Prey preparation by adult Great Tits Parus major feeding nestlings

Some birds prepare food items before giving them to their nestlings. We studied the relationships between the degree of prey preparation and prey size, nestling age, brood size and time of season. We estimated the degree of preparation of 513 animal prey items, taken by using neck collars, brought to nestling Great Tits Parus major. Prey preparation increased with prey size and decreased as the nestlings grew older, as brood size increased and as the season progressed. Other factors, such as nutrient concentration (through removal of low-quality or deleterious parts) or palatability (considering scaly moth forewings unpalatable), seem also to be important in determining prey preparation. Our results suggest that the degree of prey preparation is a compromise between the benefits gained by the nestlings (ingestion and digestion of prey is facilitated) and the costs to the parents (mainly time allocated to prey preparation).     

The significance of clutch overlap in Great Tits Parus major

We studied the effects of manipulation of the size of first broods in the Great Tit Parus major on the size and breeding success of second clutches and its relation to the degree of clutch overlap. The rearing of first brood fledglings always overlapped with the laying of the second clutch and in most cases also with the incubation period of the latter. The degree of clutch overlap depended on the size of the first brood, being less when the first brood was large. Clutch overlap also increased with season. Mechanisms affecting the timing of laying of second clutches are discussed. A large first brood imposed reproductive costs. It affected the size of the second clutch by causing it to be delayed; second clutch size decreases with season. It affected the post-fledging survival of second brood young as, in this population, this decreases with fledging date. The breeding success of second clutches was, however, not affected by the size of the first brood, but instead by the weight of the female when rearing the first brood.     

Effects of the doubly labelled water procedure on Great Tits Parus major feeding young

Capsule Female feeding rate behaviour was highly variable between individuals but no effect of the doubly labelled water (DLW) procedure on female visit rate was detected, whereas visit rates by males, which were neither trapped or manipulated, increased following trapping and manipulation of their partners.

Aims To test the assumption that the subject's behaviour is normal during the measurement period when using DLW to measure energy expenditure.

Methods Visits to the nestbox by parents feeding young were counted separately for females and males on the day before and the day after the female only was trapped to measure her energy expenditure using DLW. Visit rates were also counted for control pairs.

Results Female visit rates did not differ before and after manipulation, or between experimental and control pairs, but bird behaviour was highly variable between individuals. In contrast, the visit rates of both experimental and control males, which were not trapped, increased on the second day.

Conclusion The results for female behaviour supported the assumption of normality, but a small subset of particular individuals may be prone to adverse reactions. The response of the males may have been a reaction to disturbance at the nest.     

Pair bond and breeding success in Blue Tits Parus caeruleus and Great Tits Parus major

Data from 939 nests of the Blue Tit Parus caeruleus and 1008 nests of the Great Tit P. major from nestboxes provided in superabundance in mixed forest study sites between 1976 and 2001 were analysed to examine the effects of mate retention on breeding success and the relationship between mate fidelity and site fidelity. Most birds retained their former partner (76% in Great Tits and 65% in Blue Tits). The probability of a pair divorcing was affected by male age in Great Tits, divorce being more likely in pairs with first‐year males. Great Tit pairs breeding together for a second season bred earlier, but had no higher breeding success than pairs breeding together for the first time. In Blue Tits laying date and start of incubation tended to be earlier in pairs breeding together for a second season, but hatching and fledging dates were not earlier than in other pairs. Great Tit pairs breeding together for two consecutive seasons bred earlier in the second season than in the first, but breeding success did not differ significantly between years. In both species, breeding performance did not differ between pairs that divorced after a season and pairs that stayed together. Thus breeding success did not determine whether a pair divorced or bred together again. Neither Blue Tits nor Great Tits improved their breeding performance through divorce. Blue Tit females even had fewer fledglings in the year after divorce than in the year before. Mate retention affected breeding site fidelity. Blue Tit females had greater breeding dispersal distances between consecutive years when re‐mating than when breeding again with the same mate. In Great Tits both males and females dispersed more when re‐mating than when retaining the former partner, suggesting that mate retention increased the chance of retaining the breeding site. In both species, breeding dispersal distances did not differ between pairs that divorced and pairs in which one mate disappeared. Because no major advantage of mate retention was evident, we suggest that mate retention evolved under different conditions than those found in study sites with high breeding densities and a superabundance of artificial nesting sites.     

