Selective Prey Delivery to Incubating Females by Dominant Males, but not Helpers, in the Cooperatively Breeding Green Woodhoopoe

In cooperatively breeding bird species, one of the most conspicuous helping behaviours is the provisioning of food. Many studies have considered the feeding of nestlings, but far fewer have examined feeding of incubating females, and none have looked at the types of prey delivered at this stage. Here I show that green woodhoopoe ( Phoeniculus purpureus ) group members selectively feed incubating females with certain prey items: the diet delivered to incubating females contains a higher proportion of caterpillars, centipedes and cockroaches than that eaten by the provisioning adults themselves. The prey items selectively delivered are the largest in the diet and so might be provided in an effort to enhance the breeding female's condition, thus minimizing the time that she spends off the nest and so increasing hatching success. Intriguingly, it is only breeding males that show this adjustment in provisioning diet; helpers of both sexes simply provide the incubating female with the same proportions of different prey items that they eat when self-feeding. My results therefore offer the first evidence that members of cooperative groups may not all follow the same provisioning rules, and they also emphasize the need for studies to consider the incubation stage in just as much detail as the nestling phase if we are to understand fully the complexities of cooperative societies.     

Are colorful males of great tits Parus major better parents? Parental investment is a matter of quality

Given the known influence of parental investment on breeding success of great tits Parus major, females should be expected to use male parental quality as an essential criterion in mate choice. Since parental quality cannot usually be observed directly at the time of pairing, it has been suggested that females rely on male ornaments as indicative of their ability to provide parental care. This hypothesis, called the good parent hypothesis, has been tested repeatedly assessing only parental effort as the number of feedings made by parents. However, in evaluating parental investment, the focus should also be on the quality of prey captured rather than only on its quantity. We analyzed feeding rates and the provisioning of different prey in relation to both male yellow carotenoid-based breast coloration and the size of the black melanin-based stripe in a Mediterranean great tit population. We predicted that more carotenoid ornamented individuals would feed nestlings with a diet consisting of a higher proportion of caterpillars. However, and contrary to predictions, we found that males with higher values of hue in the yellow breast feathers, fed their offspring with a lower proportion of caterpillars and a higher proportion of spiders. In addition, nestlings that received a higher proportion of spiders showed an improved body condition after controlling for tarsus length and other variables. Male feeding rates correlated positively with brood size and tended to correlate negatively with date, although we did not find any effect of male coloration. Our data therefore support the good parent hypothesis, insofar as parental investment is also a matter of quality, and that, at least in the Mediterranean area, caterpillars are not the only key food source.     

Nestling diet variation in an insular Mediterranean population of blue tits Parus caeruleus: effects of years,territories and individuals

Mediterranean evergreen forests of Corsica are characterized by relatively high species diversity of arthropods with low population densities. Food is never superabundant for Corsican blue tits Parus caeruleus. This study focused on the composition of the food of blue tit nestlings and especially on two main components, caterpillars and spiders. The nestling diet was studied for two years using 8-mm cameras that automatically took photographs of adult birds with food. The diet was composed of c. 50% caterpillars and c. 30% spiders. There were between-year and between-individual differences in these proportions. In both years of the study the proportion of caterpillars declined during the course of the breeding season. Individual and time effects on prey sizes were also observed. Pairs and individuals were fairly constant in the proportions of prey over the feeding period. Different food items were not brought in runs. These findings suggest that strong food limitation exists on Corsica, which can considerably influence life-history traits of the blue tit.     

Seasonal insectivory by Black‐headed Trogons,a tropical dry forest frugivore

ABSTRACT Fruit‐eating birds are important seed dispersers in tropical forests, but little is known about the extent to which they rely on insects or how their diets vary seasonally. We used field observations of focal adults to quantify the diets of adult and nestling Black‐headed Trogons (Trogon melanocephalus) at nine nests in a lowland dry forest in Costa Rica. From May 2004 to August 2004, we documented 540 food deliveries to nests and 1080 food items consumed by adults. Adult and nestling trogons were largely insectivorous, feeding mainly on moth caterpillars (Lepidoptera). Fruit accounted for only 10.5% of items consumed by adults and 2.2% of items delivered to nestlings (6.1% and 0.6% of estimated dry mass, respectively). Adult and nestling diets differed significantly in both composition and prey size, with adults consuming more fruit and fewer large insects (Phasmatodea and Mantodea) than nestlings and eating more types of arthropods and fruit. Although both adults and nestlings relied heavily on moth larvae, adults preferentially consumed small caterpillars and delivered large ones to their nestlings. In addition, the proportion of large caterpillars delivered to nests remained constant throughout the nestling period, whereas the proportion of large caterpillars eaten by adults declined significantly with nestling age. Overall, arthropods delivered to nests averaged 70% heavier than those consumed by adults (estimated dry mass). Our results suggest that Black‐headed Trogons time reproduction to coincide with arthropod rather than fruit abundance, a pattern that may be more common among omnivorous forest birds than previously recognized.     

