Declining extra-pair paternity with laying order associated with initial incubation behavior, but independent of final clutch size in the blue tit

Although functional explanations for female engagement in extra-pair copulation have been studied extensively in birds, little is known about how extra-pair paternity is linked to other fundamental aspects of avian reproduction. However, recent studies indicate that the occurrence of extra-pair offspring may generally decline with laying order, possibly because stimulation by eggs induces incubation, which may suppress female motivation to acquire extra-pair paternity. Here we tested whether experimental inhibition of incubation during the laying phase, induced by the temporary removal of eggs, resulted in increased extra-pair paternity, in concert with a later cessation of laying, in blue tits (Cyanistes caeruleus). As expected, experimental females showed a more gradual increase in nocturnal incubation duration over the laying phase and produced larger clutches than controls. Moreover, incubation duration on the night after the first egg was laid predicted how extra-pair paternity declined with laying order, with less incubation being associated with more extra-pair offspring among the earliest eggs in the clutch. However, incubation duration on this first night was unrelated to our experimental treatment and independent of final clutch size. Consequently, the observed decline in extra-pair paternity with laying order was unaffected by our manipulation and larger clutches included proportionally fewer extra-pair offspring. We suggest that female physiological state prior to laying, associated with incubation at the onset of laying, determines motivation to acquire extra-pair paternity independent of final clutch size. This decline in proportion of extra-pair offspring with clutch size may be a general pattern within bird species

Evidence of the Trade-Off between Starvation and Predation Risks in Ducks

The theory of trade-off between starvation and predation risks predicts a decrease in body mass in order to improve flight performance when facing high predation risk. To date, this trade-off has mainly been validated in passerines, birds that store limited body reserves for short-term use. In the largest avian species in which the trade-off has been investigated (the mallard, Anas platyrhynchos), the slope of the relationship between mass and flight performance was steeper in proportion to lean body mass than in passerines. In order to verify whether the same case can be applied to other birds with large body reserves, we analyzed the response to this trade-off in two other duck species, the common teal (Anas crecca) and the tufted duck (Aythya fuligula). Predation risk was simulated by disturbing birds. Ducks within disturbed groups were compared to non-disturbed control birds. In disturbed groups, both species showed a much greater decrease in food intake and body mass during the period of simulated high risk than those observed in the control group. This loss of body mass allows reaching a more favourable wing loading and increases power for flight, hence enhancing flight performances and reducing predation risk. Moreover, body mass loss and power margin gain in both species were higher than in passerines, as observed in mallards. Our results suggest that the starvation-predation risk trade-off is one of the major life history traits underlying body mass adjustments, and these findings can be generalized to all birds facing predation. Additionally, the response magnitude seems to be influenced by the strategy of body reserve management

Flock formation, flock size and flock persistence in the Willow Tit Parus montanus

The study was carried out in the alpine birch region in central Norway. According to flock attachment and behaviourjuvenile Willow Tits were classified into five categories : regulars (permanent flock members), vanishers (flock members disappearing before New Year), irregulars (unsteady flock members), floaters and transients. Although the vanishers on the average arrived earlier in the area than the regulars and thereby, according to the current view, should benefit priority of being permanent flock members they nevertheless disappeared from the flocks after some time; the majority evidently emigrated from the area in October. It is suggested that the different behaviour of regulars and vanishers is hormonally based. The winter territories of flocks overlapped considerably . Flock size averaged initially 4.2 individuals (range 2- 8 birds) and was composed of 2.3 adults and 2.0 juveniles. The sex ratio irrespective of age was 1 .3, i .e . males outnumbered females. Thirty-two percent of the flocks contained more than 2 adults, some of which bred in the winter territory besides the dominant adult pair . Occasionally, even juvenile flock members bred successfully together with the adult pair within their common winter territory. Owing to strong fidelity, the birds usually kept to the same winter territory for life . Occasional territory (and flock) shifts were usually combined with pairing of individuals from different flocks, or divorces . Prior to establishment in winter flocks, the juveniles probably evaluate the resident pair and its habitat. It is furthermore suggested that juvenile flock members continuously try to improve their fitness, for example by frequently exploring areas outside their main territory, and that they are ready to change strategy if this seems beneficial

Rank-dependent winter fattening in the Willow Tit Parus montanus

The body mass in tits increases in autumn towards awinter peak in December-January and then declines towards spring . I studied the winter fattening in the Willow Tit Parus montanus at two different sites in central Norway. Body mass data were collected in the early morning, late evening and partly at noon. To make the data comparable, they were transformed to body mass indexes. In rank-structured singlespecies flocks or aggregations of tits the body mass of each individual was compared with each one of the other members, using paired t-test (2-tailed) to control for seasonal and diurnal body mass fluctuations and impact of ambient temperature. Subordinate Willow Tits carried on the whole significantly more body fat than dominants throughout the day. Dominants do not utilize their actual capacity for fat storage, suggesting that fat storage is costly . Large fat deposits obviously reduce the risk of starvation, but whether the risk of predation at the same time is reduced because of constricted manoeuvrability is dubious. Since the dominants have priority to food resources, the subordinates are forced to carry more fat as an insurance against starvation . No consistent rank-dependent differences were found with regard to the diurnal body mass cycle