Activity patterns of parent Great Tits Parus major feeding their young during rainfall

The influence of rainfall on the foraging patterns of Great Tit Parus major parents while feeding chicks at the nest was investigated using automated nest monitoring with electronic balances and photography. Great Tit females significantly reduced their visit rate to the nest during all rain intensities, while male feeding frequency did not significantly change. The female response was probably due to increased brooding requirements of young since the reduction in visit rate was most apparent at early nestling stages. At this time the chicks are incapable of thermoregulation and females significantly increased their nestbox occupancy time during rain. There was no indication that parents were compensating for periods of female inactivity during rainfall: there was no significant increase in visit rate following rainfall and no significant increase in prey size delivered to the nest during periods of rain. An analysis of data from six consecutive years revealed that the proportion of wet hours within the first week of the nestling period significantly influenced fledging weight in this species.     

The effect of old age on the reproduction of Great Tits Parus major and Blue Tits P. caeruleus

The results from two data sets show that in Great Tits Parus major and Blue Tits P. caeruleus reproduction is influenced by age. In both species, laying dates become earlier while clutch- and brood-size increase between the ages of 1 and 2, but there is no change in nesting success or post-fledging juvenile survival. Great Tits aged 5 or older are 'old' in that laying starts later and nesting success, brood-size and post-fledging juvenile survival decrease. Blue Tits become 'old' 1 year earlier than Great Tits: females aged 4 or older lay later, have a lower nesting success and smaller brood-size, and their young show a decreased post-fledging survival. The effect on laying date, in the Blue Tit, becomes apparent only for females aged 6 and older. It is concluded that ageing in small passerines is to be expected more generally, but that no extrapolations can be made as to the timing of the effect from one species to another.     

The timing of breeding and offspring size in Great Tits Parus major

The relationship between the timing of breeding, offspring weight and offspring recruitment was investigated in two populations of Great Tits Parus major. In two of six seasons a significant negative correlation was found between laying date and fledgling weight, in three seasons the opposite was true and in one season no significant correlation existed between these variables. Offspring recruitment was quantified in five seasons, and late-nesting birds were found to produce significantly fewer recruiting offspring in two of these, the same two seasons when late fledglings had significantly lower weights. No significant correlation existed between the number of recruits and laying date in the three seasons when late fledglings were as heavy as or heavier than early fledglings. The potential detriment to reproductive success of lateness thus was offset when late-nesting parents managed to produce heavy young. Between-year variations in the seasonal pattern of reproductive success has previously been suggested to contribute to the maintenance of variability in the timing of breeding in Great Tits. In this paper we suggest that producing heavy offspring late in the season might be a proximate factor influencing the seasonal patterns of reproductive success. Producing heavy offspring late in the season is probably not a "strategy" but rather an effect of chance differences between years in the seasonal availability of food or the seasonality of other factors important for the nesting birds.     

Early morning fledging improves recruitment in Great Tits Parus major

A potential key event linking the nestling phase to first‐year survival is fledging (nest leaving) because this process is characterized by a major change of environments and therefore a sudden shift in selective forces. Here we assessed whether different facets of fledging predicted subsequent survival (measured as local recruitment) in Great Tits Parus major. Nestlings had a higher recruitment probability when they fledged early in the morning and when they were heavy. The existence of selection for fledging early in the day has been suggested before, but here we provide the first empirical evidence in support of that prediction.     

