Black-headed Gull Chicks Research Articles

Sex-specific growth and morphometric sexing of chicks of the Black-headed Gull (Chroicocephalus ridibundus)

In birds, sexual size dimorphism often becomes evident as early as chicks’ growth stage. A larger sex usually has an advantage in food competition among siblings. In the smaller sex, the lesser ultimate size can be compensated by faster growth or earlier fledging. As a consequence, sex differences can manifest themselves not only in body size but also in the body condition or the rate of weight gain. This phenomenon provides additional opportunities for sexing of chicks by discriminant analysis of growth parameters. Our work deals with the influence of sex on the growth of Black-headed Gull chicks. Using logistic models, we examined growth patterns of 80 males and 76 females from hatching to departure from breeding colonies during three seasons. In males, the total head length, tarsus length, and body mass were greater than those in females from the first days of life and reached large asymptotic values. Tarsus length was a better predictor of sex than head length or body mass was. The sex of chicks did not have a significant effect on global growth rates; however, compared to males, females had a lower body condition (the difference between observed and expected chick body mass) at age 8 days and gained less weight between 21 and 28 days. Using these differences, we developed a discriminant function that allowed to correctly predict the sex of 88.5% of Black-headed Gull chicks before fledging.

The early development of aggressive behavior and rapid growth of chicks in the black-headed gull (Larus ridibundus) in conditions of diffused nesting

The effect of low nesting density caused by the fragmentation of nesting biotopes due to seasonal flooding on the development of aggressive behavior and on growth of black-headed gull chicks was investigated. An analysis of the behavior of birds nesting with different densities showed that contacts with neighboring broods were less frequent in chicks from more sparsely distributed nests, while aggressive behavior could be observed in these broods earlier and more often than in their peers from the nests located in more densely populated areas. By testing the parent birds using a standard stimulus of territorial behavior—a remotely controlled stuffed gull—we found that the hosts of widely spaced nests reacted more aggressively but left the clutches less frequently than their neighbors that nested more densely. On average, the chicks from the sparsely distributed broods grew faster and the daily increment of the skull size of the chicks correlated inversely with the frequency of parental visits. We suppose that in the case of the seasonal decrease of nesting grounds, the offspring of more aggressive young couples nesting in the most sparsely populated areas had an advantage in the development probably due to the maternal influence on the composition of the eggs. According to our data, in the conditions of low-density nesting, the parental influence on the growth of chicks and development of their aggressive behavior prevailed over the possible developing effect of the social experience.

Maternal yolk androgens stimulate territorial behaviour in black-headed gull chicks

Avian eggs contain substantial amounts of maternal androgens. The concentrations of these yolk androgens are affected by the maternal environment, such as the level of social competition, parasite exposure or food conditions. Since yolk androgens have been shown to affect a wide array of offspring traits, they may adjust the chicks to the expected post-hatching environment, but experimental evidence is still scarce. We investigate in colonial breeding black-headed gulls whether high concentrations of yolk androgens, such as those found in environments with high numbers of social interactions, facilitate aggressiveness and territorial behaviour of the chicks. Black-headed gulls are highly suitable for this, as the semi-precocial chicks defend the natal territory and food against intruders. We manipulated yolk androgen concentrations and investigated their role in both within-nest and between-nest aggression. We found that chicks hatching from androgen-treated eggs defended the natal territory more often than their nest mates from control eggs, without increasing sibling aggression. This suggests that variation in yolk androgen concentrations in relation to the social environment of the mother may indeed allow adjustment of the offspring's behaviour to the expected frequency of territorial interactions with conspecifics post-hatching.

