, the influence of social interactions and sex differences on individual differences in behaviour remains poorly understood. Using the social zebra finch (Taeniopygia guttata), we tested for behavioural differences in exploration of a novel environment and objects, between individuals of both sexes in relation to a social context; the presence of three male companions, three female companions or no companion birds. We predicted that the presence of conspecific companions should result in focal birds reacting to novelty by exploring more extensively because the companion birds contribute to anti-predator vigilance behaviour and because social isolation often causes behavioural inhibition in social species. We found that exploratory behaviour of focal individuals was significantly reduced in the presence of conspecific companions, irrespective of the companions sex. Moreover, we found a weak trend towards females being more exploratory than males, irrespective of the social context. These results demonstrate the importance of considering the social context in animal personality studies and of exploring sex differences in personalities.

Parental care in wild and captive zebra finches : measuring food delivery to quantify parental effort

Although the zebra finch, Taeniopygia guttata, has been a very important model system for the study of intrafamilial conflict and parental strategies, a detailed understanding of the variation in parental effort that can occur both within and between pairs is lacking. In part this is because many different methods have been used by individual studies to quantify parental care (i.e. nest visit rate, time in the nest and number of feeds per visit), but these have not directly been compared. We used nestbox cameras to monitor parental visit rate and the distribution of food among the nestlings in domestic, captive wild and wild free-living zebra finches. The percentage of nest visits by parents in which they fed the nestlings was consistent, with multiple feeds to different nestlings occurring within a single visit. The quantity of food delivered and its distribution among the nestlings, however, varied greatly both within and between broods. The number of regurgitations a brood received correlated significantly with the number of individual feeds when accounting for environment, but not with nest visit rate or the duration of time parents spent in the nestbox. In captive conditions, parents visited the nest at twice the rate of wild free-living birds, overall providing nestlings with twice the amount of food. Captive conditions also led to food distribution becoming less equitable among the brood, owing to changes in the number of regurgitations per individual feed and the number of overall feeds per nest visit.

The adaptive benefit of hatching asynchrony in wild zebra finches

The evolutionary consequences of parent–offspring conflict have received considerable attention, yet the extent to which parent–offspring conflict influences life history traits remains uncertain. In particular, it is unclear whether hatching patterns are parental strategies to manipulate competitive dynamics among offspring, or whether they are instead consequences of physiological or ecological variables affecting the payoffs of adjusting the onset of incubation. Here we describe an experiment in which we manipulated the hatching pattern of wild free-living zebra finches. We examined the consequences of hatching pattern on parental feeding behaviour and sibling competition, with the aim of identifying selective pressures that underlie differing hatching patterns. Increasing hatching asynchrony reduced the overall quantity of food that parents delivered and the equitability of its distribution among nestlings compared with synchronous broods. However, for each unit of food received, the begging intensity of nestlings from asynchronous broods was lower. As a consequence, the growth rate of first-hatched nestlings in asynchronous broods was significantly greater than those in synchronous broods, and there was no significant difference in growth rate between last-hatched nestlings in asynchronous broods and nestlings from synchronous broods, even though they received less food overall. The reduced scramble competition and energy wasted on begging behaviour in asynchronous broods supports the sibling rivalry hypothesis, suggesting an adaptive function for hatching asynchrony in wild zebra finches.

Parentally biased favouritism in relation to offspring sex in zebra finches

Family conflicts over parental care result in offspring attempting to exert control using solicitation behaviours, whilst the parents are potentially able to retaliate through provisioning rules. However, the evolutionary interests of one parent may not necessarily support the evolutionary interests of the other parent, and such conflicts of interest may be expressed in how the two parents allocate the same form of parental care to individual offspring. Theory suggests that such parentally biased favouritism is a universally predicted outcome of evolutionary conflicts of interest, and empirical evidence suggests that parentally biased favouritism occurs in relation to offspring size and solicitation behaviours. However, unequivocal empirical evidence of parentally biased favouritism in relation to offspring sex is absent, due to being strongly confounded by sex differences in size and solicitation behaviours. Here, we present strong evidence for parentally biased favouritism in relation to offspring sex in zebra finches (Taeniopygia guttata), independent of the effects of chick size and begging intensity. Mothers preferentially provisioned sons over daughters, whilst fathers showed no bias, meaning that sons received more food than daughters. Parentally biased favouritism in relation to offspring sex facilitates parental control over evolutionary conflicts of interest and is probably more widespread than previously realised.

Environmental and not maternal effects determine variation in offspring phenotypes in a passerine bird

Maternal and environmental effects can profoundly influence offspring phenotypes, independent of genetic effects. Within avian broods, both the asymmetric post‐hatching environment created by hatching asynchrony and the differential maternal investment through the laying sequence have important consequences for individual nestlings in terms of the allocation of resources to body structures with different contributions to fitness. The purpose of this study was to evaluate the relative importance of post‐hatching environmental and maternal effects in generating variation in offspring phenotypes. First, an observational study showed that within blue tit, Cyanistes caeruleus, broods, late‐hatched nestlings allocated resources to tarsus development, maintained mass gain and head‐bill growth and directed resources away from the development of fourth primary feathers. Second, a hatching order manipulation experiment resulted in nestlings from first‐laid eggs hatching last, thereby allowing comparison with both late and early‐hatched nestlings. Experimental nestlings had growth patterns which were closer to late‐hatched nestlings, suggesting that within‐brood growth patterns are determined by post‐hatching environmental effects. Therefore, we conclude that post‐hatching environmental effects play an important role in generating variation in offspring phenotypes.

Incubation behaviour and hatching synchrony differ in wild and captive populations of the zebra finch

Hatching asynchrony is widespread in birds laying clutches containing multiple eggs, yet is seemingly paradoxical as the age and size hierarchies result in asymmetric sibling competition and low survival prospects for late-hatched nestlings. We examined the causes of variation in hatching asynchrony between broods of zebra finch, Taeniopygia guttata, in three environments: domesticated, captive wild and wild free-living. We found that broods of both domesticated and wild birds taken into captivity hatched more asynchronously than wild free-living broods. This was directly attributable to both male and female parents of domesticated and captive wild broods initiating incubation as soon as the first egg was laid as opposed to when the clutch was virtually complete in wild free-living broods. Wild free-living birds that were transferred to captive environments immediately switched to the incubation onset behaviour seen in domesticated birds, thereby demonstrating a previously unsuspected level of intraspecific plasticity in incubation behaviour. This finding suggests that something about the captive environment is driving the early onset of incubation and contributing to an elevated level of hatching asynchrony in captive birds. Across all populations and environments males contributed almost equally to incubation, and the onset of incubation by males was highly coordinated with that of their partner.