PARENT SESSION
Contributed Oral Session 52: Avian Nesting and Breeding
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 524 A, Level 5, Palais des congrès de Montréal
Food and predator effects on passerine egg production: results from a bi-factorial experiment.
Zanette, Liana1, Clinchy, Michael*,1, Smith, James2, 1 University of Western Ontario, London, Ontario, Canada2 University of British Columbia, Vancouver, B.C., Canada
ABSTRACT- Debate over the proximate control of avian clutch size has continued for more than 50 years. Food effects are often inconsistent and predator effects have been difficult to interpret. Recent studies on both birds and mammals suggest these many inconsistencies and oddities may be resolved by considering the combined effects of food and predators. We conducted a 2 x 2 manipulative food addition plus natural predator reduction experiment on song sparrows over 3 years. On average both food addition and predator pressure significantly affected nest predation but there was also significant inter-annual variability in the effect of food addition on nest predation. Both food and nest predation determined clutch number (the number of nests built in a season). More nest predation meant more re-nesting and thus more nests per season. Fed birds built still more nests because feeding reduced the intervals between nests. Both food and clutch number determined clutch size. The more re-nesting there was (the larger clutch number) the smaller the average clutch size. Fed birds generally laid larger clutches but the size of this effect was inversely related to the size of the effect of food on clutch number. Because fed birds were able to either re-nest faster or lay larger clutches food addition had a consistently dramatic effect on the total number of eggs laid in a season. Whether food principally affected clutch number or clutch size varied between years. The source of this inter-annual variation was the inter-annual variability in the effect of food addition on nest predation. Large food effects on nest predation meant small food effects on clutch number and large food effects on clutch size; and vice versa. Thus do food and predators together govern virtually every aspect of egg production in song sparrows. We suggest much of the controversy concerning the proximate control of avian clutch size stems from focusing on either food or predators rather than their combined effects.
Key words: food-supplementation, predator-pressure, nest-predation, clutch-size