Variation in food supply, time of breeding, and energy expenditure in birds

Life-history trade-offs, density, lay date—not personality—explain multibroodedness in great tits

In various taxa, multibroodedness is a common breeding strategy. Life-history theory predicts that individuals can increase fitness by producing multiple broods within a season. Despite the apparent increase in the number of offspring parents might produce per season, not all individuals are multibrooded, suggesting a trade-off. We studied ecological and behavioral factors influencing the initiation of second clutches in great tits (Parus major), an optionally multibrooded bird species, by distinguishing two types of clutches: replacement versus true second clutches, produced after failure versus successful first breeding attempts, respectively. We predicted which lay date, density, and investment in first clutches would decrease the probability of initiating a second clutch, but which faster exploring behavioral types with a faster pace-of-life would be more likely to be multibrooded. The probability of initiating true second clutches varied negatively within-individuals with lay date and breeding density. The initiation of replacement clutches instead varied negatively among-individuals with lay date and density, suggesting nonrandom settlement of behavioral types across environments. Individuals were less likely to be multibrooded when producing many offspring from their first clutch, suggesting within-year reproductive trade-offs, similar to previous studies. No previous research has linked personality to multibroodedness; here we show which neither the repeatable nor the plastic part of an individual’s exploratory behavior predicted multibroodedness. We confirmed our prediction which the resolution of trade-offs may occur either at the within- or among-individual level. Our research contributes to the understanding of life-history evolution in the wild by studying the mechanisms shaping multibroodedness within seasons.

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

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.

Effects of climate variation on timing of nesting, reproductive success, and offspring sex ratios of red-winged blackbirds

Predicting ecological consequences of climate change will be improved by understanding how species are affected by contemporary climate variation, particularly if analyses involve more than single ecological variables and focus on large-scale climate phenomena. I used 18 years of data from red-winged blackbirds (Agelaius phoeniceus) studied over a 25-year period in eastern Ontario to explore chronological and climate-related patterns of reproduction. Although blackbirds started nesting earlier in years with warmer springs, associated with low winter values of the North Atlantic Oscillation Index (NAOI), there was no advance in laying dates over the study. Nesting ended progressively later and the breeding season lasted longer over the study, however, associated with higher spring values of NAOI. As the length of the nesting season increased, offspring sex ratios became more female biased, apparently as a result of females adjusting the sex of the eggs they laid, rather than from sex-biased nestling mortality. Clutch size did not vary systematically over the study or with climate. Opposing trends of declining nest success and increasing productivity of successful nests over the study resulted in no chronological change in productivity per female. Higher productivity of successful nests was associated with higher winter NAOI values, possibly because synchrony between nesting and food availability was higher in years with high NAOI values. Other than the association between the start of nesting and spring temperatures, local weather (e.g., temperature, rainfall) patterns that linked NAOI with reproduction were not identified, suggesting that weather patterns may be complex. Because climate affected most aspects of red-winged blackbird reproduction examined, focusing on associations between climate and single variables (e.g., first-egg dates) will have limited value in predicting how future climates will affect populations.