This species has been surveyed at regular intervals since 1968. Nowadays, the species is an important predator in the bird community at Kvismaren. Therefore, it has always been valuable to have knowledge of the population size. There was a strong focus on the crows being associated with mercury poisoning in the 1960s. A 1964 census showed that there were only nine pairs in an approximately 5000-acre survey area around Kvismare lakes. Thereafter, there has been a strong increase and stabilization in recent years.
Population Development in Crows 1964-2010
In the first four years of the survey the nesting population was around ten pairs. Between 1968-69 the population doubled as a result of mercury ban 1966. A further increase to about 40 pairs was seen in the following years and this has remained approximately at this level ever since. Of the pairs eight or ten were in Eastern Kvismaren, thus the number of Eric Rosenberg had previously mentioned (10). In 2005 only 35 pairs were recorded, but probably due to the conservation projects in Kvismaren during winter/spring of 2004-2005 therefore some pairs were deprived of their usual nesting groves. The 2010 field season saw a lull in the number of crow pairs nesting. The number of pairs had now fallen to 30. In return, five pairs of Ravens recently started nesting in the study area.
A crow population consists of breeding pairs and also non breeding flocks of crows. In 1979 nutritional choices of crows were studied closer. First, there were 49 pairs of nesting crows, and two flocks of crows, living north of Husön and Fiskinge. At the beginning of the breeding season (17 May) the flocks together numbered 28 crows and late (23 June), about 100 birds. Some individuals who had failed nesting probably joined, but also non-breeding crows from other the surrounding area. Two Black-headed Gull colonies and two-Lapwing colonies were followed daily for Crow predation. The first black-headed gull colony contained 15-20 pairs, and was locatedy in a wintercut field in Fågelsjön. When the year’s reed started to grow, the crows used its shelter to access nests. On May 20, the settlement was abandoned, the gulls moved to a smaller, open reed islet in Källviken. In the now concentrated colony of about 20 pairs the crows had more difficulty in accessing the nests, and here a dozen young gulls were reared to fledging. The crows guilty of predation on gulls were the territorial ones nearby.
The Lapwing colonies each consisting of five to ten pairs were no egg predation observed. The Lapwings used a lot of energy to keep the colony areas crow free. The spring of 1979 was relatively warm and moist, giving crows good nutritional conditions. Flocks of crows were noted mostly picking the soil surface layer. They ate probably seeds, sprouts, mollusks, worms, and both mature and larvae insects including caterpillars.
An interesting question is whether one can reduce crow predation in any way. The answer must be that this is very difficult. If the worst egg thieves – the breeding crows – are shot, their territory is rapidly taken over by a new pair from the flock of crows, or there will be a flock established in the territory. Our studies show that hunters' campaigns against the crows in the spring probably have no or very little effect in the light of division of nesting crows, and crow flocks. The latter acts as a recruitment reserve.
Various analysis’s have been done to investigate the importance of the Crows breeding results. In a study in 1973 the number of laid eggs affected by the distance between nests was examined. It turned out that when there were two or three other nests within 600 meters, clutch size fell by 0.6 eggs compared with nests without nearby nests. The reason for this has been investigated by other researchers and it appears that the crows steal eggs from the neighbors if they have nests close together. You could say that this behavior is a way to regulate territory density. Internal predation explains the crow’s preference to put the nest in small groves or single trees surrounded by open land. By placing the nest here, they can forage better, keep watch and both can rush to the intruding crows when approaching the nest.
In 1977, an analysis in which nests next to or out on the old lake bottoms were compared with nests surrounded by pure land. It turned out that in nests adjacent to lakes hatched eggs about a week later than in pairs on the acres. The young next to lakes weighed more than those out in the farmland at the same age, and the clutches at the lake were also slightly larger. It was obvious the food supply, even if it became available a little later in the spring, was better in the crow's territory that lay within or near the lakes.