José P. Veiga

NUTRITIONAL CONSTRAINTS DETERMINE THE EXPRESSION OF A SEXUAL TRAIT IN THE HOUSE SPARROW, PASSER DOMESTICUS

Many birds have distinctive patches of feathers that in some cases have been shown to function as reliable signals of phenotypic quality. It has generally been assumed that these called ‘badges’ are inexpensive to produce, so other kind of costs have been invoked to explain how they are maintained as honest signals. In the present study we addressed the hypothesis that the black throat badge of male house sparrows functions as an honest signal of condition because it is nutritionally costly to produce, a possibility not yet evaluated for badges showing status. While under field conditions adult birds exhibited larger badges than juveniles, individuals of both age classes developed badges of similar size when moulting in an aviary with ad libitum food. Fat reserves, blood proteins and body mass records of free ranging birds at the start of moult suggest that juveniles were in poorer condition than adults. The reduction of blood proteins during moult negatively correlated with badge size after moult in juveniles but not in adults, and only the variation of blood proteins, but not fat reserves, correlated with the size of the badge when using multiple regression analysis. The results suggest that: (i) a limited access to food sources during moult may account for the smaller badge size of juveniles as previously reported in the same study population; and (ii) the use of more blood proteins, that seems to depend on a worse nutritional condition, constrains the production of a larger badge. The results give support for the hypothesis that badges are costly to produce, though the magnitude of the costs may be age dependent.

Seasonal variation in sex ratio and sexual egg dimorphism favouring daughters in first clutches of the spotless starling

We investigated possible pre‐hatching mechanisms of sex‐differential investment by females that may contribute to offspring sex‐ratio adjustment enhancing the fitness return from reproductive effort in the spotless starling (Sturnus unicolor). We found a seasonal shift in sex ratio from daughters to sons as the season advances. Furthermore, the probability of breeding at 1‐year old and recruitment into the breeding population in daughters is associated with laying date but not with mass at fledging. The reverse is true for males which rarely bred at 1‐year old. We also found that eggs containing female embryos are significantly heavier than those containing males in spite of the slight sexual dimorphism in favour of males. This suggests maternal control of provisioning, favouring daughters that may balance sibling mortality and competition with their brothers. Our results on seasonal variation in sex ratio and differential egg provisioning are consistent with an adaptive tactic in which mothers increase their reproductive return by enhancing the probability that daughters survive and breed in their first year of life.

Hatching asynchrony in the house sparrow : a test of the egg-viability hypothesis

At least eight hypotheses for the evolution of hatching asynchrony have been advanced in recent years, and no single hypothesis appears to be an adequate general explanation of the function of this widespread trait (see Amundsen and Stokland 1988; Slagsvold and Lifjeld 1989). Preeminent hypotheses are the brood-reduction hypothesis (Lack 1968) and the nest-failure hypothesis (Hussell 1972; Clark and Wilson 1981). Lack proposed that hatching asynchrony facilitates the death of the last-hatched sibling if the food is scarce, thus enabling some of the young to successfully fledge. The nest-failure hypothesis states that, by starting incubation early and hatching asynchronously, the amount of time the eggs and some young are vulnerable to predation is reduced. However, the debate about hatching asynchrony has not clarified whether it represents an evolved adaptation or is the consequence of factors that constrain synchrony (see, e.g., Mead and Morton 1985; Slagsvold 1986a). Recently Arnold et al. (1987) showed that the viability of unincubated eggs declines with time. In precocial waterfowl with stringent synchronous hatching requirements, the limitation of embryo resistance to remain unincubated imposed constraints on the maximum clutch size. This egg-viability hypothesis can be modified to address the significance of hatching asynchrony in altricial birds. I suggest that hatching is asynchronous because parents must start incubation before the clutch is complete to avoid the death of the embryos that can occur when eggs remain unattended during the egg-laying period. Hence, I present the first test of the egg-viability hypothesis in altricial birds. Using data from house sparrows (Passer domesticus), I first determine whether egg hatchability declines with exposure time when eggs are experimentally maintained without parental care. Second, I test two predictions of the egg-viability hypothesis: (1) incubation is started before egg mortality rates become high, causing hatching asynchrony to increase with clutch size, and (2) because the hatchability of the eggs maintained unincubated declines over the breeding season, greater hatching asynchrony should occur late in the season. These predictions are not critical tests of the egg-viability hypothesis, and I discuss some alternative explanations.

Effects of paternal care on reproductive success in the polygynous spotless starling Sturnus unicolor

Abstract For males of socially polygynous avian species like the spotless starling, there may exist a trade-off between investing in paternal care and controlling several nests. To determine how the intensity of paternal care affects reproductive success per brood sired or expressed as the total number of young raised in all nests controlled by the same male, it is necessary to manipulate paternal care. Testosterone (T) has been shown to depress the tendency for males to care for their young, and induces them to acquire more mates. The effects of paternal care on reproductive success were studied by treating certain male starlings with exogenous T and others with the antiandrogen cyproterone acetate (CA), and comparing the parental behavior of T- and CA-males throughout the breeding season with that of controls. CA-males fed their chicks more during the first week after hatching than T-males, with controls feeding at intermediate rates, both on a per nest basis and as total effort for all nests controlled by the same male. Paternal feeding rates during the first week of chick life had a significant positive effect on the number of fledged young. The hormone treatment significantly affected the number of chicks raised per nest, CA-males having a higher breeding success per nest than T-males, and controls showing intermediate levels of success. There was no significant effect of treatment on total reproductive success attained by males throughout the season. In the polygonous spotless starling, the intensity of paternal care of young affects reproductive success per nest positively but not on a seasonal basis.

