Nest Composition and Yolk Hormones: Do Female European Starlings Adjust Yolk Androgens to Nest Quality?

Abstract

The nest is a key element of avian reproductive fitness. It provides the developmental environment for the embryo and nestling thereby affecting their quality and survival. Nests are often constructed by the male and nest quality, elaboration, and ornamentation vary among males in a species. Therefore, male nest-building behaviour is likely under sexual selection and females may use male nest-building behaviour and nest features as cues in mate choice and their own reproductive investment. We tested this hypothesis in European starlings (Sturnus vulgaris). Male starlings present green plants to females during their courtship display and furbish their nests with variable quantities of fresh green plants. The highest frequency of plant incorporation into the nest occurs during pair formation and oogenesis. We determined nest composition and measured yolk testosterone concentrations in eggs. Yolk testosterone concentrations were positively correlated with the amount of green plant material in nests. Because males use green nest material in courtship and green nest material positively affects nestlings' development, we suggest that female European starlings use male nest-building effort, ritualised courtship display of green plants, and nest characteristics in mate choice, and adjust yolk testosterone levels in the eggs to the quality of the nest to optimise offspring growth.

Distribution and concentration of maternal progesterone in the yolk of Greater Rhea eggs (Rhea americana)

“…Freshly laid eggs, especially their yolks, contain substantial levels of maternal hormones, which influence the offspring development (see Groothuis & Schwabl, 2008, for a review). Several factors modulate the yolk hormone levels, such as the genetic background, the female environment and quality, social interactions (Schwabl, 1997;Whittingham & Schwabl, 2002;Pilz & Smith, 2004), and the male quality (Gil, Graves, Hazon, & Wells, 1999;Gwinner, Yohannes, & Schwabl, 2013). Furthermore, yolk hormone levels could also vary among clutches (Gil, 2008), and even across the laying sequence within a clutch (Eising, Eikenaar, Schwabl, & Groothuis, 2001;Schwabl, 1993Schwabl, , 1997.…”

‘Green incubation': avian offspring benefit from aromatic nest herbs through improved parental incubation behaviour

“…Herbs could also affect incubation via sexual and social signalling [29,33]. In starlings the males' provisioning with herbs during courtship may signal their quality [35,46].…”

“…As nests throughout the study population reached incubation onset, we substituted experimental nests for natural nests in an alternating fashion. Nest exchange after the start of incubation, our established procedure over 15 study years, causes no detectable brood desertion [24,29]. We replaced 36 natural nests by artificial nests, retaining the dry nest bowl woven by the female to provide a familiar sight and sensation for the incubating bird at its return.…”

“…For example, female great tits (Parus major) in experimentally flea-reduced nests slept longer than those in flea-infested nests [24]. Finally, because herbs are also used as social and sexual signals [33] (see electronic supplementary material, movie), they may affect offspring through modified parental physiology or behaviour [29,33].…”

“…Here, we hypothesize that inclusion of green herbs benefits offspring already before they hatch through efficient parental incubation. Parents could pass on thermal or parasiterepelling benefits obtained from herbs to their offspring by improving incubation performance; they could modify their behaviour in response to signals conveyed by herbs or respond directly to pharmacological effects [28,29,33]. To the best of our knowledge, no study has investigated whether herb inclusion modifies incubation behaviour, thereby explaining fitness consequences for offspring and incubating birds.…”