Feeding ecology and specialist diet of critically endangered Ridgway's Hawks

Ridgway's Hawks (Buteo ridgwayi) are critically endangered forest raptors endemic to the Caribbean island of Hispaniola, with ～100 pairs remaining in the world. The species is ecologically little known yet such studies are important for understanding critical habitat needs and population dynamics. We studied the provisioning behavior of adults at 22 nests on the northeast coast of the Dominican Republic from 2005 to 2008. Mean brood size was 1.80 ± 0.45, and the mean number of fledglings per nest was 1.10 ± 0.97. We found that 80% of the prey items delivered to nestlings were reptiles, with lizards accounting for 65% of the prey and those in the genus Celestus accounting for nearly 35% of prey. Other prey items included snakes (14%), rats (9%), and smaller proportions of birds, frogs, and centipedes. The number of prey items and amount of biomass delivered to nestlings did not vary with brood size, but adults delivered more prey to 3‐ to 5‐week‐old nestlings and more biomass to 5‐week‐old nestlings. Food delivery rates did not differ between successful or failed nests, suggesting that food availability did not influence nest outcome. Given that most prey items delivered to nestlings in our study were reptiles, conservation strategies developed for Ridgway's Hawks (e.g., translocations and habitat conservation) should take into consideration their specialist reptile diet.     

Size-biased allocation of prey from male to offspring via female: family conflicts, prey selection, and evolution of sexual size dimorphism in raptors

In birds with bi-parental care, the provisioning link between prey capture and delivery to dependent offspring is regarded as often symmetric between the mates. However, in raptors, the larger female usually broods and feeds the nestlings, while the smaller male provides food for the family, assisted by the female in the latter part of the nestling period, if at all. Prey items are relatively large and often impossible for nestlings to handle without extended maternal assistance. We video-recorded prey delivery and handling in nests of a raptor with a wide diet, the Eurasian kestrel Falco tinnunculus, and simultaneously observed prey transfer from male to female outside the nest. The male selectively allocated larger items, in particular birds and larger mammals, to the female for further processing and feeding of nestlings, and smaller items, in particular lizards and smaller mammals, directly to the nestlings for unassisted feeding. Hence, from the video, the female appeared to have captured larger prey than the male, while in reality no difference existed. The female’s size-biased interception of the male’s prey provisioning line would maximize the male’s foraging time, and maximize the female’s control of the allocation of food between her own need and that of the offspring. The male would maximize his control of food allocation by capturing smaller prey. This conflict would select for larger dominant females and smaller energy-efficient males, and induce stronger selection the longer the female depends on the male for self-feeding, as a proportion of the offspring dependence period.     

Parental Sex Differences in Food Allocation to Junior Brood Members as Mediated by Prey Size

Individual offspring within a brood may receive different amounts of provisioning from the male and female parents. Some hypotheses suggest that this bias is the result of an active and adaptive choice by parents. An alternative hypothesis is that feeding biases arise as a result of a constraint of fitting large prey items into small gapes. In an experiment with pied flycatchers, Ficedula hypoleuca , we tested for sex-biased allocation to junior nestlings in asynchronous broods and whether this could be explained by active parental choice or by passive allocation according to prey size and gape size. In both control broods and broods with experimentally increased degree of asynchrony, prey types did not differ between parents but females brought smaller prey than males at younger but not older nestling stages. At younger but not older nestling stages, the majority of feeds to junior nestlings were from females, and the smaller nestlings consumed smaller prey than older siblings. However, there was no evidence of active preference of small nestlings by females as parents did not differ in the tendency to bypass a begging senior nestling in order to feed a junior nestling. Provisioning rates by females were lower than those by males when nestlings were young and we suggest that foraging time constraints caused by the need to brood offspring result in females bringing smaller prey than males. In turn, the larger prey brought by males was more often transferred to larger offspring after the smaller ones failed to swallow it. In such cases, 'preferential' feeding of small nestlings by females may simply be a passive side effect of foraging constraints and gape-size limitations.     