Reproductive effort affects immune response and parasite infection in a lizard : a phenotypic manipulation using testosterone

Life-history theory predicts that there is a trade-off between reproductive effort and several traits that determine fitness. Infectious disease has gained acceptance as a crucial factor linking both variables. In most instances phenotypic manipulation is necessary to convincingly demonstrate a causal relationship of reproductive effort on parasitism. However, experimental studies that manipulate reproductive effort or parasite load have been rarely conducted in reptiles. In this study. we manipulated reproductive effort of male lizards (Psammodromus algirus) through testosterone implants, and measured the associated response in some haematological variables and parasite load. Testosterone-supplemented males had lower scores than control males in factor I of a PCA for different blood parameters. This factor is correlated with the number of white blood cells, especially lymphocytes, and with plasma glucose levels. Experimental males also had higher scores in factor 3 that is mainly related to protein catabolism. Scores of males in component I tended to be correlated negatively with tick load, while scores in component 3 were correlated positively with the number of haemogregarines in the blood. These results suggest that higher investment in reproduction decreases the immune defences. and conduces to the use of structural resources, which may render individuals more susceptible to some haemoparasites. This is consistent with the idea that an increase in reproductive effort mediated by testosterone has a negative effect on the ability to counteract parasite infections.

A test of the hypothesis of mate choice based on heterozygosity in the spotless starling

The hypothesis of mate choice based on heterozygosity predicts that sexual traits will reach their extreme expression in males with the greatest average heterozygosity, and that both a male’s ornament expression and his mating success will correlate positively with degree of individual heterozygosity. In agreement with the first prediction, we found a negative correlation between degree of homozygosity and length of the throat feathers (a secondary sexual trait) in male spotless starlings, Sturnus unicolor. However, we found a quadratic relationship between degree of homozygosity and both mating success and mean number of young sired per year. Hence, males of intermediate heterozygosity were more successful in mating and reproduction than either more homozygous or more heterozygous males. These results do not support the hypothesis of mate choice based on heterozygosity.

Infanticide by male and female house sparrows

Abstract Of 211 breeding attempts of house sparrows, Passer domesticus, 9–12% were affected by successful or apparently attempted infanticide. Among males, infanticide appeared to be committed by territorial individuals that had recently lost their mates. Since there was a large population of female floaters, such males had access to mates, suggesting that intrasexual competition for mates was not a necessary precondition for male infanticide in this population. Infanticidal males bred with the females whose chicks they killed sooner than replacement males that refrained from infanticide. Females that mated with bigamous males lost eggs and nestlings due to infanticide by females more frequently than did monogamous females. Moreover, primary females (the only ones assisted by the male) tended to suffer from infanticide by females more often than did secondary females. This suggests that females commit infanticide in order to monopolize paternal investment.

Settlement and fat accumulation by Migrant Pied Flycatchers in Spain

Pied Flycatchers migrating through Spain showed a marked tendency to settle in some areas. During the stay their weight and fat reserves increased. The density of settled birds and some unidentified seasonal features were major factors determining the settling rate of newly arrived birds. The level of fat reserves at arrival also seemed to be involved in the decision as to whether to stay or not. The tendency to settle was stronger in juveniles than in adults, apparently as a consequence of their lighter weight on arrival. The length of stay was related to the rate of gain in weight. The role of age‐related factors on the decision to depart is discussed in the light of differences between the settlement and movement patterns of juveniles and adults.

Why are house sparrows predominantly monogamous? A test of hypotheses

Observational and experimental evidence collected in a 4-year study of house sparrows, Passer domesticus, was used to test hypotheses about the evolution of monogamy. Reproductive success increased for males and decreased for females with increasing harem size. This rejects hypotheses that state that monogamy is advantageous for both sexes. Only 9·6% of 197 control males were polygynous. However, after removal of 30 males, between 36·7 and 73·3% of potential bigamous males acquired an additional female; 43–65% of the experimental widows mated with already mated males while only 23·3% rejected polygynous bonds. This contradicts the prediction, derived from the hypothesis based upon the polygynythreshold model, that widowed females prefer to remate with unmated males. Of 86 males, 27·9%, more than the percentage of polygynous males, remated after a first breeding attempt with a female that had not bred in the colony that season. Hence, the limitation of the polygyny level cannot be explained by a lack of females. When nestboxes were put closer together, males held more of them, but they did not increase their harem size. This suggests that the limitation of nest sites is not a proximate determinant of monogamy. Resident females were very aggressive towards a caged female placed close to their nest. The way in which the aggression rate varied with breeding phase and intruder persistence favours the hypothesis that females protect their monogamous status. However, further direct tests are required to confirm that monogamy in this population is maintained by aggression between females.

Importance of rabbits in the diet and reproductive success of Black Kites in southwestern Spain

The rabbit Oryctolagus cuniculus is the most common prey of many predators in Mediterranean ecosystems, but how its abundance affects the productivity of its predators has been poorly studied. The Black Kite Milvus migrans is a generalist predator, but in our study area eats mainly rabbits. There was considerable year to year variation in the proportions of the nine prey types examined in the diet. The abundance of young and sick rabbits seemed to determine the consumption rate of rabbits by Black Kites. Interannual variation in reproductive performance (chick starvation mortality, fledging weight and length of the nestling period) was related to the proportion of rabbits in the diet, a year of high performance coinciding with a high proportion of rabbits in the diet. Some factors explaining the importance of rabbits for the reproductive success of this raptor are discussed.