Indicator models of sexual selection suggest that costly ornaments signal reliable information regarding an individual's quality to potential mates. In species that produce altricial offspring, the amount of parental care provided by both males and females can impact reproductive success. The Good Parent Hypothesis proposes that ornamentation in biparental species can act as an honest signal of parental ability to potential mates. We tested this hypothesis using the mountain bluebird (Sialia currucoides), a sexually dichromatic, socially monogamous species in which both sexes have structurally based ornamental plumage coloration. A male's plumage color predicted neither the rate at which it provisioned nestlings nor brood growth rate. The same was true for females. We also found no indication of assortative mating by color or body condition. Feeding rates within pairs were positively correlated, which we suggest may be due to pairs responding similarly to the perceived needs of nestlings or to local area prey availability. In sum, our results do not support the Good Parent Hypothesis as an explanation for the evolution of ornamental plumage color in mountain bluebirds. We suggest alternative hypotheses for the evolution of ornamental plumage color in this species.

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Female birds are hypothesized to mate outside the pairbond to secure alleles that enhance the fitness potential of their offspring. To test this, researchers typically compare fitness-related attributes of extra-pair (EP) and within-pair (WP) half-siblings. Often neglected, however, is the possibility that females may provide EP offspring with certain non-genetic advantages. For example, in species in which eggs hatch asynchronously, females may place EP offspring amongst earlierlaid eggs in the clutch. Because they tend to hatch first, chicks from earlier-laid eggs are often larger and more developed than their nestmates and thus have a competitive advantage. We tested for an association between offspring paternity and position in the laying/hatching sequence in a Wyoming population of the house wren Troglodytes aedon. Eggs in this population always hatch asynchronously, usually in the order laid, over 24Á48 h, setting up a stair-step-like size hierarchy within broods early in the nestling stage. This suggested that we could thus use a chick's mass relative to that of its nestmates as an index of the chick's position in the laying sequence. We first confirmed this, showing that position in the laying sequence explains 70% of the variance in chick size relative to nestmates. We then compared masses of EP and WP offspring shortly after hatching in 27 broods with mixed paternity. On average, EP offspring weighed 14% more than their WP counterparts, a highly significant difference. Our results therefore suggest that EP offspring are more likely than WP offspring to occur in earlier-laid eggs, thus gaining a size-based competitive advantage. We recommend that, when comparing EP and WP half-siblings in species in which eggs hatch asynchronously, researchers test for this potential maternally derived effect on offspring performance.

Polymerase chain reaction (PCR)-based methods to determine the sex of birds are well established and have seen few modifications since they were first introduced in the 1990s. Although these methods allowed for sex determination in species that were previously difficult to analyse, they were not conducive to high-throughput analysis because of the laboriousness of DNA extraction and gel electrophoresis. We developed a high-throughput real-time PCR-based method for analysis of sex in birds, which uses noninvasive sample collection and avoids DNA extraction and gel electrophoresis.

Few studies have examined how avian life-history traits vary within populations as elevation increases and climate becomes more severe. We compared egg and clutch sizes of Mountain Bluebirds (Sialia currucoides) nesting at two elevations (1500 m and 2500 m above sea level) in the Bighorn Mountains of Wyoming over two years. Eggs laid by females at the high-elevation site were, on average, significantly (6%) smaller in volume than eggs laid by their lower-elevation counterparts. Across elevations, egg size showed a significant positive correlation with female body condition (weight relative to size), and high-elevation females had significantly lower indices of condition than low-elevation females. Temperatures during clutch formation were colder at the high-elevation site, and egg size was negatively related to temperature after controlling for the effects of female condition. Clutches of females at high elevations were, on average, marginally smaller (by 5%, 0.3 eggs) than clutches of low-elevation females. Unlike egg size, clutch size was unrelated to either female condition or temperature during clutch formation. This suggests that, when under energetic or nutritional stress at high elevations, females sacrifice egg size before sacrificing clutch size.