Extra Chick Research Articles

An Experimental Test of the Information Model for Negotiation of Biparental Care

BackgroundTheoretical modelling of biparental care suggests that it can be a stable strategy if parents partially compensate for changes in behaviour by their partners. In empirical studies, however, parents occasionally match rather than compensate for the actions of their partners. The recently proposed “information model” adds to the earlier theory by factoring in information on brood value and/or need into parental decision-making. This leads to a variety of predicted parental responses following a change in partner work-rate depending on the information available to parents.Methodology/Principal FindingsWe experimentally test predictions of the information model using a population of long-tailed tits. We show that parental information on brood need varies systematically through the nestling period and use this variation to predict parental responses to an experimental increase in partner work-rate via playback of extra chick begging calls. When parental information is relatively high, partial compensation is predicted, whereas when parental information is low, a matching response is predicted.Conclusions/SignificanceWe find that although some responses are consistent with predictions, parents match a change in their partner's work-rate more often than expected and we discuss possible explanations for our findings.

Breeding ecology of the Chestnut-crowned Babbler: a cooperative breeder in the desert

Cooperative breeding systems provide a rich testing ground for evolutionary theory because of the apparent paradox posed by individuals caring for the offspring of others. Such breeding systems are unusually common among Australian birds, but most species have been studied within the temperate and tropical zones. Here we provide the first detailed study of the breeding ecology of the Chestnut-crowned Babbler (Pomatostomus ruficeps), which was conducted in an arid-zone setting. Chestnut-crowned Babblers bred in groups of 2–13. Breeding occurred between late winter and early summer, 81% of 16 groups fledged offspring and 38% of groups fledged two broods. Clutch-size ranged from two to five, and one to five chicks fledged from successful nests after 21–25 days of provisioning. The size of the breeding unit had a significant effect on breeding success, with an extra chick fledged in the season for every three helpers in the group. These helper effects are among the strongest reported for pomatostomid babblers, perhaps reflecting the extreme environment in which this study was conducted. This study will form a basis for future research into the cooperative breeding system of Chestnut-crowned Babblers and aid with our understanding of variation in the breeding ecology of Australian babblers.

Low or no occurrence of extra-pair paternity in the Black Guillemot Cepphus grylle

We investigated extra-pair paternity (EPP) among 46 young in 32 broods of 19 social pairs of Black Guillemot Cepphus grylle, using a combination of three polymorphic microsatellite markers. The analysis was based on blood samples collected over eight years from a population of individually colour-ringed birds in North Norway. All chicks shared an allele on all three loci with both social parents. Hence, there were no cases of extra-pair parentage. Given our sample sizes, the upper 95% confidence limit of EPP was estimated to 6.2% of the young and 8.9% of the broods. Among seven extra chicks, for which we lacked a DNA sample from the social mother, there was one case of genotype mismatch with the social father, but this could also be because of egg dumping. Low occurrence of EPP has now been documented in five of the six species of Atlantic auks.

Adverse effects of instrumentation in incubating Adélie penguins (Pygoscelis adeliae)

The use of data-loggers has permitted to explore the biology of free-ranging animals. However, this method has also been reported to reduce reproductive success while the reasons of this deleterious effect remain poorly documented. In this study, we aimed to identify critical periods of the breeding cycle of Adelie penguins (Pygoscelis adeliae) when the reproductive success may decrease because of instrumentation. For this purpose, we monitored 40 pairs, where one parent was instrumented before egg laying and 30 pairs without devices (controls). These pairs were followed at least during the incubation period but the majority was monitored during the entire breeding season. Reproductive success was affected in pairs where males were instrumented. This was not due to extra chick mortality during chick rearing but to a significantly lower hatching success. Moreover, the use of artificial eggs recording incubation temperatures and egg rotation indicated that in instrumented incubating males, eggs spent as much time at optimal incubation temperatures as control eggs but were rotated at a higher frequency. In Adelie penguins, males initiate incubation and it has been established that the early stage of incubation is one of the most critical periods for embryonic development. The low hatching rate observed in instrumented males was associated with a higher egg rotation rate, perhaps as a stress response to the presence of the instrument. Even though the causal effects remain unclear, instrumentation severely affected hatching success. For these reasons, we recommend equipping birds after the early incubation.

