Summary 1.We present a multivariate model of the post-fledging survival of juvenile great and coal tits ( Parus major L. , P. ater L. ) in relation to chick body condition and timing of breeding. Radio-telemetry and colour marks were used to track tit families during 20 days from fledging, that is, the period of post-fledging dependence. Data on 342 chicks of 68 broods were obtained. 2. Forty-seven per cent of juveniles died during the observation period, predation being the main cause of mortality. In the first 4 days after fledging the mortality rate was 5-10% per day. 3. Survival of juveniles was positively correlated with fledging mass. Furthermore, survival strongly decreased during the season. In the second half of June, mortality was five times the rate of mid-May. The differential survival resulted in selection for both early fledging and high fledging mass. Juvenile condition was less important for survival in birds that had fledged early in the season. Their survival rates exceeded 70% in all weight classes, whereas in late broods only the heaviest individuals survived equally well. The survival of birds fledging both late and in poor condition was below 20%. Thus, selection for high fledging mass was much stronger in the late season than in early broods. 4. We conclude that the impact of predation after leaving the nest results in selection for early breeding and, particularly in the late season, for high fledging mass. This may explain why the earliest broods have been found to produce most recruits into the breeding population even if they did not profit from maximum food availability during the nestling period. On the other hand, energetic limitations may constrain the begin of egg laying in adult birds. Thus, counteracting evolutionary responses to the seasonal development of food availability (the caterpillar peak) and to the risk of post-fledging mortality (the peak in post-fledging mortality) may have focused the period of optimal reproduction to a narrow time-window.

The design of artificial nestboxes for the study of secondary hole-nesting birds: a review of methodological inconsistencies and potential biases. Acta Ornithol. 45: 1-26.

Summary 0[ We analysed the e}ect of prey density and size on the foraging performance of great and blue tit "Parus major L[\ P[ caeruleus L[# parents\ and its consequences for the growth and~edging weight of nestlings[ Because~edging weight is a determinant of subsequent survival and therefore _tness\ foraging decisions of the parents play a key role in the reproductive system of tits[ The analysis quanti_es "i# the rate at which energy is delivered to the nestlings in relation to prey size and abundance\ and "ii# the growth rates of nestlings and the resulting~edging weight in relation to the rate of food delivery by the parents[ 1[ The searching time per prey item increased exponentially with decreasing prey biomass[ During the peak abundance of caterpillars\ the average searching time per item was 1=4Ð2 min instead of 4Ð5 min before and after the peak[ Searching time was signi_cantly reduced when the birds returned to the foraging site where the preceding prey was found[ This accords with the clumped distribution of caterpillars within the canopy[ 2[ The foraging performance "in mg caterpillars per min# was maximal when cater! pillars were both abundant and large\ i[e[ shortly before they left the trees for pupation[ The high feeding frequency and the large prey then caused a peak energy~ow rate to the nestlings of 3Ð4 times the rate before or after the caterpillar peak[ This suggests that the foraging success and rate of food delivery by tit parents was primarily determined by the abundance and size of prey[ 3[ The growth rate of nestlings\ as well as their~edging weight was correlated with the rate of food delivery[ Low feeding performance of the parents resulted therefore in poor relative growth rates of only 9=2Ð9=5 of the rate achieved under optimal conditions and\ as a consequence\ in a low~edging weight[ This indicates that tit parents have restricted options to adjust prey delivery rates according to the require! ments of the brood[ 4[ The results give insight into the chain of causal mechanisms through which an environmental factor "availability of food# has a strong and immediate e}ect on _tness "growth\~edging weight and\ thus\ survival of the nestlings#[ The importance of caterpillar size for foraging success and prey delivery rates of parent tits makes clear\ why the phase of best foraging conditions is shorter than the period during which caterpillars are available[ The relationships we quanti_ed give a proximate explanation for the great e}ects that temperature and caterpillar growth have on the between! year variation in selection intensity for laying date observed in other studies[

We studied the nestling diet and the foraging performance of Great Tits in relation to prey abundance in the field. Numerous experimental studies present data on foraging decisions in captive Great Tits. Little is, however, known about prey selection in the field in relation to the food available and the consequences this has for the food delivery rate to nestlings. Since the foraging performance of the parents is one of the main determinants of fledging weight and juvenile survival, foraging behaviour is an important part of Great Tit reproduction. During the early breeding season up to 75% of the prey biomass delivered to the nestlings were spiders, which is in contrast with other studies. Only when caterpillars reached a size of 10–12 mg (approximately the average size of the spiders caught at that time) did the Great Tits change their preferences and 80–90% of the delivered prey masses were caterpillars, as reported by other authors. This ‘switching’ between prey occurred within a few days. It was not related to the changes in abundance but to size of caterpillars. The rate at which caterpillars were delivered to the nestlings (in mg/nestling/h) was strongly correlated with the caterpillar biomass available (in mg/m of branches) and nestling growth rate was significantly influenced by the mass of available caterpillars. The results provide evidence why perfect timing of breeding is so important for the Great Tit, and contribute to the understanding of the causal link between food supply, growth and breeding success.

For altricial young, fledging is an abrupt step into an unknown environment. Despite increasing numbers of studies addressing the post-fledging period, our current knowledge of the causes and consequences of post-fledging survival remains fragmentary. Here, we review the literature on post-fledging survival of juvenile altricial birds, addressing the following main questions: Is low post-fledging survival a bottleneck in the altricial reproductive cycle? What is known of proximate and ultimate causal factors such as trophic relations (food and predation), habitat conditions, or abiotic factors acting in the post-fledging period? We analyzed weekly survival estimates from 123 data series based on studies of 65 species, covering weeks 1-13 post-fledging. As a general pattern, survival of fledglings was low during the first week post-fledging (median rate = 0.83), and improved rapidly with time post-fledging (week 4 median rate = 0.96). For ground-nesting species, survival immediately after leaving nests was similar to egg-to-fledging survival. For species breeding above-ground, survival during the first week post-fledging was substantially lower than during both the nestling period and later post-fledging stages. Thus, the early post-fledging period is a bottleneck of markedly elevated mortality for most altricial species. Predation was the main proximate cause of mortality. Various factors such as habitat, annual and seasonal variation in the environment, and the physical condition of fledglings have been found to affect post-fledging survival. Individual survival depended strongly on physical traits such as mass and wing length, which likely influence the ability of fledglings to escape predation. Trophic relationships at various levels are the main ultimate driver of adaptation of traits relevant to survival during the pre-and post-fledging periods. Spatiotemporal dynamics of food resources determine the physical development of juveniles and, in turn, their performance after fledging. However, predators can cause quick and efficient selection for fledgling traits and adult breeding decisions. Parental strategies related to clutch size and timing of breeding, and the age and developmental stage at which young fledge have substantial effects on post-fledging survival. The intensity and duration of post-fledging parental investment also influences fledgling survival. Post-fledging mortality is therefore not a random and inevitable loss. Traits and strategies related to fledging and the post-fledging stage create large fitness differentials and, therefore, are integral, yet poorly understood, parts of the altricial reproductive strategy. RESUMEN Supervivencia de aves altriciales luego de salir del nido: determinantes ecol ogicos y adaptaci on Para juveniles altriciales, salir del nido es un paso abrupto hacia un ambiente desconocido. A pesar del incremento en los estudios investigando el periodo posterior a la salida, nuestro conocimiento actual de las causas y consecuencias de la supervivencia luego de salir del n...