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In this thesis, I analysed the mating behaviour of the solitary-nesting American kestrel (Falco sparverius) in southern Quebec (Canada) and the colony-nesting lesser kestrel (F. naumanni) in Aragon (Spain). DNA fingerprinting of 26 families of lesser kestrels revealed that 3.4% of nestlings were extra-pair, which may have arisen through either extra-pair copulation or mate replacement. Two nestlings in two different nests were also the result of intraspecific brood parasitism. DNA fingerprinting of 21 American kestrel families showed that all the nestlings in two nests were extra-pair (10% extra-pair young overall), most probably due to mate replacement. / I analysed the mating behaviour of both species in two studies with a similar aim, i.e. to test why mated pairs copulate so frequently. Sixteen pairs of wild American kestrels and 12 pairs of "solitary" lesser kestrels (14 nests per 0.3 km$ sp2)$ were analysed in terms of four hypotheses that explain high frequency of within-pair copulations both outside and during the fertile period. First the Paternity Assurance Hypothesis, i.e. males control timing and frequency of copulations to best assure fertilization, was rejected because extra-pair copulation attempts were low in both species ($<$1% of all copulations observed), within-pair copulation frequencies did not increase with nest density in the lesser kestrel, and copulation and mate attendance did not increase as the fertile period approached. Second, the Immediate Material Benefits Hypothesis, i.e. females trade copulations for food, was refuted because copulation most often occurred without food transfers. Third, the Female Mate-Guarding of Males Hypothesis, i.e. females distract their mates from other mating opportunities by copulating frequently, was rejected because male loss was low, males and females solicited similar amounts of copulations, and females did not differ in the timing or frequency of solicitations. Finally, the Mate Assessment Hypothesis, i.e. assessment of mate quality is mediated by copulation, most closely predicted the behaviour observed since within-pair copulation was high outside the fertile period and during pair formation in both species.