Sven-Axel Bengtson

Effect of Bird Predation on Lumbricid Populations

The effects of predation by golden plover Pluvialis apricaria (L.) on populations of Allolobophora caliginosa (Sav.) and Lumbricus rubellus Hoffm. in early spring on a hayfield in Iceland were estimated by two independent methods. Field experiments (lasting for 22 days) with nets preventing bird predation showed that the mean abundance of lumbricids was 238 ind m-2 on protected sites and 107 ind m-2 on exposed sites (P < 0.001). Observations of feeding plovers showed that about 4.5 lumbricids were taken per m2 and day; i.e. about 100 ind m-2 in the experimental period. This agrees closely with the result obtained by the first method. No selective predation could be confirmed but certain differences between the two species of lumbricids are discussed.

Selective predation on lumbricids by golden plover Pluvialis apricaria.

Predation by golden plovers Pluvialis apricaria (L.) on lumbricids was studied on a hayfield in Iceland in early June 1977 by means of stomach analyses and formalin sampling. Lumbricids, Allolobophora caliginosa (Sav.) and Lumbricus rubellus Hoffm., constituted about 90% (fresh wt) of food remains in stomachs. The plovers had taken relatively more A. caliginosa than L. rubellus and a relatively larger portion of medium-sized (by biovolume) than smaller and larger lumbricids. Thus the predation provided a selective advantage to L. rubellus over A. caliginosa and to the smallest and largest over the medium-sized individuals. The results are discussed in terms of degree of pigmentation (cryptic colouration), burrowing behaviour, activity, and density of the lumbricids. It is suggested that the selective effects of predation may vary considerably from one year to another.

Short-Term Colonization Success of Lumbricid Founder Populations

Small founder units (two or four individuals) of adult Allolobophora caliginosa (Sav.) and Lumbricus rubellus Hoffm. kept in netbags were introduced into a hayfield in NE Iceland. After 14 months 82% of the A. caliginosa units and 35% of the L. rubellus units still contained live adults, and the rates of individual survival were of the same magnitude. Differences in survival between units of A. caliginosa were presumably associated with position of the netbags in the field (exposure, etc.) and initial density of founders; thus, survival seemed to decrease with increasing density. A. caliginosa produced significantly more cocoons and newly hatched individuals than L. rubellus. Colonization success is discussed in relation to the experimental conditions and the difference between the two species in terms of differences in their ecology. It is suggested that A. caliginosa achieves a relatively high post-dispersal colonization success by having a relatively long life-span as adult rather than having a high birth rate.

Thermal emissivity of avian eggshells.

The hypothesis has been tested that evolution has resulted in lower thermal emissivity of eggs of birds breeding openly in cold climates than of eggs of birds that nest under protective covering or in warmer climates. Directional thermal emissivity has been estimated from directional-hemispherical reflectance spectra. Due to several methodological difficulties the absolute emissivity is not accurately determined, but differences between species are obvious. Most notably, small waders of the genus Calidris, breeding in cold climates on the tundra, and in most cases with uniparental nest attendance, have low directional emissivity of their eggshells, about 0.92 when integration is carried out for wavelengths up to 16μm. Species belonging to Galloanserinae have the highest directional emissivity, about 0.96, of their eggs. No differences due to climate or breeding conditions were found within this group. Eggs of most other birds tested possess intermediate emissivity, but the values for Pica pica and Corvus corone cornix are as low as for Calidris. Large species-dependent differences in spectral reflectance were found at specific wavelengths. For instance, at 4.259μm the directional-hemispherical reflectance for galliforms range from 0.05 to 0.09, while for Fratercula arctica and Fulmarus glacialis it is about 0.3. The reflection peaks at 6.5 and 11.3μm due to calcite are differentially attenuated in different species. In conclusion, the hypothesis that evolution has resulted in lower thermal emissivity of bird eggs being exposed in cold climates is not supported by our results. The emissivity is not clearly related to nesting habits or climate, and it is unlikely that the small differences observed are ecologically important. The spectral differences between eggs that nevertheless exist should be taken into account when using infrared thermometers for estimating the surface temperature of avian eggs.