Blackbird

The blackbird (Turdus merula) is a much loved species, ubiquitous across the UK with the exception of some areas in northern Scotland. Males are particularly identifiable - living up to their name, they are entirely black with a striking yellow-orange beak and eye rings. Females are less conspicuous as they are all brown with some speckling on the breast. Both sexes weigh 80-100g. Originally a woodland bird, the blackbird has been particularly successful in gardens - their breeding season can start 2 weeks earlier in gardens than in woodland, and breeding success is often higher. This may be because of food provided in gardens, or lower nest predation rates in urban areas compared to rural. 

A male blackbird taking food to his chicks in the nest.

Across their woodland, grassland, and garden habitats blackbirds eat a variety of foods, including insects, fruit and berries. Earthworms constitute a large part of this species' diet, and they find them by listening for their movement beneath the soil. Often, blackbirds can be heard rummaging through the leaf litter in search of invertebrates. More rarely these birds have even been seen taking tadpoles and newts from ponds. The feeding habits of blackbirds has been shown to differ between rural and urban areas. Generally, blackbirds are the first to arrive at feeding stations in the morning, enabled by their relatively large eyes. In towns and cities they arrive later than in rural areas. This may be due to urban heat islands meaning blackbirds use less energy to stay warm overnight, reducing the urgency of finding food in the morning.

There are believed to be around 5.1 million breeding pairs of blackbird in the UK. In winter, the number of individuals increases to between 10 and 15 million as birds arrive from Europe. Around 12% of winter residents have flown from Finland, Sweden, Denmark, the Netherlands and Germany. Some of these migrating birds continue further south to Spain, France or Portugal. 

Male blackbirds establish their territory in their first year, and will continue to reside there throughout their lives. The exception to this is after the fledging of the last brood of the year, when the adults are moulting; territory boundaries are disregarded and blackbirds feed in close proximity. Territories are re-established and defended from late autumn until July. Blackbird pairs can have 2-3 broods a year, with the start of the breeding season controlled primarily by temperature, and the end by water, and therefore food, availability. The female spends up to 2 weeks building a nest out of grass, small twigs and other plant material and plasters the inside with mud, then lines it with fine grass. Between 3 and 5 blue-green speckled eggs are laid in the nest and hatch 2 weeks later. Only the mother broods the chicks, but both parents collect food for them; mostly caterpillars in woodland and earthworms in gardens. Chicks beg for food from their parents with their bright yellow gapes open.

The progression of a Turdus merula brood: top left - 4 blueish eggs sat in the cup-shaped nest,
top right - recently hatched chicks ( ~2 days old), bottom left - chicks after about 10 days,
bottom right - young blackbirds almost ready to fledge. 

In their first week T. merula chicks learn to fly and begin to forage further, testing foods but will still follow the parents to beg for food. Three weeks after fledging the young are independent and begin to leave the natal area. Juvenile blackbirds are a similar colour to female adults, but are more speckled and may appear larger due to fluffed up feathers. 

Blackbird song is beautiful and varied, often projected from a high perch. It serves to attract a mate and ward off any potential rivals (mate guarding). The timing and intensity of a blackbird's song is affected by multiple factors. As food availability increases, males sing earlier, for longer and with a higher peak intensity, suggesting singing is costly and an honest communication of the male's condition. Males also sing earlier and for longer when their mate is most fertile. In areas with more light pollution blackbirds can often be heard singing at night. Light pollution is also thought to affect the reproductive cycles of blackbirds - in one study (Dominoni et al. 2013)  males exposed to constant low levels of light at night failed to produce gonads in their second year. This could be due to unresponsiveness to photoperiods, or chronic stress induced by light pollution. 

The population of blackbirds in the UK appears to be currently stable, although it has fluctuated in the past; from the 1970s to mid-90s there was a population decline, then a 26% increase to 2008. Due to their large and stable population, the UK conservation status of T. merula  is green, though they are protected by the Wildlife and Countryside Act 1981. Although this species hasn't been as severely affected by changes in farming practices as some birds, blackbirds are impacted by management such as hedgerow removal, farmland drainage and use of agrichemicals. Individuals can help ensure the blackbird remains as common as it is by avoiding garden chemicals and planting trees and shrubs to provide nesting opportunities. Caterpillar food plants and plants with berries are especially good as they also provide food for blackbirds. 

More information on blackbirds and the issues surrounding them can be found here:

https://www.bto.org/understanding-birds/species-focus/blackbird
https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/animals/birds/blackbird/
https://www.rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z/blackbird/
Experimental manipulation of the dawn and dusk chorus in the blackbird Turdus merula
Dominoni et al. 2013 - Long-Term Effects of Chronic Light Pollution on Seasonal Functions of   European Blackbirds (Turdus merula)