Prenatal androgen exposure modulates cellular and humoral immune function of black-headed gull chicks

Avian eggs contain considerable amounts of maternal yolk androgens, which have been shown to beneficially influence the physiology and behaviour of the chick. As androgens may suppress immune functions, they may also entail costs for the chick. This is particularly relevant for colonial species, such as the black-headed gull (Larus ridibundus), in which the aggregation of large numbers of birds during the breeding season enhances the risk of infectious diseases for the hatching chick. To test the effect of maternal yolk androgens on the chick's immune function, we experimentally manipulated, in a field study, yolk androgen levels within the physiological range by in ovo injection of either androgens (testosterone and androstenedione) or sesame oil (control) into freshly laid eggs. We determined cell-mediated immunity (CMI) and humoral immunity of the chicks at the beginning of the nestling period to evaluate early modulatory effects of yolk androgens on immune function. Embryonic exposure to elevated levels of androgens negatively affected both CMI and humoral immunity in nestling gull chicks. Consequently, maternal yolk androgens not only entail benefits of enhanced competitiveness and growth as previously shown, but also costs in terms of immunosuppression. The outcome of embryonic yolk androgen exposure thus likely depends on the post-hatching circumstances for the developing offspring such as parasite exposure and degree of sibling competition.

The hormonal control of begging and early aggressive behavior: Experiments in black-headed gull chicks

The hormonal control of begging and sibling competition is largely unknown, but recent evidence suggests a role for steroid hormones. We tested the influence of the aromatizable androgen testosterone (T), the non-aromatizable androgen 5α-dihydrotestosterone (DHT), and 17β-estradiol (E) on both begging behavior and aggressive behavior in black-headed gull chicks ( Larus ridibundus). Chicks of this species have a conspicuous begging display, while their frequently performed early aggressive behavior is facilitated by testosterone and important for territorial defense. Hormone treatment was applied by implants between days 6 and 16 after hatching. Behavior was tested by means of standard stimulus tests. The results were validated in a second experiment under semi-natural conditions. Begging was suppressed by T and DHT and not affected by E. Aggressive Pecking was strongly facilitated by T. The erect threat posture, characteristic for older chicks, was facilitated by T, DHT, and E and the nest-oriented threat display, typical for young chicks, only by T and DHT. Growth was suppressed in the T group. The results indicate that androgen production, needed for territorial defense, has costs in terms of a suppression of begging and growth. It is discussed to what extent older chicks may avoid these costs by converting testosterone to estrogen and why pre-natal and post-natal exposure to androgens differ in their effect on begging behavior.

Balancing between costs and benefits of maternal hormone deposition in avian eggs

Avian eggs contain substantial amounts of maternal androgens, and several studies have indicated that these are beneficial for the chick. Nevertheless, there is a large and systematic variation in maternal hormone concentrations both within and between clutches. If maternal androgens also involve costs, this might explain why not all mothers put high levels of androgens in their clutches. However, the simultaneous occurrence of both benefits and costs has not yet been convincingly demonstrated. We show experimentally that yolk androgens suppress immune function and simultaneously stimulate growth in black-headed gull chicks. Thus, mothers face a trade-off between these costs and benefits and may tune hormone deposition to prevailing conditions that influence chick survival.

Yolk androgens and begging behaviour in black-headed gull chicks: an experimental field study

Black-headed gulls, Larus ridibundus, produce clutches of three eggs, which contain high levels of maternal androgens in the yolk. These levels increase with laying order and the eggs hatch asynchronously. Experiments have supported the hypothesis that this within-clutch variation in maternal androgens mitigates the disadvantage of last-hatched chicks in sibling competition, by enhancing embryonic development and early posthatching growth. We hypothesized that these effects come about by the stimulating effects of maternal androgens on begging behaviour and competitive ability. In the field, we injected first-laid eggs of a clutch (which have a low androgen level) with either an androgen solution (T eggs) or vehicle (Oil eggs). We then created pairs of chicks hatched from Oil and T eggs, matched for egg mass and hatching date. Parent–chick interactions were recorded from observation hides. Chicks from T eggs hatched almost half a day sooner than those from Oil eggs. Furthermore, chicks from T eggs were more active during the first week after hatching, were more often the first to react to the approaching parent, begged more frequently, and obtained the larger share of food. We conclude that the enhancing effect of yolk androgens on growth in this species arises at least partly through androgen-mediated effects on the chicks' behaviour. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Inter-sexual differences in T-cell-mediated immunity of black-headed gull chicks ( Larus ridibundus ) depend on the hatching order