Foraging ecology of the California gnatcatcher deduced from fecal samples

The California gnatcatcher is a threatened species essentially restricted to coastal sage scrub habitat in southern California. Its distribution and population dynamics have been studied intensely, but little is known about its diet. We identified arthropod fragments in 33 fecal samples of the California gnatcatcher to gain insight into its foraging ecology and diet. Fecal samples were collected from adult males, adult females, fledglings, and nestlings. Leaf- and planthoppers (Homoptera) and spiders (Araneae) predominated numerically in samples. Spider prey was most diverse, with eight families represented. True bugs (Hemiptera) and wasps, bees, and ants (Hymenoptera) were only minor components of the gnatcatcher diet. Gnatcatcher adults selected prey to feed their young that was larger than expected given the distribution of arthropod size available in their environment, and chicks were provisioned with larger prey items and significantly more grasshoppers and crickets (Orthoptera) and spiders than adults consumed themselves. Both adults and young consumed more sessile than active prey. Further studies are needed to determine whether arthropods sampled in coastal sage scrub that are common in fecal samples are good indicators of California gnatcatcher habitat. Received: 30 December 1998 / Accepted: 28 April 1999     

Timing of breeding and nestling diet of Wood Warbler Phylloscopus sibilatrix in relation to changing food supply

Capsule Wood Warblers did not match their reproduction to the caterpillar peak

Aims To study the timing of Wood Warblers’ breeding and nestling diet in relation to caterpillar abundance under primeval conditions in the Bia?owie?a National Park, Poland.

Methods Observations of food brought by parents. Inter‐year and seasonal changes in availability of folivorous caterpillars were assessed by direct counts and caterpillar frass collection.

Results Maximum food requirements occurred two weeks after the peak of caterpillar abundance. The mismatch had no effect on nestlings’ development. Diet varied little across years and habitats, but varied strongly within a season. Following the decline of ‘green’ caterpillars, their proportion in the Wood Warbler diet strongly declined. Caterpillars were replaced by winged insects. Small nestlings received more spiders than older ones.

Conclusion Timing of Wood Warbler breeding in Bia?owie?a National Park was constrained by the females’ arrival time. Birds did not match their reproduction to the caterpillar peak. ‘Green’ caterpillars were the preferred food for nestlings; birds responded to the caterpillar decrease by feeding smaller proportions of them and switching to alternative prey, namely winged insects.     

Effects of habitat fragmentation on provisioning rates, diet and breeding success in two species of tit (great tit and blue tit)

The aim of this study was to examine the effects of forest fragmentation on the ability of parent birds to provide their young with an adequate food supply. To examine whether prey population densities of the great tit (Parus major L.) and the blue tit (P. caeruleus L.) vary between study areas in different forest size classes we compared provisioning rates and chick diet and related these parameters to breeding success. We filmed 217 nests over two breeding seasons and collected data on frass fall as a general estimate of caterpillar availability. Nests which were attended by none or one parent only during filming (n = 46) were excluded from the analyses. In both years and for both species feeding rates were highest in the smallest fragments and lowest in the large forest. There was also a suggestion that differences in feeding rates between areas vary between years. We found no consistent tendency for prey size to change with forest size, although both species brought slightly smaller prey items to the nest in the smallest forest fragments and feeding rates correlated negatively with prey size. Caterpillars were the main item fed to nestlings, in both species. We found no evidence to suggest that either frass fall or the proportion of caterpillars in the diet varied with forest size. There was also no correlation between mean frass fall and the total number of caterpillars brought to the nests, in either species. Breeding success, as measured by clutch size, brood size, fledging weight and fledging success, did not differ between the small fragments and the large forest, in either species. There was also no relationship between provisioning rate (as concerns volume of prey fed to nestlings and the quality of chick diet) and breeding success parameters. In conclusion, this study does not suggest suboptimal foraging or breeding conditions in small fragments compared to a nearby large forest, for either species. Received: 23 June 1997 / Accepted: 29 December 1997     

Prey selection and foraging performance of breeding Great Tits Parus major in relation to food availability

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.     

