Demographic changes in populations, such as skewed sex ratios, are of concern to conservationists, especially in small populations in which stochastic and other events can produce declines leading to extirpation. We documented a decline in one of the few remaining populations of Southwestern Willow Flycatcher (Empidonax traillii extimus) in southern California, USA, which dropped from 40 to 5 adults between 2000 and 2015. Declines were unequal between sexes (94% for males, 82% for females). Adult sex ratios were female-biased in 10 of 16 yr. The proportion of paired males that were polygynous ranged from 0% to 100%, depending on the ratio of females to males in the adult population. Some males paired with up to 5 females simultaneously. We investigated the role of nestling sex ratio in the female-biased adult sex ratio by using genetic techniques to determine sex from blood samples collected from 162 nestlings in 72 nests from 2002 to 2009. Both population-level and within-brood nestling sex ratios were female-biased, and were not influenced by nest order (first or subsequent), parental mating type (monogamous or polygynous), or year. Disproportionately more females than males were recruited into the breeding population, mirroring nestling and fledgling sex ratios. It thus appears that a skewed nestling sex ratio has contributed to a female-biased adult population, which in turn has influenced mating behavior. We propose that the capacity for polygyny, which generally occurs at low levels in Southwestern Willow Flycatchers, has allowed this population to persist through a decline that might otherwise have resulted in extinction.
Demographic changes in small populations are of concern to conservationists, particularly in endangered species in which stochastic and other events can produce declines to extinction. Random fluctuations in reproduction, mortality, dispersal, and sex ratios exacerbate existing stressors and put populations on downward trajectories that can be difficult to reverse. Skewed adult sex ratios affect the ability of individuals to find mates, and, depending on the mating system, reduce individual fitness and population growth rate (Bessa-Gomes et al. 2004). Male-biased sex ratios may have less of an effect on population dynamics than female-biased ratios because females are the limiting sex for population growth. When females outnumber males, given populations with the same size and adult sex ratio, monogamous breeders are predicted to decline faster than polygynous breeders because female mating potential is more strongly limited by male availability under monogamy than under polygyny (Legendre et al. 1999, Engen et al. 2003, Sæther et al. 2004).
Detecting shifts in demography and mating behavior in natural populations requires long-term study, and empirical evidence for evaluating and informing theoretical treatments of the effects of demographic stochasticity on population persistence is limited. The increasing attention that is being given to the population dynamics of endangered species has created both the need for and the opportunity to examine demographic theory within the context of species recovery. Longitudinal investigations of species of conservation concern are yielding datasets with which to test the predictions of models in natural settings, and thereby to simultaneously advance species conservation and our understanding of the biology of small populations (Nisbet and Hatch 1999, Gerlach and Le Maitre 2001, Paxton et al. 2002, Budden and Beissinger 2004, Ferrer et al. 2008, Veran and Beissinger 2009).
The Southwestern Willow Flycatcher (Empidonax traillii extimus), an endangered migratory passerine restricted to riparian habitat in the southwestern U.S., is considered monogamous, but has the capacity for facultative polygyny (Sedgwick 2000, Davidson and Allison 2003), producing variability in mating behavior within and among years and individuals. We document a skewed adult sex ratio over a 16-yr period in a declining population of the Southwestern Willow Flycatcher. To identify factors contributing to the skewed sex ratio, we examined the sex ratio of nestlings, fledglings, and adults that were later recruited into the breeding population. We also related the sex ratio of adults each year to the degree of polygynous mating in the population.
Field Data Collection
We studied Southwestern Willow Flycatchers (hereafter, flycatchers) annually from 2000 to 2015 at Marine Corps Base Camp Pendleton (MCBCP; 33.36°N, 117.42°W), California, USA, which at the beginning of the study supported the second-largest population of flycatchers in the state. Our investigation was part of a long-term intensive demographic study of flycatchers that included annual censuses, color banding, and monitoring of nesting activities. MCBCP supports 3,300 ha of willow- (Salix spp.) dominated riparian habitat, all of which was surveyed annually using a standardized protocol (Sogge et al. 2010) to locate all resident (breeding) flycatchers in the site. Four or 5 surveys were conducted between May 15 and July 31 each year to locate flycatchers and determine their status. Intensive surveys for Least Bell's Vireos (Vireo bellii pusillus) conducted in a parallel study in the same habitat provided additional opportunities to detect flycatchers. We assigned status as transient (migrant in passage), floater (nonterritorial, nonbreeding individual, typically present only briefly in the study area), or resident breeder. Transients were typically seen only once during surveys prior to June 25, the date after which detected flycatchers were likely to be resident. In contrast, floaters were typically seen once, but occasionally multiple times, late in the breeding season. Floaters often occurred in the vicinity of occupied breeding territories, but did not exhibit territorial behavior such as singing or defense. In our study, floaters often returned and entered the breeding population the following year. Birds were deemed resident based on observations of territorial defense, interactions with a mate, or nesting activities. Sites historically occupied by resident flycatchers received weekly visits early in the season to detect the arrival of breeding birds. Flycatchers in our site exhibited high site fidelity and consistently used a subset of the riparian habitat in MCBCP for breeding. Once located, all resident flycatchers were visited at least weekly through late August to determine their breeding status (paired or unpaired) and to monitor their nesting activity following a standardized protocol (Rourke et al. 1999). Birds that defended territories throughout the season but exhibited no evidence of breeding or presence of a mate were considered unpaired. We attempted to locate and monitor all nests of all pairs, and determined the mating type (monogamous or polygynous) of parents for each nest. Because nearly all of the adults in our population were uniquely col