Nest protectors provide a cost-effective means of increasing breeding success in Giant Ibis Thaumatibis gigantea

SummaryA main cause of nestling loss of the ‘Critically Endangered’ Giant Ibis Thaumatibis gigantea appeared to be mammalian predation. Predator-exclusion devices were added to a randomly selected half of 52 nesting trees, with the other half acting as controls. The number of young fledged per nest was 50% higher for protected nests than control nests. Each protected nest produced almost two-thirds (0.63) of an extra chick. During the nestling period, the daily nestling survival rate was (99.9%) (equivalent to 90% survival over the nestling period) for protected nests and 99.3% (61.3% survival over the whole period) for control nests. Nest protection devices cost US$5 per tree. Each extra nestling fledged as a result of this intervention therefore cost $8. If it was also necessary to pay to locate the nests, then the cost of protecting each nest would be $140 and the cost of producing each extra chick $224. Trees with protectors added were significantly more likely to be reused in the next year than unprotected trees.

VARIATION IN BLUE TIT BREEDING PERFORMANCE ACROSS GRADIENTS IN HABITAT RICHNESS

Food supply is widely considered to be a major factor in determining life history traits and reproductive performance of birds. However, large spatial and temporal variation in natural available food supply is not always paralleled by proportional changes in energy demand by breeding birds. This necessarily results in variation in the supply–demand ratio and the amount of food available per unit mass of nestling. Because reproductive performance should respond to increases in available supply as a saturation curve, reaching a plateau above a certain threshold of food supply, we predict that variation in supply should change the intensity of selection on reproductive traits. We first tested this prediction using long-term data on nestling growth and survival in Blue Tits (Parus caeruleus) breeding over a gradient of habitat richness in Corsica, France. This long-term data analysis evaluates the effect of variation in food supply available to breeding tits using three surrogate variables: interannual variation in peak caterpillar abundance (caterpillar frass fall), offset between breeding date and peak caterpillar abundance, and natural variation in clutch size. We also used an experimental brood size manipulation (±3 chicks) to test the effect of varying brood demand on nestling growth. Results of the long-term data analysis show that all three variables affect fledging mass and fledging success in poor habitats, while only fledging mass is affected by variation in offset in rich habitats. Moreover, mean annual fledging success and fledging mass is strongly affected by annual variation in peak caterpillar abundance at low levels of abundance, but these effects disappear when food becomes abundant (saturation threshold level of 373 and 560 mg frass·m−2·d−1 for fledging success and fledging mass, respectively). Brood size manipulations confirm these results. In rich habitats, breeding birds can raise three extra chicks without any apparent effects on chick growth, while in poor habitats, chick growth is significantly reduced with brood enlargement. Our study shows that although food limitation can play an important role, it may not always be the primary force shaping life history traits.

Clutch size limitation in waders: experimental test in redshank Tringa totanus.

Several hypotheses have been raised to explain the upper limit of clutch size at four eggs in waders (suborder Charadrii), which may play an important role in the evolution of the variety of mating and parental care systems in this group. Experimental tests of the hypotheses have produced conflicting results. It was recently suggested that the combined effects of several incubation costs of a larger clutch suffice to limit its size to four eggs in this group. Here we test the incubation-limitation hypothesis in a field experiment, in redshank Tringa totanus. We created five-egg clutches by adding one egg from another nest to a just completed four-egg clutch. Four-egg control clutches were created by replacing one of the eggs by an egg from another nest. All egg removals, additions and replacements were done before incubation started. Incubation time in five-egg clutches increased by 1day to 24.3±0.23days, compared to 23.3±0.32days in four-egg clutches. Egg hatchability and nest predation rates did not differ significantly between treatments. On average five-egg clutches produced one extra chick at hatching (4.5±0.26chicks) compared to four-egg clutches (3.5±0.27chicks). Also when several additional costs from incubating enlarged clutches are added, redshanks by laying a fifth egg would on average increase their reproductive success at hatching by an estimated 22%. The incubation-limitation hypothesis therefore is clearly rejected in this species. Possible mechanisms behind the four-egg clutch limit in waders and ways of testing the alternatives are discussed.