Forest fragmentation is associated with primary brood sex ratio in the treecreeper (Certhia familiaris)

We studied the primary brood sex ratio of an old-growth forest passerine, the Eurasian treecreeper (Certhia familiaris), along a gradient of forest fragmentation. We found evidence that male nestlings were more costly to produce, since they suffered twofold higher nestling mortality and were larger in body size than females. Furthermore, the proportion of males in the brood was positively associated with the provisioning rate and the amount of food delivered to the nestlings. During the first broods, a high edge density and a high proportion of pine forests around the nests were related to a decreased production of males. The densities of spiders, the main food of the treecreeper, were 38% higher on spruce trunks than on pine trunks. This suggests that pine-dominated territories with female-biased broods may have contained less food during the first broods. The observation was further supported by the fact that the feeding frequencies were lower in territories with high proportions of pines. In the second broods, territories with a high forest patch density produced female-biased broods, whereas high-quality territories with a large amount of deciduous trees and mixed forests produced male-biased broods. Our results suggest that habitat quality as measured by habitat characteristics is associated with sex allocation in free-living birds.     

Diets of adult and nestling starlings (Sturnus vulgaris) in Hawke’s Bay,New Zealand

Abstract

Gizzards were examined from 334 adult and 62 nestling starlings collected in mixed farmland during 1971–72. The birds ate insects, spiders, earthworms, snails, millipedes, centipedes, seeds, and fruits. Starlings ate fewer subterranean animals (7%) than those usually living partially hidden (45%), on the ground (31%), or on vegetation (18%). About half the invertebrates eaten were 2–5 mm long and about a quarter 6–10 mm long. Nestlings tend to be fed significantly larger items than were eaten by adults. The commonest items in adult starlings were Coleoptera adults, Lepidoptera larvae, Hemiptera, and fruits; in nestlings, Coleoptera adults and Diptera adults and larvae were important. Earthworms were found in all the nestlings. More than 50% of adult gizzards contained earthworm chaetae in wetter months, but fewer in drier months. The diet of starlings, despite considerable overlap with the foods eaten by mynas, rooks, and magpies, included somewhat different components and proportions of the food supply. Fruits were found in adults only; most were probably eaten after harvest. Although predation on two major pasture pests—Costelytra zealandica and Wiseana cervinata larvae—was insubstantial, nearly 40% of the total invertebrates eaten by adult starlings were insect pest species.     

Habitat quality, nestling diet, and provisioning behaviour in great tits Parus major

Variation in early nutrition is often a strong predictor of offspring condition and fitness. In the case of woodland passerine birds, nestling diet is determined by the spatiotemporal distribution of prey items such as caterpillars during the nestling period, and is usually quantified as differences in provisioning behaviour between habitats. However, the habitat level does not account for variation between individual territories, the level at which competition and selection are assumed to operate. Here we use nestbox cameras and Radio Frequency Identification technology (RFID) to simultaneously assess variation in both nestling diet (components) and provisioning rates (quantity) among a sample (n=22) of different quality great tit Parus major territories selected from a larger breeding population (n=310 fledged broods) in a single year. Caterpillars were by far the most numerous item provisioned to nestlings (mean=75% of prey items), as expected given the known importance of this food source for this species. Broods raised close to an oak tree, or far from the woodland edge, were provisioned the highest proportion of caterpillars. Provisioning rates declined seasonally and there was a weak association between low provisioning rates and caterpillar rich diets. During the first week of the nestling stage, nestling condition was unrelated to the proportion of caterpillars in the diet, provisioning rates and oak proximity. Condition at fledging was slightly improved in broods fed a higher proportion of caterpillars in the diet and in broods raised close to an oak tree. However, in our data early breeding was the only predictor of recruitment success, although power was low for this test. Analyses of long‐term data (41 years) from the same population confirmed a relationship between oak proximity and fledgling mass, but not recruitment success. Our results suggest that territory level environmental variation can affect offspring condition, probably through observed changes in nestling diet, but that such variation does not necessarily produce discernable effects on offspring fitness.     

