The sex steroid hormones that affect development in birds have been thought to be produced exclusively by the embryo or neonate. I used radioimmunoassay measure amounts of androstenedione, 5 alpha-dihydrotestosterone, testosterone, 17 beta-estradiol, and corticosterone yolk freshly laid canary (Serinus canaria) zebra finch (Poephila guttata) eggs. Testosterone was found both eggs, but its contents were much higher former than latter. testosterone content eggs a same clutch increased with...

We tested the hypothesis that mother birds counterbalance negative effects of hatching asynchrony for later–hatched chicks by increasing yolk androgen concentrations in consecutive eggs their clutch. In doing so, they may adaptively tune each offspring's competitive ability and, thus, growth and survival. However, evidence support this is contradictory. The maternal androgens black–headed gulls increase significantly with laying order a experimentally functional consequences on chick...

Females may favour some offspring over others by differential deposition of yolk hormones. In American kestrels (Falco sparverius), we found that yolks eggs laid late in the sequence a clutch had more testosterone (T) and androstenedione (A4) than first–laid eggs. To investigate effects these androgens on nestling 'fitness', injected both T A4 into compared their hatching time, growth survival with those which vehicle as control. Compared to controls, injection at dose intended increase...

Past studies have shown that the yolk of canary (Serinus canaria) egg contains maternal testosterone, its concentrations increase in subsequently formed eggs a clutch, and testosterone influences development. The present study investigated 1) if levels vary female during formation; 2) how such putative variations may be related to yolk; 3) environmental factors, as day length, can modify mother her eggs. Maternal levels, measured females' feces, increased formation laying, decreased...

Summary 1. Females of egg‐laying vertebrates may adjust the development their offspring to prevailing environmental conditions by regulating deposition hormones into eggs. Within‐ and amng‐clutch variation in levels steroid were studied egg yolks Black‐Headed Gull ( Larus ridibundus , Linnaeus) relation at nest site. This species breeds colonies different densities habitats, chicks hatch asynchronously. 2. Egg contained very high androstenedione, substantial testosterone moderate...

It has been proposed that the maternal androgens in avian egg yolk enhance offspring fitness by accelerating growth and improving competitive ability. Because quality is strongly influenced condition, we predicted females good condition would produce high-quality eggs with relatively high androgen content. We experimentally enhanced supplementary feeding lesser black-backed gulls (Larus fuscus) during formation compared concentrations of androstenedione (A4), 5alpha-dihydrotestosterone (DHT)...

During long-term fasting at rest, protein utilization is maintained low levels until it increases a threshold adiposity. This study examines 1) whether such shift in energy substrate use also occurs during endurance exercise while fasting, 2) the role of corticosterone, and 3) adrenocortical response to an acute stressor. Ten species migrating birds caught after flight over least 500 km were examined. Plasma uric acid corticosterone with fat stores >5% body mass high smaller stores....

Maternal yolk androgens can promote growth and competitive abilities of nestling birds but are also suggested to increase susceptibility parasites or suppress immune function. We tested the hypothesis that females exposed ectoparasites during egg formation will adjust content in yolk. predicted when anticipating high levels parasitism, deposit (i) less into all eggs their clutch (ii) smaller amounts late laying sequence facilitate brood reduction. In a field experiment we female great tits...

Abstract Despite the well‐known, long‐term, organizational actions of sex steroids on phenotypic differences between sexes, studies maternal in vertebrate egg have mainly focused effects seen early life. Long‐term yolk hormones adult behavior and underlying mechanisms that generate them been largely ignored. Using an experiment which hand‐reared house sparrows ( Passer domesticus ) from testosterone‐ or control‐treated eggs were kept under identical conditions, we show testosterone treatment...

Greater offspring predation favors evolution of faster development among species. We hypothesized that greater exerts selection on mothers to increase levels anabolic androgens in egg yolks achieve development. Here, we tested whether (1) concentrations yolk passerine species were associated with and (2) embryo nestling rates androgen concentrations. examined three potency along the synthesis pathway: androstenedione (A(4)) testosterone (T) 5 alpha -dihydrotestosterone (5 -DHT)....