Brood size manipulations within the natural range did not reveal intragenerational cost of reproduction in the Willow TitParus montanus

To test for the existence of a reproductive cost, we manipulated brood sizes (‐2 and +2 nestlings) over 6 years in a northern population of Willow TitsParus montanusbreeding in natural holes. Possible effects were sought in subsequent survival and fecundity of the parents. Parents given extra chicks made more feeding visits than did parents with reduced and control broods. However, this was not reflected in differences in parental body‐weight between groups at the end of the nestling period. Brood size manipulation did not significantly affect female or male survival. In 4 out of 6 years, there was a weak and nonsignificant effect on male survival, consistent with a cost of reproduction. Female and male fecundity in the year following the experiment was not affected by the manipulations. Thus, the data do not give evidence of an intragenerational cost of reproduction in the Willow Tit. Parents appeared unwilling to increase their breeding effort to a level which jeopardized their own survival or future breeding success. It is possible that, because of the time constraints in northern latitudes, females work under their capacity and lay smaller clutches than would otherwise be most profitable. Thus, no costs to the parents would be expected as a consequence of manipulations. These results suggest that the current reproductive rate is not maintained by reproductive cost in the Willow Tit. However, the results do not rule out the possibility that selection has operated outside the current range of reproductive rates during evolutionary history of the species.

Nesting Energetics of House Wrens (Troglodytes aedon) in Relation to Maximal Rates of Energy Flow

We tested the hypothesis that brood size in House Wrens (Troglodytes aedon), which have a mean mass of 10.6 g, is confined by digestive limits to maximal rate of energy flow in parents feeding nestlings. Using doubly-labeled water (DLW), we measured field metabolic rate during feeding of nestlings (FMR,), tested whether it increased with brood size, and compared it with the wrens' near-maximal rate of intake, digestion, and assimilation measured in the laboratory. To expand the range of House Wrens' effective brood sizes (i.e. brood size/parent), we manipulated some brood sizes by removal of the male parent or by addition of extra chicks or eggs. The rate of nest visits by parent wrens was positively correlated with mass of chicks (P < 0.001). Various measures of brood size were regressed against FMR,. However, when brood size and overnight minimum air temperature were entered together in a stepwise multiple-regression model, only minimum air temperature was significantly related to FMR, (P = 0.013, r2 = 0.201). Mean FMR, (60.8 3;2;± 2.0 kJ /day) was lower than the near-maximal rate of intake, digestion and assimilation measured in the laboratory (92.7 kJ I day). Therefore, we reject the hypothesis that brood size is limited by digestive constraints on maximum metabolic rate in House Wrens.

Motives for Parental Infanticide in White Storks Ciconia ciconia

~.,i' , larger broods and laid larger clutches than non-infanticidal birds. Males killed the p,~, chick in 8 out of 9 cases. Victims were bom from the last-hatched egg in 4- and 5-egg .clutches, they were the lightest in their brood and grew at lower rates than their nestmates during the days preceding their elimination. The last-hatched nestlings in 4-chick broods had lower pre-fledging survival rates than their elder sibs. Potential victims contributed a low fraction to parents' reproductive output, and 4-chick broods were especially costly to raise because parents provisioned them both more frequently and for longer nestling periods. Hence, the presence of an extra chick seems to lower the benefit/cost ratio to parents rearing a large brood once its elder siblings have hatched successfully. If nestlings do not compete aggressively for food, parents would be selected to eliminate the extra chick themselves. This hypothesis could provide an explanation for the existence of parental infanticide algo in other species.

Food Provisioning, Nestling Growth and Experimental Manipulation of Brood Size in the African Redbreasted Sparrowhawk Accipiter rufiventris

Up to two extra chicks were added to a Redbreasted Sparrowhawk Accipiter rufiventris nest to determine whether the parents would increase their biomass provisioning rate. During 46 days of manipulations (day 19 to day 64) no response occurred in either of the adults' rate, or that combined. Provisioning mainly small passerines, the adults provided between 43 and 122 g d-' during the first 64 days. The adult male which remained provisioning, weaned the one remaining fledgling until the 76th day following hatch when no more prey was delivered. The male provisioned significantly (p < 1 x 10-6) lighter prey than his mate but provided about 3.67 kg of prey in contrast to the female's 1.95 kg during the hatch-to-independence period. The rate of provisioning was significantly (p < 0.05) lowered during steady or torrential rain, but temperature extremes did not affect the rate. Since available daylight hours, temperature, instrinsic prey size, and rain could not account for the smaller amount of biomass delivered (to two nests), relative to temperate breeding sparrowhawks, I suggest in conjunction with other comparisons that some subtropical raptors may be reproductively limited by food abundance.