Weight loss of the female during the first brood as a factor influencing second brood initiation in Great Tits Parus major and Blue Tits P. caeruleus

We tested the hypothesis that the weight lost by female Great and Blue Tits Parus major and P. caeruleus while raising their first brood influences their ability to start a second brood. The evening weight of female parents was recorded when the nestlings were 5 and 13 days old, in different years and habitats. Several predictions were tested: (1) both species lose weight while raising nestlings and Great Tit females which start a second brood lose less weight than females which do not; (2) differences in the average weight lost between years and areas correlate with differences in the proportion of second broods; (3) the relative weight loss in Blue Tits, which only rarely undertake second broods, is higher than in Great Tits in which second broods are more common. Other factors also are related to the probability of undertaking a second brood: more second broods are undertaken by more successful females, adult females and females that lay earlier.
The comparison of Great and Blue Tits suggests that the two species use different reproductive strategies.     

Body temperature responses of Great Tits Parus major to handling in the cold

Animals typically respond to stressful stimuli such as handling by increasing core body temperature. However, small birds in cold environments have been found to decrease body temperature (T_b) when handled over longer periods, although there are no data extending beyond the actual handling event in such birds. We therefore measured both the initial T_b decrease during ringing and standardized T_b sampling, and subsequent recovery of T_b after this handling protocol in wild Great Tits Parus major roosting in nestboxes in winter. Birds reduced their T_b by 2.3 °C during c. 4 min of handling. When birds were returned to their nestboxes after handling, T_b decreased by a further 1.9 °C over c. 2 min, reaching a T_b of 34.6 °C before taking 20 min to rewarm to 2.5 °C above their initial T_b. The T_b reduction during handling could be a consequence of increased heat loss rate from disrupted plumage insulation, whereas T_b reduction after handling might reflect reduced heat production. These are important factors to consider when handling small birds in the cold.     

Temperature and foraging success of Great Tits Parus major hunting for spiders

The number of spiders caught by caged Great Tits Parus major in a 10 minute test increased in a sigmoid fashion with ambient temperature between 2 and 13°C. In control tests with immobile prey Calliphora pupae there was no significant effect of temperature. These results indicate that increasing activity of arthropod prey with temperature (Kacelnik 1979b) renders them more vulnerable to predators. We suggest that one reason why small temperate-zone birds such as Great Tits do not forage but sing when they get up in the morning is because low ambient temperatures reduce prey activity and hence the profitability of foraging.     

Extra-pair paternity among Great Tits Parus major following manipulation of male signals

Female Great Tits are known to eavesdrop on the singing behaviour of males. It is unknown, however, whether manipulation of these signals is sufficient to influence extra-pair copulations, or whether such potentially costly reproductive decisions are unaffected by altering short-term signals of condition. Using interactive playbacks, we systematically engaged males in territorial contests in which we could control whether the focal male won or lost the interaction, regardless of the males' true potential. We then determined the levels and patterns of extra-pair paternity among experimental and neighbouring pairs using DNA microsatellite analysis. Extra-pair young were found in equal frequency among the nests of males allowed to win interactions as among those who lost interactions. However, cuckolded males were significantly less variable in allele sizes across the five microsatellite alleles tested than males who were not cuckolded. As measures of genetic variability are increasingly being found to correlate with individual fitness, this may suggest that females are attentive to underlying measures of condition when making extra-pair decisions. Short-term modification of the males' perceived quality may be insufficient to cause females to alter extra-pair decisions.     

Increasing the perceived predation risk changes parental care in female but not in male Great Tits Parus major

In birds, little is known about how the presence of predators alters parental food distribution decisions among nestlings. We found that experimentally increasing perceived predation risk changed parental care in female but not in male Great Tits Parus major. Females fed the lightest and average nestlings at similar rates under control conditions when predation risk was not manipulated but ignored the lightest nestling under increased perceived predation risk. Moreover, females reduced the duration of nest visits greatly after encountering a model predator, suggesting that the perception of predators may facilitate brood reduction mechanisms.     

Car traffic along hedgerows affects breeding success of Great Tits Parus major

Capsule?In hedgerows near roads with fast and frequent traffic, the mortality of Great Tit Parus major broods was higher than in hedgerows with less traffic and hedgerows with no disturbance.     

