Feeding experience and relative size modify the begging strategies of nestlings

The offspring of birds and mammals use a combination of movementsand vocalizations, known as begging, to solicit food from theirparents. A widespread interpretation of begging is that itconstitutes an honest signal of offspring need. But we knowthat in the house sparrow (Passer domesticus) the intensityof begging calls reflects the past experience of offspringin addition to their need. Here we show that this result generalizesto other species. An experiment with hand-reared magpies (Pica pica) and great spotted cuckoos (Clamator glandarius) indicates that the begging strategies depend on the past experience ofchicks and the composition of their brood. In asynchronoustwo-magpie broods, both chicks begged at the same intensitywhen the large chick obtained food more easily than its sibling,but the large chick begged at higher intensity when it was easier for the smaller chick to obtain food. Cuckoo chicks beggedat higher intensity than magpies.     

Chick begging as a signal: are nestlings honest?

Begging by dependent avian offspring is known to correlate withhunger level, and parents use this as a signal of brood demandto adjust their chick feeding behavior. While there is informationon how each chick adjusts its begging to its own condition,little is known of how chicks adjust to the state of their nestmates. In two experiments we manipulated the competitive environmentof individual European starling (Sturnus vulgaris) chicks byaltering the state of nest mates while holding the state oftarget chicks constant In the first experiment we placed thetarget chick's nest mates in neighboring nests with brood sizesof two, five, or eight chicks. Following the manipulation wereturned them to their own nests and recorded begging behavioron videotape. In the second experiment we separated a targetchick from its siblings and manipulated feeding level in thelaboratory. The siblings were fed at one of three levels; meanwhile,all the target chicks were fed at the intermediate level. Afterthe manipulation we placed the target chicks with their siblingsand recorded their begging in response to an artificial stimulus.In neither experiment was the begging effort of the unmanipulatedtarget chicks affected by the changes in begging behavior oftheir siblings. This result supports the view that begging isa reliable signal of individual chick state and does not involveresponses to the effort of nest mates.     

Food restricted Tufted Puffin (Fratercula cirrhata) nestlings increase vocal activity during handling without modulating total or free corticosterone

In some species, corticosterone (CORT) appears to play a role in the control of begging behavior. Because of the potentially high costs associated with chronic elevation of CORT, it has also been proposed as a mechanism to ensure begging is an honest signal. We determined the effects of moderate food restriction (50% of high calorie treatment) on vocal behavior during handling, and on baseline levels of both total and ‘free’ unbound CORT in Tufted Puffin (Fratercula cirrhata) nestlings. Chick vocalizations during handling were similar to begging calls, and we assumed they were representative of begging behavior. We also measured total and free CORT in free-living Tufted Puffin chicks to determine if hormone levels in our experiment were comparable to natural levels. We found no effect of caloric restriction on either total or free baseline CORT, yet food-restricted nestlings vocalized more intensely during handling than chicks in the high calorie group. Mean plasma concentrations of total and free CORT in experimentally manipulated birds did not differ from levels in free-living nestlings. These results suggest that CORT does not play a role in modulating begging behavior in this species.     

The hormonal control of begging and early aggressive behavior: experiments in black-headed gull chicks

The hormonal control of begging and sibling competition is largely unknown, but recent evidence suggests a role for steroid hormones. We tested the influence of the aromatizable androgen testosterone (T), the non-aromatizable androgen 5alpha-dihydrotestosterone (DHT), and 17beta-estradiol (E) on both begging behavior and aggressive behavior in black-headed gull chicks (Larus ridibundus). Chicks of this species have a conspicuous begging display, while their frequently performed early aggressive behavior is facilitated by testosterone and important for territorial defense. Hormone treatment was applied by implants between days 6 and 16 after hatching. Behavior was tested by means of standard stimulus tests. The results were validated in a second experiment under semi-natural conditions. Begging was suppressed by T and DHT and not affected by E. Aggressive Pecking was strongly facilitated by T. The erect threat posture, characteristic for older chicks, was facilitated by T, DHT, and E and the nest-oriented threat display, typical for young chicks, only by T and DHT. Growth was suppressed in the T group. The results indicate that androgen production, needed for territorial defense, has costs in terms of a suppression of begging and growth. It is discussed to what extent older chicks may avoid these costs by converting testosterone to estrogen and why pre-natal and post-natal exposure to androgens differ in their effect on begging behavior.     

