Inter-individual variation in provisioning rate,prey size and number,and links to total prey biomass delivered to nestlings in the Collared Flycatcher(Ficedula albicollis)

Background:In bird species where offspring growth and survival rely on parents’food provisioning,parents can maximise their fitness by increasing the quantity and/or the quality of preys delivered to their offspring.Many studies have focused on inter-individual variation in feeding rate,yet this measure may not accurately reflect the total amount of food(i.e.energy)provided by parents if there is large variation in the quantity and quality of preys at each feeding.Here,we explored the relative role of individual(sex,age,body condition),breeding(hatching date,brood size)and environmental(temperature)factors on feeding rate,prey number,size and quality,and their contribution to total prey biomass delivered to the nestlings of 164 Collared Flycatcher(Ficedula albicollis)parents in 98 nests.Results:Preys delivered to the nest were mainly larvae(53.6%)and flying insects(45.6%).Feeding rate increased with brood size and age,and was higher in males than females.Mean prey number decreased,but mean prey size increased,as the season progressed and parents feeding their brood with primary larvae brought more preys per visit.Relationships between feeding rate,mean prey number and size remained when taking into account the provision-ing quality:parents brought either a large number of small prey or a small number of larger items,and the force of the trade-offs between feeding rate and mean prey number and size depended on the quality of the provisioning of the parents.Whatever the percentage of larvae among preys in the provisioning,the variance in total prey biomass was foremost explained by feeding rate(65.1%to 76.6%)compared to mean prey number(16.4%to 26%)and prey size(2.7%to 4%).Conclusions:Our study shows that variation in feeding rate,prey number,size,but not quality(i.e.percentage of lar-vae),were influenced by individual factors(sex and age)and breeding decisions(brood size and timing of breeding)and that,whatever the provisioning strategy adopted,feeding rate was the best proxy of the total biomass delivered to the nestlings.

A diagnosis model of parental care:How parents optimize their provisioning strategy in brood reduction?

Altricial birds often display biased preferences in providing parental care for their dependent offspring,especially during food shortages.During this process,such inflexible rules may result in provisioning errors.To demonstrate how parents optimize their provisioning strategies,we proposed a“diagnosis model”of parental care to posit that parents will undergo a diagnosis procedure to test whether selecting against some particular offspring based on phenotype is an optimal strategy.We tested this model in an asynchronous hatching bird,the Azure-winged Magpie Cyanopica cyanus,based on 10 years of data about demography and parental provisioning behaviors.Given their higher daily survival rates,core offspring(those hatched on the first day)merits an investment priority compared with their marginal brood mates(those hatched on later days).However,a marginal offspring also merited a priority if it displayed greater weight gain than the expected value at the early post-hatching days.Parents could detect such a marginal offspring via a diagnosis strategy,in which they provisioned the brood at the diagnosis stage by delivering food to every nestling that begged,then biased food toward high-value nestlings at the subsequent decision stage by making a negative response to the begging of low-value nestlings.In this provisioning strategy,the growth performance of a nestling became a more reliable indicator of its investment value than its hatching order or competitive ability.Our findings provide evidence for this“diagnosis model of parental care”wherein parents use a diagnosis method to optimize their provisioning strategy in brood reduction.