Cooperatively breeding birds employ a variety of mating strategies, and we do not fully understand breeding group structure and the range of reproductive strategies used by group members. We examined group structure and parentage in a population of Brown-headed Nuthatches (Sitta pusilla). We genotyped 346 adults and nestlings banded at 59 nests using nine microsatellite loci to determine parentage and relatedness and compared the results to field observations of marked individuals from 282 nests monitored from 2006–2010. Based on field observations, 23% of nests were cooperative, and only 13% of the cooperative groups had more than one helper. Consistent with field observations, genetic analyses indicated that 83% of helpers were related to at least one parent at the nest, and that most helpers were male. Extra-group paternity appeared to be common at nests of both pairs (45%) and cooperative groups (32%, but did not differ in frequency between the two nest types. Although male helpers unrelated to the breeding female might be expected to sire offspring in cooperative groups, we did not observe this phenomenon. Instead, we found one, and possibly two, cases of incestuous breeding involving related helpers.