Table 1. Sample sizes of the field experiment to study the effect of photostimulation on laying dates in wild living great tits (Parus major).

		Treatment	Sample size	Birds with laying date	Percentage with lay date
Oosterhout	2009	Indoor – one long day	10	6	59%
		Indoor – natural day length	10	6
		Not taken indoors	7	4
	2010	Indoor – one long day	10	7	72%
		Indoor – natural day length	10	5
		Not taken indoors	16	14
Hoge Veluwe	2009	Indoor – one long day	15	7	53%
		Indoor – natural day length	14	7
		Not taken indoors	11	7
Total indoor			69	38	55%
Total not taken indoor			34	25	73%
Totals			103	63

We created 3 experimental groups: (i) photostimulated females (ii) females which experienced natural day length in captivity and (iii) females which experienced natural day length in the field. Females from group (i) and (ii) were kept indoors for one day and two nights after which they were released in the field at the location where they were caught.