Date Published: May 22, 2019
Publisher: Public Library of Science
Author(s): Richard T. Reynolds, Jeffrey S. Lambert, Shannon L. Kay, Jamie S. Sanderlin, Benjamin J. Bird, Antoni Margalida.
One measure of habitat quality is a species’ demographic performance in a habitat and the gold standard metric of performance is reproduction. Such a measure, however, may be misleading if individual quality is a fitness determinant. We report on factors affecting lifetime reproduction (LR), the total number of lifetime fledglings produced by an individual, and long-term territory-specific reproduction in a multi-generational study of northern goshawks (Accipiter gentilis). LR increased with longer lifespans and more breeding attempts and was strongly correlated with the number of recruits in two filial generations indicating that LR was a good fitness predictor. Extensive differences in LR attested to heterogeneity in individual quality, a requisite for the ideal pre-emptive distribution model (IPD) of habitat settling wherein high quality individuals get the best habitats forcing lower quality individuals into poorer habitats with lower reproduction. In response to 7‒9-year prey abundance cycles, annual frequency of territory occupancy by breeders was highly variable and low overall with monotonic increases in vacancies through low prey years. Occupancy of territories by breeders differed from random; some appeared preferred while others were avoided, producing a right-skewed distribution of total territory-specific fledgling production. However, mean fledglings per nest attempt was only slightly lower in less versus more productive territories, and, contrary to IPD predictions of increases in annual territory-specific coefficients of variation (CV) in reproduction as breeder densities increase, the CV of production decreased as density increased. Rather than habitat quality per se, conspecific attraction elicited territory selection by prospecting goshawks as 70% of settlers comprised turnovers on territories, resulting in occupancy continuity and increased territory-specific reproduction. Top-producing territories had as few as 2 long-lived (high LR) and up to 6 short-lived (low LR) sequential breeders. While individual quality appeared to effect territory-specific heterogeneity in reproductive performance, our data suggests that differences in individual quality may be washed-out by a random settling of prospectors in response to conspecific attraction.
Lifetime reproduction (LR) reveals the full extent of variation in fitness potential among individuals, and differences among individuals are the sources of variation on which natural selection works [1, 2]. Common to many studies of LR in birds are extensive among-individual variation in LR and strong correlations between LR and lifespan (longevity) and number of breeding attempts [3, 4]. Other influencing factors include phenotype, habitat composition and structure, food abundance, weather, interspecific competition, weather during different stages of individual life histories, mate quality, predation, population dynamics, and individual covariates such as body size and condition [3, 5–12]. Estimates of variance in LR require data from complete life cycles of individuals  but the propensity of juveniles in many species to disperse from a study area make LR data difficult to obtain. Despite this, several studies in which locally-born individuals recruited as breeders into a study population showed that lifetime fledgling production and subsequent numbers of recruits were correlated, indicating that lifetime production of fledglings is a good predictor of fitness [13–15]. Studies of rate-sensitive fitness metrics report that early breeding in life should be favored by selection because starting to breed early can improve an individual’s LR by increasing the number of breeding attempts [1, 16, 17] and changes in reproductive rates are most pronounced in the early years [3, 18, 19]. On the other hand, delayed reproduction may be favored if costs of early reproduction (i.e., reduced survival, lowered future reproduction, and somatic maintenance) outweigh the benefits [20–22]. Alternatively, focusing on the entire lifespans is fundamental for understanding a species’ life history, population ecology, and trade-offs among life history traits. For example, longer lifespans allow more reproductive attempts and increased chances of reproducing during periods of favorable environmental conditions [23–25].