According to Wikipedia: "The untidy nest is composed of hay, grass, wool or other material and lined with feathers, which improve the thermal insulation. A complete nest consists of three layers; base, lining and dome. The typical clutch is five or six eggs, white to pale grey and heavily marked with spots, small blotches, or speckling; they are 20 x 14 mm in size and weigh 2.1 g, of which 7% is shell. The eggs are incubated by both parents for 12–13 days before the altricial, naked chicks hatch, and a further 15–18 days elapse before they fledge. Two or three broods may be raised each year; birds breeding in colonies produce more eggs and fledglings from their first broods than solitary pairs, but the reverse is true for second and third clutches. Females which copulate frequently tend to lay more eggs and have a shorter incubation time, so within-pair mating may be an indicator of the pairs' reproductive ability. There is a significant level of promiscuity; in a Hungarian study, more than 9% of chicks were sired by extra-pair males, and 20% of the broods contained at least one extra-pair young."
In my opinion the nest is not untidy, actually. I can confirm the three layers though. I can also confirm that both parents incubated the eggs. 15-18 days is quite a short period (it is ca. 3 weeks, i.e. 18-21 days with Tit hathclings). I cannot say how many eggs there were. I wonder if there will be a second clutch.
I found a couple of papers on Eurasian Tree Sparrows, so I might have some more to report soon.
Here some interesting links:
http://blx1.bto.org/birdfacts/results/bob15980.htm
http://www.ibercajalav.net/img/421_TreeSparrowP.montanus.pdf
http://www.iucnredlist.org/apps/redlist/details/149116/0
http://ibc.lynxeds.com/species/eurasian-tree-sparrow-passer-montanus
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