European starling chicks benefit from high yolk testosterone levels during a drought year

Avian egg yolk contains androgenic hormones, such as testosterone, of maternal origin. Experimental elevation of yolk testosterone levels enhances growth of canary chicks. Success in sibling competition, due to increased begging, is presumed to underlie this growth enhancement, because canary hatchlings from testosterone-treated eggs beg longer in response to vibrational stimuli than controls. Furthermore, experimental elevation of both yolk androstenedione and testosterone increased chick growth and begging in black-headed gulls. We measured daily growth of European starling (Sturnus vulgaris) chicks hatching from testosterone-treated or vehicle-treated (control) eggs until 14 days of age, and measured begging behavior at hatching and at 5 days of age. A temporary drought caused relatively high levels of early brood reduction for this population; 2- and 3-day-old chicks were most likely to starve. We found that chicks from testosterone-treated eggs were less likely to starve than control chicks, and were heavier on the days when most brood reduction occurred. However, chicks from testosterone-treated eggs begged less than control chicks on the day of hatching, and begged similarly at 5 days of age. Thus, while yolk testosterone did increase growth during periods of (presumably) high competition, increased begging does not appear to mediate this effect. Instead, testosterone may induce more efficient energy use, for example, by decreasing ineffective begging. While our results indicate that elevated yolk testosterone enhances survival, and thus offspring and parental fitness, further evidence regarding the fitness consequences of yolk androgens are vital to understanding their role in avian life history.     

Responses of breeding Cory's shearwater Calonectris diomedea to experimental manipulation of chick condition

We studied the regulation of provisioning in Cory's shearwaterat Selvagem Grande during the chick rearing period. Provisioningwas examined in terms of feeding frequency and amount of fooddelivered to chicks. Two groups of chicks were subjected toshort-term contrasting manipulations of their nutritional status:one group of chicks was given a food supplement of about 30g, and another group was deprived of up to 30 g of food. Adultstending deprived chicks increased the frequency of feedingvisits (but not the size of feeds), which resulted in an increasein the net rate of food delivery. At the end of this study,deprived chicks were growing at the same rate as fed chicks. Parents attending fed chick did not change their provisioningrates in response to the treatment. Our results indicate thatCory's shearwaters are able to adjust their provisioning ratein response to short-term variation in the nutritional statusof their chicks. We also examined the change in the beggingrate of fed and deprived chicks in response to the treatment.There was no relationship between the begging rate and thecondition of chicks, which is taken to be a measure of thechick's physiological condition, related to its ability towithstand imposed periods of fasting. However, fed chicks decreasedtheir begging rate after the increase in their condition dueto supplementary food. Conversely, deprived chicks, which wereonly able to sustain their condition before the onset of thetreatment, maintained high levels of begging. To some extent,these results suggest that parental provisioning can be influencedby the begging behavior of chicks.     

Darwin's Finch Begging Intensity Does Not Honestly Signal Need in Parasitised Nests