Environmental drivers of roosting body mass variation in boreal Great Tits Parus major

Small wintering passerines adaptively modulate daily body mass acquisition as part of their energy management policy. However, whether birds optimize overnight mass loss or body mass at dawn remains poorly understood. We studied environmental correlates of individual variation in body mass at dusk, overnight mass loss and body mass at dawn in a wild population of Great Tits Parus major wintering in northern Fennoscandia. Body mass at dusk, overnight mass loss and body mass at dawn were independent of prevailing conditions despite extremely low night ambient temperatures. Body mass at dusk was higher in males than in females, and decreased throughout winter and when snowfall was higher in the previous month. Overnight mass loss increased with precipitation during the previous week and tended to be higher in mid‐winter, when nights were longest. However, birds reduced overnight mass loss with higher temperatures in the previous week and higher precipitation in the previous 2 weeks. Dawn body mass was strongly correlated with dusk body mass and overnight mass loss, and showed only mild associations with weather variables once dusk mass was accounted for. Body mass in roosting boreal Great Tits seems to be constrained by recent snowfall as the winter progresses, but otherwise appears to be mostly unaffected by previous and current temperatures, suggesting a regular use of facultative hypothermia.     

Interactive vocal communication at the nest by parent Great Tits Parus major

Although most bird species show monogamous pair bonds and bi‐parental care, little is known of how mated birds coordinate their activities. Whether or not partners communicate with each other to adjust their behaviour remains an open question. During incubation and the first days after hatching, one parent – generally the female – stays in the nest for extended periods, and might depend on acoustic communication to exchange information with its mate outside. The Great Tit Parus major is an interesting study system to investigate intra‐pair communication at the nest because males address songs to their mate while she is in the nest cavity, and females answer the male from the cavity with calls. However, the function of this communication remains unknown. In this study, we recorded the vocalizations and observed the resulting behaviour of Great Tit pairs around the nest at different breeding stages (laying, incubation and chick‐rearing). We observed vocal exchanges (vocalization bouts, alternated on the same tempo, between the female inside the nest and her male outside) in three contexts with different outcomes: (1) the female left the nest, (2) the male entered the box with food, and the female then used specific call types, (3) mates stopped calling but did not leave or enter the nest. The structure of vocal exchanges was globally stable between contexts, but females used calls with an up‐shifted spectrum during exchanges, at the end of which they left the nest or the male entered the nest. Birds vocalized more and at higher tempo during exchanges that ended up in feeding inside the nest. Birds also vocalized more during exchanges taking place during laying – a period of active mate guarding – than during incubation. We conclude that vocal exchanges could signal the females’ need for food and the males’ mate guarding behaviour, and discuss other possible functions of this communication.     

Thermal conditions for successful breeding in Great Tits (Parus major L.)

Summary The development of temperature regulation in relation to the growth and age of the nestlings is described in a way permitting use of the data in a model designed to predict the range of temperature tolerance of broods of Great Tits in the nestling stage. Such a model is described in a second paper. The physiological part of that model is made up mainly of six equations (nos. 6, 10, 11, 12, 14 and 15), which are all presented and discussed here. It is shown in this paper that the development of temperature regulation is a function of body weight rather than of age. The level of the basal metabolic rate of nestling Great Tits is lower than that of adult passerines of comparable size. The basal metabolic rate of a newly hatched Great Tit is only about one fourthe of the metabolic rate expected from Lasiewski and Dawson's equation for adult passerine birds. This discrepancy diminishes gradually during the nestling period and disapears shortly before fledging.Basal and maximum metabolic rates, as well as the body temperatures coinciding with these rates, are described in allometric equations as functions of nestling body weight. The evaporative heat loss of the nestlings is described as a function of body weight and body temperature, and an estimate of the maximum amount of water available to them for evaporative heat loss is given. A distinction is made between a long-term risk of hyperthermia, which results in mortality through dehydration of the nestling body, and an immediate risk of hyperthermia, which occurs when the maximum rate at which nestlings can evaporate water is insufficient to cope with the required heat loss by water evaporation. It is concluded that this immediate risk of hyperthermia is the most important of the factors affecting the upper limit of the range of temperature tolerance.