Foraging behavior of Cerulean Warblers during the breeding and non‐breeding seasons: evidence for the breeding currency hypothesis

Birds require additional resources for raising young, and the breeding currency hypothesis predicts that insectivorous species exploit large soft‐bodied prey during the breeding season, but shift to small, likely hard‐bodied, prey during the non‐breeding season. To test this hypothesis, we examined prey use by Cerulean Warblers (Setophaga cerulea), foliage‐gleaning Nearctic‐Neotropical migrants, during the breeding and non‐breeding seasons. We collected data on foraging behavior during the breeding season (including observations of prey items fed to young) in upland mixed‐oak forest in southeastern Ohio in 2009 and 2010 and, during the non‐breeding season, in shade coffee in the Cordillera de Merida, Venezuela, in 2008–2009. Cerulean Warblers captured 7% more large prey (visible prey extending beyond the bill) during the breeding than the non‐breeding season, but foraged at similar rates during both seasons. Large, soft‐bodied prey appeared to be especially important for feeding young. We found that adults delivered large prey on >50% of provisioning visits to nests and 69% of identifiable large prey fed to nestlings were greenish larvae (likely Lepidoptera or caterpillars) that camouflage against leaves where they would tend to be captured by foliage‐gleaning birds. Availability of specific taxa appeared to influence tree species foraging preferences. As reported by other researchers, we found that Cerulean Warblers selected trees in the genus Carya for foraging and our examination of caterpillar counts from the central Appalachian Mountains (Butler and Strazanac 2000 ) showed that caterpillars with greenish coloration, especially Baileya larvae, may be almost twice as abundant on Carya than Quercus. Our results provide evidence for the breeding currency hypothesis, and highlight the importance of caterpillars to a foliage‐gleaning migrant warbler of conservation concern.     

The importance of a main dish: nestling diet and foraging behaviour in Mediterranean blue tits in relation to prey phenology

Insectivorous birds rely on a short period of food abundance to feed their young; they must time their reproduction to match the timing of Lepidoptera larvae, their main prey. Apart from the net result (i.e. birds are timed or mistimed with respect to the food's peak), an important aspect is the possible influence of other factors, such as the seasonality of the environment or the abundance and diversity of species contributing to the caterpillar peak, on birds' phenology and their ability to cope well with unpredictable food supplies. In a 2-year study, we explored the seasonal variation of nestling diet in Mediterranean blue tits Cyanistes caeruleus and how reproductive parameters (nestling condition, provisioning rates) are affected by the phenology and composition of food. We also examined the influence of the synchrony between offspring needs and caterpillar peak in shaping the composition of the nestlings' diet. We found that the effect of synchrony on nestling condition varied between years which may be partially due to differences in food peak attributes. The adequacy of birds' timing in relation to prey phenology affected foraging decisions; those birds that were not able to correctly adjust their timing were forced to rely on less preferred prey (tortricids). In this sense, we found that relative contribution of tortricids (smaller caterpillars but easier to get) and noctuids (preferred prey but more difficult to find) to the diet influenced nestling condition and parental provisioning effort; parents performed fewer feeding events and reared heavier nestlings as the contribution of noctuids to the diet increased. The relationship between the proportion of caterpillars and nestling mass was curvilinear, whereas that parameter was negatively affected by the percentage of pupae. Our results show how changes in diet composition may contribute to explain the effect of mismatching on birds' breeding performance.     

The diet of great tit nestlings: Comparing observation records and stable isotope analyses

The diet of wild animals has been studied using many different strategies, approaches and methods in recent decades. In this regard, stable isotopes analysis (SIA) is becoming a widespread tool, but no study has yet, to our knowledge, compared diet estimations from SIA with direct observations of the diet of passerine nestlings. Accordingly, our aim was to test the predictive power of SIA for this purpose and identify potential confounding factors such as habitat effects. To do this, we compared isotopic signatures of δ¹³C and δ¹⁵N in the feathers of great tit (Parus major) nestlings, and the corresponding estimates of their diet based on stable isotope mixing models, with prey proportions delivered by their parents obtained through video-recordings. Between-nest differences in isotopic signatures of δ¹⁵N were larger than within-nest differences. We found that δ¹⁵N signatures of nestling feathers correlated positively with the proportion of spiders and negatively with the proportion of caterpillars in the nestlings’ diet, the most important prey types. On the other hand, between-nest and within-nest differences in δ¹³C ratios were of similar magnitude and δ¹³C ratios correlated mainly with the proportion of trees surrounding nest-boxes that were Quercus spp. Estimates of diet composition based on mixing models correlated with the observed nestling diet, yet effect sizes were quite low. Although mixing models are commonly used to ascertain diets, our data show that they can provide valuable information on the relative intake of prey types from different trophic levels; but when complex dietary patterns are recorded (e.g. due to the confounding effects of habitat and/or temporal variation) it can be difficult to draw firm conclusions about diet composition.     