Parental care should be selected to respond to honest cues that increase offspring survival. When offspring are parasitised, the parental food compensation hypothesis predicts that parents can provision extra food to compensate for energy loss due to parasitism. Chick begging behaviour is a possible mechanism to solicit increased feeding from attending parents. We experimentally manipulated parasite intensity from Philornis downsi in nests of Darwin's small ground finch (Geospiza fuliginosa) to test its effects on chick begging intensity and parental food provisioning. We used in‐nest video recordings of individually marked chicks to quantify nocturnal parasite feeding on chicks, subsequent diurnal chick begging intensity and parental feeding care. Our video analysis showed that one chick per brood had the highest parasite intensity during the night (supporting the tasty chick hypothesis) and weakest begging intensity during the day, which correlated with low parental care and rapid death. We observed sequential chick death on different days rather than total brood loss on a given day. Our within‐nest video images showed that (1) high nocturnal larval feeding correlated with low diurnal begging intensity and (2) parent birds ignored weakly begging chicks and provisioned strongly begging chicks. Excluding predation, all parasite‐free chicks survived (100% survival) and all parasitised chicks died in the nest (100% mortality). Weak begging intensity in parasitised chicks, which honestly signalled recent parasite attack, was not used as a cue for parental provisioning. Parents consistently responded to the strongest chick in both parasitised and parasite‐free nests.     

Sibling competition and siblicide in asynchronously-hatching broods of the cattle egret Bubulcus ibis

Feeding behaviour and sibling competition were observed in nine families of the cattle egret (Bubulcus ibis) from blinds during 1359 nest-h throughout the nestling period. During days 0–19, size differences among siblings were clear; begging behaviour of chicks changed with time. At least one parent always attended the nest. Food boluses regurgitated early within a feeding period were received by senior chicks more often than by juniors. When any two siblings begged for food at the same time, the elder and younger received the first bolus on 65% and 35% of occasions respectively. Between days 20 and 39, the frequency of begging reached a peak. Begging behaviour became intense and stereotyped. The number of boluses received per begging declined rapidly, especially for junior chicks. In large broods, the success rate of begging was lower and fights occurred among siblings, especially among juniors. Out of 256 dyadic fights, the elder sibling won 85, lost one, and tied 171. The youngest chick died in two broods, apparently as the result of these fights (siblicide). No parents interfered in fights among their offspring. After day 40, the frequency of begging decreased gradually and ceased by day 80, No chicks died in the last period, although the frequency of fights in all large broods remained high.     

Shared parental care is costly for nestlings of common cuckoos and their great reed warbler hosts

Obligate avian brood parasitism typically involves one of 2strategies: parasite chicks are either 1) virulent and evictall other eggs and nest mates to be raised alone or 2) moretolerant and share foster parental care with host chicks forsome or the entirety of the nestling period. We studied theconsequences of experimentally forced mixed broods of age-matchedone common cuckoo (Cuculus canorus) and 2 great reed warbler(Acrocephalus arundinaceus) chicks. In these broods, both cuckooand host chicks grew slower than did either individual cuckoosor great reed warblers in broods of 1 parasite or 3 host chicks,respectively. Video records showed that in mixed broods, cuckoochicks received feedings less frequently than the 33% predictedby chance at 4 days of age but parental food allocations increasedto chance levels at 8 days of age. The consistent patterns oflower growth rates arose even though chicks in broods of 1 parasiteand 2 hosts received the largest prey items per feeding. Inaddition, several other measures of parental provisioning alsodid not predict species and brood-specific differences in nestlinggrowth rates across the different treatments. However, variationin begging displays and its specific costs on host and parasitechicks in the different nest treatments were not quantifiedin this study. We conclude that young of nest mate–evictorcommon cuckoos benefit from the sole occupancy of host nestsin part owing to an initial competitive disadvantage for parentalcare in broods with age-matched great reed warbler chicks.     

Parent-offspring conflict and the coordination of siblings in gulls

Offspring solicit food from their parents by begging behaviours. Studies on birds suggest that these displays are 'honest signals of need' and adults provide food according to the begging level. However, siblings may compete for parental resources and the begging intensity is expected to change with brood size. Here, we show that in the black-headed gull (Larus ridibundus) an increase of the numbers of siblings can result in a decrease of individual begging cost through nestlings' synchronized signalling. This is in accordance with some mathematical models. As parents respond to the total solicitation emerging from the nest, the probability to get food increases with the number of chicks begging together. The more siblings there are, the more they coordinate their begging while decreasing the number of individual begging bouts. Intra-brood synchronization of begging enables chicks to reduce their effort and hence exerting an important role in parental-offspring negotiation.