Severe bottlenecks can reduce genetic diversity and increase inbreeding as individuals are forced to mate with close relatives, but it is unknown at what minimum population size the negative fitness consequences of bottlenecks are expressed. The New Zealand avifauna contains a large number of species that have gone through bottlenecks of varying severity, providing an exceptional opportunity to test this question by using the comparative method. Using decreased hatchability as a measure of fitness costs, we found that hatching failure was significantly greater among both native and introduced species that had passed through bottlenecks of <150 individuals. Comparisons between pre-and postbottleneck populations of introduced species suggest that hatching problems arise even in populations founded by <600 individuals. Our study confirms that hatching failure is widespread and persistent among birds passing through severe bottlenecks and that the population sizes at which this fitness cost is expressed are several times greater than the number of individuals currently used to found most new populations of endangered species. We recommend that conservation managers revise the protocols they use for reintroductions or they may unwittingly reduce the longterm viability of the species they are trying to save. H abitat destruction and exploitation are causing catastrophic population declines in many species around the world. Even if endangered populations recover, severe bottlenecks may reduce genetic diversity and increase inbreeding as survivors are forced to mate with close relatives, resulting in lowered heterozygosity, increased genetic load, and increased expression of deleterious alleles (1). Inbreeding may yield significant costs to fitness and decrease population survival (2), a process termed inbreeding depression, but its importance has been questioned (3-5), and examples of the negative fitness consequences due to inbreeding in small populations of wild animals are few (6, 7). Despite the potential importance of bottleneck size to conservation biology, the number of individuals required to avoid the fitness costs of small population size and maintain the viability of a population has been difficult to test in free-living animals (1).Theoretical models suggest that minimum effective population sizes range from 50 to 5,000 individuals, depending on levels of acceptable loss of genetic variability and the timeframe of population persistence (8, 9). The exact number is not a trivial question because the survival of many endangered species depends on the reliability of such guidelines. It has even been suggested that severe bottlenecks may be advantageous because they reduce inbreeding depression by purging deleterious alleles (10) although whether such benefits are great enough to justify deliberate inbreeding have been questioned (11,12). The problem for conservation biologists is to understand whether bottlenecks create fitness costs and at what population size these costs become so severe that they threaten t...

Sperm competition should select for increased sperm production if the probability of fertilization by a specific male is proportional to the relative number of sperm inseminated. A review of the literature generally supports the predicted positive association between sperm production or allocation and various measures of the presumed intensity of sperm competition. However, it is not clear how increased sperm competition is related to extra-pair paternity, and it remains unknown whether certainty of paternity should be associated with relative testis size. Based on a large sample of bird species with information on extra-pair paternity gathered from the literature, we demonstrate that testis mass is related positively to the level of extra-pair paternity, after controlling for body size and phylogeny. Although large testes may be necessary to avoid sperm depletion in species in which males frequently engage in multi-pair copulations, we argue that selection has favoured increased testis mass in situations of more intense sperm competition in order to retaliate against copulations by rival males. The fact the males are not always successful in retaliating against rival ejaculates further suggests that females may largely control the allocation of paternity in birds and that increased sperm production by males may simply be a male strategy to make the best of a bad situation.

Sperm size varies enormously among species, but the reasons for this variation remain obscure. Since it has been suggested that swimming velocity increases with sperm length, earlier studies proposed longer (and therefore faster) sperm are advantageous under conditions of intense sperm competition. Nonetheless, previous work has been equivocal, perhaps because the intensity of sperm competition was measured indirectly. DNA profiling now provides a more direct measure of the number of offspring sired by extrapair males, and thus a more direct method of assessing the potential for sperm competition. Using a sample of 21 species of passerine birds for which DNA profiling data were available, we found a positive relation between sperm length and the degree of extrapair paternity. A path analysis, however, revealed that this relationship arises only indirectly through the positive relationship between the rate of extrapair paternity and length of sperm storage tubules (SSTs) in the female. As sperm length is correlated positively with SST length, an increase in the intensity of sperm competition leads to an increase in sperm length only through its effect on SST length. Why females vary SST length with the intensity of sperm competition is not clear, but one possibility is that it increases female control over how sperm are used in fertilization. Males, in turn, may respond on an evolutionary time scale to changes in SST size by increasing sperm length to prevent displacement from rival sperm. Previous theoretical analyses predicting that sperm size should decrease as sperm competition becomes more intense were not supported by our findings. We suggest that future models of sperm-size evolution consider not only the role of sperm competition, but also how female control and manipulation of ejaculates after insemination selects for different sperm morphologies.

Begging by nestling birds can be conspicuous and loud. Such displays are thought to function in signalling nestling condition and securing parental care, but they also may inadvertently attract the attention of predators. We compared the structure of nestling begging calls to the risk of predation among 24 species of birds breeding in a forest community in central Arizona. After controlling for body size and phylogeny, we found that species subject to greater nest predation had calls with higher frequency (pitch) and lower amplitude (loudness) than species subject to lower rates of nest predation. As these acoustic features make it di¤cult for potential predators to pinpoint the source of a sound, our results suggest that an increased risk of predation has led to the evolution of begging calls that minimize locatability. The relationship between call structure and the risk of predation also supports the hypothesis that attracting predators is a direct cost of begging and that such costs can constrain any evolutionary escalation in the intensity of nestling begging.

There is growing evidence that inbreeding can negatively affect small, isolated populations. This contrasts with the perception in New Zealand, where it has been claimed that native birds are less affected by inbreeding depression than threatened species from continental regions. It has been argued that New Zealand's terrestrial birds have had a long history of small population size with frequent inbreeding and that this has 'purged" deleterious alleles. The rapid recovery of many tiny and inbred populations after introduced predators have been controlled, and without input from more genetically diverse populations, has further supported the view that inbreeding is not a problem. This has led to a general neglect of inbreeding as a factor in recovery programs for highly endangered species such as the Black Robin (Petroica traversi) and Kakapo (Strigops habroptilis). We examined the reasons for this situation and review the New Zealand evidence for genetic purging. Complete purging of the genetic load and elimination of inbreeding depression are unlikely to occur in natural populations, although partial purging may be more likely where small populations have become inbred over an extended period of time, such as on small isolated islands. Recent molecular data are consistent with the view that island endemics, including New Zealand's threatened birds, have low genetic variation and hence have possibly gone through longer periods of inbreeding than threatened species from continental regions. Nevertheless, results from recent field studies in New Zealand indicate that, despite the opportunity for purging, inbreeding depression is evident in many threatened species. Although inbreeding depression has not prevented some populations from recovering from severe bottlenecks, the long-term consequences of inbreeding and small population size--the loss of genetic variation--are potentially much more insidious. The degrees to which genetic factors reduce population viability generally remain unquantified in New Zealand. Although minimizing ecological risks (e.g., preventing reinvasion of islands by mammalian predators) will continue to receive high priority in New Zealand because of their much larger impacts, we advocate that genetic considerations be better integrated into recovery plans.