Parental food provisioning is related to nestling stress response in wild great tit nestlings: implications for the development of personality

Background

Variation in early nutrition is known to play an important role in shaping the behavioural development of individuals. Parental prey selection may have long-lasting behavioural influences. In birds foraging on arthropods, for instance, the specific prey types, e.g. spiders and caterpillars, matter as they have different levels of taurine which may have an effect on personality development. Here we investigated how naturally occurring variation in the amounts of spiders and caterpillars, provisioned to nestlings at day 4 and 8 after hatching, is related to the response to handling stress in a wild passerine, the great tit (Parus major). Broods were cross-fostered in a split-brood design allowing us to separate maternal and genetic effects from early rearing effects. Adult provisioning behaviour was monitored on day four and day eight after hatching using video recordings. Individual nestlings were subjected to a handling stress test at an age of 14 days, which is a validated proxy for exploratory behaviour as an adult.

Results

Variation in handling stress was mainly determined by the rearing environment. We show that, contrary to our predictions, not the amount of spider biomass, but the amount of caterpillar biomass delivered per nestling significantly affected individual performance in the stress test. Chicks provisioned with lower amounts of caterpillars exhibited a stronger stress response, reflecting faster exploratory behaviour later on in life, than individuals who received larger amounts of caterpillars.

Conclusions

These results suggest that natural variation in parental behaviour in wild birds modulates the developmental trajectories of their offspring's personality via food provisioning. Since parental provisioning behaviour might also reflect the local environmental conditions, provisioning behaviour may influence how nestlings respond to these local environmental conditions.

     

The manipulation of food resources reveals sex-specific trade-offs between parental self-feeding and offspring care

Parent Palestine sunbirds (Nectarinia osea) feed on flower nectar that is not fed to their nestlings. This phenomenon provided a unique opportunity to manipulate self-feeding rates of parent birds independently of the rate at which they feed arthropod prey to their offspring. Based on provisioning models, we predicted that parents would invest more in their young as the energy content of their own food increased. From our earlier work, we also predicted that the levels of sex-specific activities of males and females would differ as the energy content of their food increased. Sunbird pairs with two or three nestlings were provided with feeders containing a low-, medium- or high-concentration sucrose solution. As the sugar concentration increased, the females delivered arthropods at a greater rate to their nestlings, removed proportionally more faecal sacs and spent longer at the nest, while the males increased their mobbing effort. Nestling food intake and body mass, but not tarsus length or bill size, were larger in small broods than in large broods, and increased with increasing feeder sugar concentration. These results imply that increasing the energy content of food consumed by parent sunbirds allows them to increase the rate at which other foods are delivered to their young and to increase other parental care activities as well. The results also add credence to the idea that behavioural decisions reflect life-history trade-offs between parental self-feeding and investment in current young.     

Obtaining accurate measurements of the size and volume of insects fed to nestlings from video recordings

Video recordings are commonly used to study the types, amount, and size of food items provided to nestling birds. However, the accuracy and repeatability of estimates of the size of food items from video recordings has not been examined. We assessed three aspects of the reliability of measuring prey size from video recordings of Great Tits (Parus major) provisioning nestlings. To test the accuracy of measurements of prey size (length and width) used to determine prey volume, we molded artificial plasticine caterpillars and compared their size and volume as determined using measurements of length and width on screenshots of video recordings (using the vertical diameter of nest‐box entrance holes as a size reference) to their actual size and volume. We also examined within‐ and among‐observer repeatability of measurements of the size and volume of actual prey items delivered to nestlings by adult Great Tits. We found that observers were able to accurately measure prey size and determine volume, with high agreement between the actual size and volume of plasticine caterpillars and the size and volume as determined from measurements made on screenshots from video recordings (r_ICC = 0.99). In addition, within‐ and among‐observer repeatability were also high (r_ICC = 0.98 and 0.93, respectively). Overall, our results suggest that the size of prey items delivered to nestlings by adults in video recordings can be accurately measured and those measurements, in turn, can be used to accurately determine the volume of those insect prey.