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Birds that breed opportunistically maintain partial activation of reproductive systems to rapidly exploit environmental conditions when they become suitable for breeding. Maintaining reproductive systems outside of a breeding context is costly. For males, these costs are thought to include continual exposure to testosterone. Males of seasonally breeding birds minimise these costs by downregulating testosterone production outside of a breeding context. Opportunistically breeding birds trade off the need to rapidly initiate reproduction with the costs of elevated testosterone production. One way opportunistically breeding males could minimise these costs is through fine scale changes in testosterone production across discrete reproductive stages which have a greater or lesser requirement for active sperm production. Although spermatogenesis broadly depends on testosterone production, whether changes in testosterone levels across the reproductive stages affect sperm quality and production is unknown. Here, we measured testosterone, sperm quality, and body condition in male zebra finches at discrete stages within reproductive bouts (egg laying, incubation, nestling provisioning, and fledging) and across two consecutive reproductive events in captive male zebra finches (Taeniopygia castanotis). We also examined associations between male testosterone, sperm quality/production, body condition, and nestling body condition. We found that testosterone levels varied across discrete reproductive stages with the lowest levels during incubation and the highest following chick fledging. Testosterone levels were positively associated with sperm velocity and the proportion of motile sperm but were not associated with male body condition. We found no associations between paternal body condition, testosterone levels, or sperm traits with nestling body condition (a proxy for the reproductive quality of a male and his partner). This study is the first to show that opportunistically b

Rates of embryonic and post-natal growth vary extensively among species and geographic space. This variation is well represented in songbird offspring from different latitudes and can strongly influence organismal quality and fitness. However, environmental and evolutionary causes and consequences of variation in embryonic and post-natal growth remain unclear. Here we experimentally show in the field that, within the constraints imposed by physiological trade-offs, warmer incubation temperatures shortened embryonic period length among nine species of songbirds from two latitudes. Yet, the magnitude of the response varied and species-specific reaction norms of embryonic reduction in response to our treatment positively correlated with the natural temperature experienced during incubation. Furthermore, we found little evidence for potential metabolic costs imposed on offspring by faster development, but we detected benefits for size at hatching instead. These results question the generality of theories considering avian development to be strictly dictated by intrinsic trade-offs and suggest that shorter embryonic periods caused by warmer temperature may not be as detrimental as traditionally thought. Costs of shorter development due to warmer embryonic temperature may appear later in life as stunted post-natal growth via influences on offspring metabolism and parental feeding and brooding effort. Our treatment increased metabolic rate without producing appreciable changes in parental care yielding slower post-natal growth rates in two species, faster growth in one and no effects for the majority of the species studied. These results suggest that shorter embryonic periods are not generally associated to costs paid during the post-natal stages but also question the role of metabolism for growth. We tested for the association between metabolism and growth using a comparative approach. We discovered that metabolic rate and body mass of nestlings predicted variation in pos