Hans Källander

The Trade-Off Between Offspring Number and Quality in the Great Tit Parus major

(1) Trade-offs between brood size and offspring growth and survival were manipulating the size of 221 great tit Parus major L. broods between 1983 (2) Nestling mass, wing length and tarsus length were inversely related to Hatching date also affected nestling growth, but its effect differed between Intrabrood variability in nestling size increased with brood size. (3) Nestling survival was inversely related to brood size. Small nestlings suffered mortality than large ones. The difference in nestling survival rate between categories was too small to equalize their productivity. (4) Fledgling survival to the autumn and to the following breeding season inversely related to brood size; fledglings from reduced broods survived better from control broods, which in turn survived better than fledglings from enlarged This resulted in the most productive category being the control broods. The fledglings increased with their size as nestlings and decreased with their hatching (5) The sex-ratio among independent fledglings was affected by brood size tion; proportionally more males survived in enlarged than in reduced broods. (6) Dispersal distance of juveniles was not affected by brood size, size hatching date, whereas it was affected by sex. The effects of starvation competition on fledgling survival and dispersal (Less)

Feeding frequency and parental division of labour in the double-brooded great tit Parus major

SummaryWe studied the relative contribution of each sex and total effort expended in feeding nestlings in the great tit Parus major in relation to artificially altered brood size. A recent model suggests that feeding frequency should reflect the optimal trade-off between parental and fledgling survival, the former being negatively, the latter positively, influenced by high feeding frequencies. In both sexes weight loss was linearly related to feeding frequency. Since fledgling survival increases with nestling weight, the conditions of this model are fulfilled. However, in contrast to the predictions of the model, the total feeding frequency for both sexes combined did not differ between control and enlarged broods, but was lower for reduced ones. This outcome was not the result of a physiologically related inability of the parents to increase their delivery rate. Instead, we suggest that parents with enlarged broods could not find sufficient amounts of prey large enough to be economically worth transporting to the nest. Differences in brood-provisioning rates between the sexes may arise because costs and benefits of feeding nestlings may differ. Females lost more weight than males during the nesting period, but maintained a relatively higher weight during the incubation period. The relationship between weight loss and feeding frequency was similar for both sexes. Male and female brood-feeding frequency was related to brood size in a similar way. This is discussed in light of the great tits mating system and the fact that the great tit is facultatively double-brooded.

Female nutritional state affects the rate of male incubation feeding in the pied flycatcher Ficedula hypoleuca

SummaryMale pied flycatches Ficedula hypoleuca regularly feed their mates during incubation. By experimentally supplying some females with extra food we studied how the females nutritional state affected her incubation schedule and the rate at which her mate fed her. Females that received extra food spent more time on the nest and shorter periods away from it, compared with control females. This suggests that nest attentiveness is governed by the amount of energy available to the female. When females reccived extra food, males decreased their rate of incubation feeding. They also did so in response to increasing ambient temperatures, whereas incubation schedules were unaffected. We, therefore, conclude that our results support the “female nutrition hypothesis”, i.e., that the food provided by the male constitutes a significant nutritional contribution to the incubating female.

Piracy by Black-headed Gulls on Lapwings

A description and a quantitative analysis are given of Black-headed Gull piracy on Lapwings, as observed in Sweden where such behaviour occurs mainly in autumn and early spring. It is concluded that Blackheaded Gulls are able, at least at times, to subsist by kleptoparasitism alone.

Manipulation of the Brood Size of Pied Flycatchers

Most studies on passerines have shown a positive correlation between clutch size and the production of fledglings. They have also shown that clutches larger than the modal one fledge more young than those of modal size (Klomp 1970, Murphy and Haukioja 1986, Alatalo and Lundberg 1989). This observation apparently contradicts Lack’s (1947, 1954) original hypothesis of clutch size determination in nidicolous birds, according to which clutch size is determined by the largest number of young that the parents can adequately nourish.

Experimental evidence for low intra-pair cache pilfering rates in European nuthatches

European nuthatches, Sitta europaea, live all year in permanent pair-territories. In winter, hoarded food is an important food source. To measure the rate of cache pilfering within pairs, each member of six pairs was supplied with either white- or dark-shelled sunflower seeds which the birds cached. When each bird had cached some 100–300 seeds, the feeder was removed and the birds were monitored during their normal foraging. For each sunflower seed that the nuthatches retrieved, the colour of the seed and the sex of the retrieving bird were recorded. In all pairs, each member almost exclusively retrieved seeds of the colour it had itself cached, pilfering amounting to less than 5%. As no clear differences in hoarding niches were detected between the sexes, these results suggest that caches were relocated with the aid of memory.

Food hoarding and use of stored food by Rooks Corvus frugilegus

Capsule In autumn, Rooks cached large quantities of acorns, which were retrieved during winter. Aims To describe the autumn storing of acorns by Rooks in a natural situation and to quantify the retrieval of acorns during winter. Methods The collection, transportation and caching of acorns were studied during four autumns, and cache retrieval during two winters, in an extensive grassland area of southern Sweden. Results Rooks transported acorns from a few tens of metres up to 4 km from the source oaks and carried a mean number of 3.56 acorns (range 2–7) during outward flights. Permanent grass appeared to be the preferred caching habitat. In winter, 1 to 1.5 acorns per Rook per hour were retrieved during active foraging. Some were re-cached, with a much lower proportion being re-cached in late than in early winter. In snowy conditions Rooks foraged in the grasslands only when the snow cover was broken or very thin, suggesting that they were unable to use their stores in deep snow. Conclusion Observations strongly indicate that Rooks knew the exact location of their caches and probably possess the long-term memory capacity necessary to relocate caches made several months earlier. It is concluded that Rooks are the most specialized food hoarders of the Corvus species.