The timing of birds' breeding seasons: a review of experiments that manipulated timing of breeding

Abstract

Reproductive success usually declines in the course of the season, which may be a direct effect of breeding time, an effect of quality (individuals with high phenotypic or environmental quality breeding early), or a combination of the two. Being able to distinguish between these possibilities is crucial when trying to understand individual variation in annual routines, for instance when to breed, moult and migrate. We review experiments with free-living birds performed to distinguish between the 'timing' and 'quality' hypothesis. 'Clean' manipulation of breeding time seems impossible, and we therefore discuss strong and weak points of different manipulation techniques. We find that the qualitative results were independent of manipulation technique (inducing replacement clutches versus cross-fostering early and late clutches). Given that the two techniques differ strongly in demands made on the birds, this suggests that potential experimental biases are limited. Overall, the evidence indicated that date and quality are both important, depending on fitness component and species, although evidence for the date hypothesis was found more frequently. We expected both effects to be prevalent, since only if date per se is important, does an incentive exist for high-quality birds to breed early. We discuss mechanisms mediating the seasonal decline in reproductive success, and distinguish between effects of absolute date and relative date, for instance timing relative to seasonal environmental fluctuations or conspecifics. The latter is important at least in some cases, suggesting that the optimal breeding time may be frequency dependent, but this has been little studied. A recurring pattern among cross-fostering studies was that delay experiments provided evidence for the quality hypothesis, while advance experiments provided evidence for the date hypothesis. This indicates that late pairs are constrained from producing a clutch earlier in the season, presumably by the fitness costs this would entail. This provides us with a paradox: evidence for the date hypothesis leads us to conclude that quality is important for the ability to breed early.

Telomere length reflects phenotypic quality and costs of reproduction in a long-lived seabird

“…After an advancement in the first few years of life, arrival date has low intra-individual variability [33,43]. Timing of arrival and reproduction onset are closely related parameters, with an impact on reproductive success in the common tern as in many other bird species [44,45]. Brood size for each nest was recorded every 2 -3 days.…”

Introduction. Adaptation to the annual cycle

“…Several contributions to this volume draw attention to the physiological basis of various tradeoffs (e.g. Barta et al 2008;Martin et al 2008;Speakman 2008;Verhulst & Nilsson 2008). We believe that incorporating this basis will lead to a new generation of analysis that will provide a deeper understanding of life histories and suggest new lines of physiological research.…”

“…Individuals can also be expected to respond to their physiological condition. Cues and associated plasticity are central to most of the papers in this issue and are the main focus of Barta et al (2008), Shine & Brown (2008) and Verhulst & Nilsson (2008).…”

“…Breeding is a crucial part of an annual cycle and is considered by all papers but is the particular concern of Shine & Brown (2008; year-to-year variation and cues), Ball & Ketterson (2008; sex differences), Verhulst & Nilsson (2008;timing) and Speakman (2008;costs).…”

Stage-dependent survival in relation to timing of fledging in a migratory passerine, the Northern Wheatear (Oenanthe oenanthe)

“…Survival of juveniles fledged early in the breeding season is often higher than that of juveniles fledged later in the season, which is hypothesized to be due to higher food abundance, lower predation pressure and/or because parents breeding early may be of higher quality and provide more food (Smith et al 1989;Naef-Daenzer et al 2001;Verhulst and Nilsson 2008;Vitz andRodewald 2011 but see Anders et al 1997;Yackel Adams et al 2006). Early fledglings may also arrive earlier at stopover and wintering sites and thereby experience more favourable conditions and gain competitive advantage on non-breeding sites, resulting in increased survival rates.…”