Hatching asynchrony in avian species leads to age and size differences between nestlings within a brood, handicapping last-hatched chicks in the sibling rivalry. Starvation due to this competitive disadvantage has been regarded as the primary cause of an increase in mortality with hatching order. However, for gulls it has also been suggested that disease is the cause of mortality for last-hatched chicks, possibly through reduced immunocompetence and thereby an enhanced susceptibility to infection. In addition, the male-biased mortality reported for several gull species may be related to a higher vulnerability to diseases in males compared to females. To determine the potential influence of the immune system on these mortality patterns, we investigated the T-cell-mediated immunity (CMI) of black-headed gull chicks in relation to hatching order and sex. We found a significant decrease in the CMI with hatching order. This result may be causally related to systematic changes in maternal yolk steroids and carotenoids within the laying sequence. For second-laid eggs, male CMI was significantly lower than female CMI. This is possibly linked to higher plasma levels of testosterone in male embryos which might have an immunosuppressive effect. If so, this effect is masked in eggs of either high (first egg) or low (last egg) quality. Chicks with low CMI showed enhanced mortality rates. Thus the differences in immune response are likely to contribute to the observed mortality patterns. However, hatching order significantly affected mortality independently of CMI, suggesting that competitive disadvantage due to hatching asynchrony is also important.

Feeding frequency of Black-headed Gull chicks

Black-headed Gulls feed their chicks at intervals of about 45 min. This feeding rate is independent of nestling age, brood size and time of day. Adults swallow food that the chicks do not eat. This ‘back feeding’ decreases with the age and number of nestlings. The observed feeding and the size of meals (estimated from what chicks regurgitate when handled) are enough to cover the energy requirements judged from the intake of hand-reared young. A statistical analysis of growth rate, feeding frequency, asymptote weight and brood size of non-passerine families shows that high feeding frequencies are associated with increased growth rates. Feeding frequency and body size are the most important variables in predicting growth rates of birds.

The ‘peck/no-peck decision-maker’ in the black-headed gull chick

1. 1. In companion papers a model called the Choice Threshold Model was described, and its applicability to percentage preference data tested. The present paper extends the treatment to absolute numbers of responses in a choice situation. 2. 2. It is postulated that while the Choice Threshold Model functions as a ‘directing’ decision-maker, determining how choices are to be directed, the determination of when and how often such choices shall occur may be the function of a separate ‘go/no-go decision-making mechanism’, the rate of whose activity is represented by the symbol K. 3. 3. Data on rates of pecking at coloured stimuli by black-headed gull chicks Larus ridibundus, are used to determine the relationship between K and the stimulus situation. 4. 4. It is found that K is determined only by the most preferred stimulus available. 5. 5. Thus a stimulus affects the animal in two different ways. By influencing the ‘directing decision-maker’ (the Choice Threshold Model), it affects the relative number of times it is chosen compared with other simultaneously available stimuli. By influencing the ‘go/no-go decision-maker’, which it only does when it is the most preferred of the available stimuli, it affects the absolute rate of choosing regardless of which stimulus is chosen. This dichotomy is reminiscent of that of classical ethology between ‘directing’ and ‘releasing’ roles of stimuli. 6. 6. Paradoxically it is found that stimuli which are preferred with respect to their ‘directing’ function tend to have low ‘releasing’ value and vice versa. 7. 7. A quantitatively precise prediction concerning the relative merits of two methods of studying selective responsiveness, simultaneous or successive presentation of stimuli, can be derived from the model if it is assumed that K is constant. In the light of the findings about the dependence of K on the stimulus situation, this prediction has to be modified. The present data are used in an indirect way to verify the modified prediction. 8. 8. The actual values of K (i.e. ‘releasing’ value) for each colour are estimated from the data, and are plotted graphically against the corresponding threshold values (i.e. ‘directing’ value). The inverse relationship appears to be a regular one, for the points all fall on a smooth curve. 9. 9. These findings can all be explained by a Lorenzian hydraulic type of model. Various versions of the model have been simulated, and the parameters of one version adjusted to achieve a precise fit to the gull data.