Unpredictable food supply modifies costs of reproduction and hampers individual optimization

Abstract

Investment into the current reproductive attempt is thought to be at the expense of survival and/or future reproduction. Individuals are therefore expected to adjust their decisions to their physiological state and predictable aspects of environmental quality. The main predictions of the individual optimization hypothesis for bird clutch sizes are: (1) an increase in the number of recruits with an increasing number of eggs in natural broods, with no corresponding impairment of parental survival or future reproduction, and (2) a decrease in the fitness of parents in response to both negative and positive brood size manipulation, as a result of a low number of recruits, poor future reproduction of parents, or both. We analysed environmental influences on costs and optimization of reproduction on 6 years of natural and experimentally manipulated broods in a Central European population of the collared flycatcher. Based on dramatic differences in caterpillar availability, we classified breeding seasons as average and rich food years. The categorization was substantiated by the majority of present and future fitness components of adults and offspring. Neither observational nor experimental data supported the individual optimization hypothesis, in contrast to a Scandinavian population of the species. The quality of fledglings deteriorated, and the number of recruits did not increase with natural clutch size. Manipulation revealed significant costs of reproduction to female parents in terms of future reproductive potential. However, the influence of manipulation on recruitment was linear, with no significant polynomial effect. The number of recruits increased with manipulation in rich food years and tended to decrease in average years, so control broods did not recruit more young than manipulated broods in any of the year types. This indicates that females did not optimize their clutch size, and that they generally laid fewer eggs than optimal in rich food years. Mean yearly clutch size did not follow food availability, which suggests that females cannot predict food supply of the brood-rearing period at the beginning of the season. This lack of information on future food conditions seems to prevent them from accurately estimating their optimal clutch size for each season. Our results suggest that individual optimization may not be a general pattern even within a species, and alternative mechanisms are needed to explain clutch size variation.

Seasonal patterns in reproductive success of temperate‐breeding birds: Experimental tests of the date and quality hypotheses

“…These results indicate that the cost of raising enlarged broods depends on environmental conditions during the nestling period. Likewise, in a 6‐year brood size manipulation study of collared flycatchers ( Ficedula albicollis ), nestling condition was lower in experimentally enlarged broods and this effect was more pronounced in years with less food (Török, Hegyi, Tóth, & Könczey, 2004). Collectively, these findings underscore the importance of directly evaluating relationships between indices of environmental quality and fitness‐related traits, and annual fluctuations in these indices, when assessing the implications of late breeding.…”

“…Furthermore, we detected a cost of enlarged brood sizes in the form of nestling mortality, independent of food abundance. Although enlarged broods may produce more fledglings (Shutler et al., 2006), this relationship may be affected by food supplies, with a greater number of fledglings produced from enlarged broods in food‐rich years and a threshold in the number of fledglings produced occurring in average food years (Török et al., 2004). Nestling survival was lower at PG than SDNRA possibly due to overall lower food supplies at PG (Figure 3); however, our work and a previous study (Harriman, 2014) suggest that parents at PG are in lower body condition than parents at SDNRA, which may also account for these results.…”

“…Recruitment of offspring is generally higher when nestlings are raised during periods of greater food supply (Török et al., 2004). A fitness‐related cost of the delay treatment was reflected in reduced local recruitment of nestlings in delayed nests of early‐initiating parents when compared to their non‐delayed counterparts.…”

Lifetime offspring production in relation to breeding lifespan, attractiveness, and mating status in male collared flycatchers

“…This species is ideal for long-term studies of reproductive success. It shows a preference for nestboxes, can easily be captured, and has high breeding-site fidelity (Pärt and Gustafsson 1989;Könczey et al 1992;Hegyi et al 2002) and considerable local recruitment rates (Pärt 1990;Török et al 2004). Nestboxes were checked multiple times a week throughout the nesting period, so breeding attempts were followed from nest building to fledging.…”

Reduced compensatory growth capacity in mistimed broods of a migratory passerine

“…Due to the unavoidable, accidental loss of a proportion of individual samples, 214 peak time had to be determined by pooling data from all trees, but this does not cause bias as 215 medians of tree-level peak dates are nearly always the same as the pooled-data peak for the 216 given plot (our unpublished data; only one plot was used in this experiment). The height of 217 the peak was highly variable (see Török et al 2004) but it showed little correlation with peak 218 timing (log transformed peak height, N = 23 years, r = -0.075, p = 0.735), and there was little 219 difference in peak height between the three experimental years (data not shown), so we did 220 not consider peak height in our analyses. Frass collection was done at several distinct 221 locations over our greater study area.…”