Reproductive responses of birds to experimental food supplementation: a meta-analysis

Abstract

Introduction: Food availability is an important environmental cue for animals for deciding how much to invest in reproduction, and it ultimately affects population size. The importance of food limitation has been extensively studied in terrestrial vertebrate populations, especially in birds, by experimentally manipulating food supply. However, the factors explaining variation in reproductive decisions in response to food supplementation remain unclear. By performing meta-analyses, we aim to quantify the extent to which supplementary feeding affects several reproductive parameters in birds, and identify the key factors (life-history traits, behavioural factors, environmental factors, and experimental design) that can induce variation in laying date, clutch size and breeding success (i.e., number of fledglings produced) in response to food supplementation. Results: Food supplementation produced variable but mostly positive effects across reproductive parameters in a total of 201 experiments from 82 independent studies. The outcomes of the food effect were modulated by environmental factors, e.g., laying dates advanced more towards low latitudes, and food supplementation appeared not to produce any obvious effect on bird reproduction when the background level of food abundance in the environment was high. Moreover, the increase in clutch size following food addition was more pronounced in birds that cache food, as compared to birds that do not. Supplementation timing was identified as a major cause of variation in breeding success responses. We also document the absence of a detectable food effect on clutch size and breeding success when the target species had poor access to the feed due to competitive interactions with other animals. Conclusions: Our findings indicate that, from the pool of bird species and environments reviewed, extra food is allocated to immediate reproduction in most cases. Our results also support the view that bird species have evolved different life-history strategies to cope with environmental variability in food supply. However, we encourage more research at low latitudes to gain knowledge on how resource allocation in birds changes along a latitudinal gradient. Our results also emphasize the importance of developing experimental designs that minimise competition for the supplemented food and the risk of reproductive bottle-necks due to inappropriate supplementation timings.

Food restriction negatively affects multiple levels of the reproductive axis in male house finches, Haemorhous mexicanus

“…This is especially true for females, who experience an additional energetic cost associated with egg formation (Nager, 2006). During periods of energetic scarcity, birds may delay egg laying and have smaller clutches (Meijer et al, 1990;Rodenhouse and Holmes, 1992), whereas food supplementation can result in birds advancing laying and producing larger clutches (Ruffino et al, 2014). Food availability, therefore, acts both as the ultimate factor and as a proximate factor to control the timing of breeding.…”

Sex- and melanic-specific variations in the oxidative status of adult tawny owls in response to manipulated reproductive effort

“…Accordingly, there are numerous examples showing that experimental restriction or supplementation of organisms with energy or particular nutrients induces a change in their life histories (e.g. Catoni et al, 2008;Prevedello et al, 2013;Ruffino et al, 2014). The conversion of nutrients into energy usable by the organisms for trait allocation is also known to come at a cost.…”

Experimental manipulation of food availability leads to short‐term intra‐clutch adjustment in egg mass but not in yolk androgen or thyroid hormones

“…Effects of egg size on offspring development have been extensively studied in birds: a recent meta‐analysis reveals that egg size positively correlates with offspring quality from hatching to post‐fledging (Krist ). Nevertheless, there is large variation in egg size among females (ca 70% of the variation) and even within clutches (Bernardo , Christians , Williams , Ruffino et al ). Resource limitation via trade‐offs between self and eggs, especially on protein, has been suggested as one of the explanations for this variation in egg size (constraint hypothesis, Lack , Christians ).…”