The breeding season for the Blackbird lasts from March to June. Temperature influences the timing of the start of breeding, and rainfall probably influences the ending of breeding, and breeding begins later in woodland than in gardens. Incubation of eggs starts gradually while the clutch is being laid, so consequently hatching is usually spread over a period of a day or two. Hatching success is about 90% in gardens, old parents being more successful than yearlings. Hatching success in woodland is 92 to 95%. In gardens, nestlings are fed mainly on earthworms in the early part of the breeding season, later on a greater variety of food, especially insects. In woodland, nestlings are fed especially on green caterpillars and a variety of other invertebrates taken from the woodland floor (Snow 1958).Experiments suggest that Blackbird react to humans as if they were a potential predator, changing the spatial and temporal use of their habitat. For example, Blackbirds increased their scanning rate with increasing number of pedestrians in the proximity of their feeding patches – a common reaction of birds foraging in exposed part of their habitats. A consequence of this process may be a reduction in breeding densities as the amount of disturbance by pedestrian’s increases (Juricic and Telleria 2000).
In animals, increased body mass can increase survival and reproductive success through the availability of stored energy reserves as insurance against unpredictable foraging opportunities. Yet animals, particularly birds, usually maintain considerably lower body mass than the maximum possible. This is theoretically predicted to be the result of mass-dependent costs, most notably in birds, mass-dependent predation due either to body mass having a detrimental effect on the ability to take off quickly and escape from predators or to the increased foraging time necessary to maintain a greater mass.
What has been found, is that during the breeding season, female blackbirds have a far greater body mass than throughout the rest of the year and also far greater than the males. This seasonal changes in the importance of sex in determining body mass suggest increased mass-dependent predation risk as a cost of breeding for female blackbirds, as has been shown in recent studies of blue tits Parus caeruleus and pied ﬂycatchers Ficedula hypoleuca. Another potential explanation of the greater mass of females in the breeding season could be that they are responding to a reduced predation risk resulting from being less conspicuous and that body reserves can be increased to reduce starvation risk.
Local weather, particularly temperature, is another major factor in determining how much weight and therefore energy reserves blackbirds can carry. Considering that blackbirds forage principally on the ground for soil-living invertebrates, whose activity will be linked to soil temperature, it is likely that the birds are responding to this, rather than the mean maximum air temperature (Macleod et al. 2005)
- Juricic.F..E., & Telleria.L.J., (2000) Effects of Human Disturbance on Spatial and Temporal Feeding Patterns of Blackbird (Turdus merula) in Urban Parks in Madrid, Spain. Bird Study, 47, 13-21.
- Macleod.R., Barnett.P., Clark.A.J., & Cresswell.W., (2005) Body mass change strategies in blackbirds Turdus merula: the starvation–predation risk trade-off. Journal of Animal Ecology, 74, 292-303.
- Snow.W.D., (1958) The Breeding Behaviour of the Blackbird (Turdus merula) in Oxford. IBIS, 100, 1-30.