Energetic limitation of avian parental effort:Field experiments in the kestrel ( Falco tinnunculus )

• Published in: Journal of Evolutionary Biology
• Main Authors: Masman, Dirkjan, Dijkstra, Cornelis, Daan, Serge, Bult, Ab
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1989
• Subjects: reproductive strategies; brood size; avian energetics; Microtus arvalis
• Online Access: https://hdl.handle.net/11370/3345348b-a2c4-4edf-8095-699d8f9511af; https://research.rug.nl/en/publications/3345348b-a2c4-4edf-8095-699d8f9511af; https://doi.org/10.1046/j.1420-9101.1989.2060435.x; https://pure.rug.nl/ws/files/3361067/1989JEvolBiolMasman.pdf

We studied the limiting factors for brood size in the kestrel, Falco tinnunculus, by measuring parental effort in natural broods of different size and parental response to manipulation of food satiation of the brood. Parental effort was quantified as total daily time spent in flight, and total daily energy expenditure, from all-day observations. During nestling care males with different natural brood sizes (4 to 7 chicks), spent an average of 4.75 h · d-1 in flight independent of brood size, and expended an average total daily energy of 382 kJ · d-1. Due to a higher flight-hunting yield (mammal-prey caught per hour hunting), males with larger natural broods were able to provision their broods with the same amount of food (mainly Microtus arvalis) per chick (62.6 g · d-1), with the same effort as males with smaller broods. This provisioning rate was close to the mean feeding rate of hand-raised chicks in the laboratory, that were fed ad libitum, (66.8 g · d-1 · chick-1). Our food deprivation experiments revealed that male kestrels strongly respond to food shortage in the nest. In the older nestling phase males on average increased their daily rate of food delivery to the nest as a response to experimental food deprivation by almost three times to 646.4 g · d-1, by increasing their flight activity level from 4.46 to 8.41 h · d-1. This increased energy expenditure was sustained, for as long as eleven days, by increasing the metabolizable energy intake up to what is presumed to be the maximum rate. Even under considerable experimental food stress (chicks not being satiated due to continuous removal of delivered food by the observers) about half of the available daylight time remained unused for foraging. We conclude 1) that the mean daily energy expenditure of males during nestling care - to which clutch size is apparently initially adjusted - is well below the maximum they are able to sustain and 2) that the energy expenditure they can sustain under extremely high nestling demand is not set by the available time ...