Oviposition drives hatching order and developmental disparities with brood mates

Birth or hatching order can affect fitness. It has long been assumed that the order in which a bird's egg is laid (its oviposition) and first exposed to incubation relative to other eggs in the nest determines the order in which it hatches and the subsequent effects on development and survival. To my knowledge, this cause of hatching order has not been tested while controlling for laying-order effects on egg composition. Like those of many species, the last-laid eggs in clutches of the Lincoln's sparrow ( Melospiza lincolnii ) hatch last and produce nestlings with reduced growth and survival rates. Last-hatched Lincoln's sparrows also develop wider bills that are associated with reduced attractiveness of adult courtship songs. Using wild, free-ranging Lincoln's sparrows, I replaced the first or fourth freshly laid egg in four-egg host nests with a freshly laid first egg from a donor nest. I discovered that the experimental elevation of oviposition—which controlled for egg composition—delayed hatching, inhibited growth and widened bill gape, thus confirming the mechanism for hatching order and revealing a potential role for oviposition in reproductive success. Similar effects of postnatal family position could affect fitness in other taxa, as well.

Sex Differences in Forebrain Monoaminergic Response to Song Performance

In many species, successful reproduction is dependent on the ability to adjust social behavior in response to an ever-changing social environment. Because a sexual signal's value and meaning can differ between females and males, responses to those signals should also differ. One way individuals can modulate social behavior is through experience-dependent modulation of the sensory systems that process social signals. Central monoamines (norepinephrine, dopamine, serotonin) modulate neural sensitivity to social stimuli and are key regulators of experience-dependent neuroplasticity in vertebrate sensory systems. However, few studies have examined how exposure to different sexual signals influences monoaminergic activity in female compared to male sensory systems. We used Lincoln's sparrows (Melospiza lincolnii) to examine sex differences in how variation in the trill performance of song influences central monoaminergic activity in the auditory telencephalon. Trill performance measures the rate at which a song syllable is produced relative to the syllable's frequency bandwidth and is thought to reflect the difficulty with which songs are produced. High-performance trills are more threatening to males but more attractive to females. We found that the effects of trill performance on monoaminergic activity were sex-dependent. Relative to the response to low-performance songs, exposure to high-performance songs decreased noradrenergic activity in the caudomedial nidopallium, and tended to decrease serotoninergic activity in the caudomedial mesopallium and caudomedial nidopallium of the auditory telencephalon in females, but in males, the monoamine measurements were indistinguishable between song treatments. These results suggest that the mechanisms underlying sensory processing of male sexual signals differ between the sexes.

Contrast influences female attraction to performance-based sexual signals in a songbird

Animals do not make decisions in a bubble but often refer to previous experience when discriminating between options. Contrast effects occur when the value of a stimulus affects the response to another value of the stimulus, and the changes in value and response are in the same direction. Although contrast effects appear irrational, they could benefit decision makers when there is spatial or temporal variation and autocorrelation in the value of stimuli that elicit decisions. Here, we examined whether contrasts influence female evaluation of male performance-based sexual signals. We exposed female Lincoln's sparrows ( Melospiza lincolnii ) to one week of songs that we had experimentally reduced or elevated in performance, followed by a novel song of intermediate performance. We found that high-performance songs were more attractive to females than low-performance songs. Moreover, the intermediate songs were more attractive following exposure to low- than to high-performance songs. These results indicate that contrast can influence evaluation of performance-based sexual stimuli. By examining contrast effects in the ecologically relevant context of mate choice for performance, we can better understand both the adaptive value of comparative evaluation as well as the mechanisms that underlie variation in mate choice and sexual selection.

Song in the cold is ‘hot’: memory of and preference for sexual signals perceived under thermal challenge

The environmental conditions under which signals are perceived can affect receiver responses. Many songbird populations produce a song chorus at dawn, when, in cold habitats, they would experience thermal challenge. We recorded temperature and the song activity of Lincoln's sparrows ( Melospiza lincolnii ) on a high-elevation meadow, and determined that song behaviour is concentrated around the coldest time of the day, at dawn. We hypothesized that this is because male song in the cold is more attractive to females than song in the warm. To test this, we exposed laboratory-housed Lincoln's sparrow females to songs at 1°C and 16°C, which they naturally experience in the wild. Females spent 40 per cent more time close to the speaker during playback at 1°C than at 16°C. When tested at 16°C 1–2 days later, females biased their movement towards the speaker playing songs previously heard at 1°C over 16°C. Thus, female Lincoln's sparrows remembered and affiliated with songs they heard under thermal challenge, indicating that the thermal environment can affect the attractiveness of a sexual signal.