As biparental care is crucial for breeding success in Procellariiformes seabirds (i.e., albatrosses and petrels), these species are expected to be choosy during pair formation. However, the choice of partners is limited in small-sized populations, which might lead to random pairing. In Procellariiformes, the consequences of such limitations for mating strategies have been examined in a single species. Here, we studied mate choice in another Procellariiforme, Bulwer’s petrel Bulweria bulwerii, in the Azores (ca 70 breeding pairs), where the species has suffered a dramatic population decline. We based our approach on both a 11-year demographic survey (capture-mark-recapture) and a genetic approach (microsatellites, n = 127 individuals). The genetic data suggest that this small population is not inbred and did not experience a genetic bottleneck. Moreover, pairing occurred randomly with respect to genetic relatedness, we detected no extrapair parentage (n = 35 offspring), and pair fecundity was unrelated to relatedness between partners. From our demographic survey, we detected no assortative mating with respect to body measurements and breeding experience and observed very few divorces, most of which were probably forced. This contrasts with the pattern previously observed in the much larger population from the Selvagens archipelago (assortative mating with respect to bill size and high divorce rate). We suggest that the Bulwer’s petrels from the Azores pair with any available partner and retain it as long as possible despite the fact that reproductive performance did not improve with pair common experience, possibly to avoid skipping breeding years in case of divorce. We recommend determining whether decreased choosiness during mate choice also occurs in reduced populations of other Procellariiform species. This might have implications for the conservation of small threatened seabird populations.

Field work was conducted on Vila islet, Santa Maria island, Azores archipelago, from 2002 to 2012 included. Adults were captured in their nesting burrows each year during incubation, and ringed for identification. Chicks were ringed before fledging. These capture-mark-recapture sessions enabled us to know the life-history of each ringed individual, year after year, that is, the nest it was occupying (nesting cavities were marked with individual numbers), whether or not it was breeding, the outcomes of its breeding attempts, the identity of its social partner(s) and its offspring. Adults were measured  (wing length using a stopped ruler to the nearest mm; tarsus length, culmen length and bill depth at the gonys using a vernier calliper to the nearest 0.1 mm).

Blood samples (50-100 µl) were collected from adults upon their first capture in 2002, 2003 and 2004. . Chicks were sampled a few days after hatching. We extracted bird DNA using the QIAmp Tissue Kit (QIAGEN). Eleven microsatellite loci (autosomal loci Bb2, Bb3, Bb7, Bb10, Bb12, Bb20, Bb21, Bb22, Bb23, Bb25, plus the sex-linked Bb11, Molecular Ecology Resources Primer Development Consortium 2010) were amplified by Polymerase Chain Reaction (PCR). Genotypes (number of base pairs at each allele for each locus) were analysed using GeneMapper 4.0 (Applied Biosystems). 118 adults (57 males, 61 females), including those that were genotyped, plus the offspring from 2002 to 2004 included, were sexed using molecular methods (Fridolfsson and Ellegren 1999, cited in our MS). The sex of 48 other adults (18 males, 30 females), including some chicks that later recruited into the breeding population, was inferred from that of their partner for which molecular sexing had been conducted.

To check if the demographic bottleneck experienced by Bulwer’s petrels in the Azores was associated with a genetic bottleneck, we used the BOTTLENECK software, which relies on the method of Cornuet and Luikart (1996, cited in our MS). Relatedness between social partners was estimated using MER (Wang 2002; version 3 downloadable from http://www.zoo.cam.ac.uk/ioz), after excluding the sex-linked locus Bb11.

We tested if there was an assortative mating based on body measurements or structural body size (PC1 scores of a Principal Component Analysis conducted on wing length, tarsus length and culmen length). To do this, we used two methods. First, we considered the pairs that were observed each year and we analysed our study years separately, after conducting Generalized Linear Models (GLMs) or Spearman rank correlations, according to whether or not the conditions for GLMs were met (that is, whether or not model residuals were normally distributed, Kéry and Hatfield 2003, cited in our MS). Second, we considered all the sexed pairs that were observed in our study together. In this situation, however, a given individual could be involved in several pair bonds (after e.g., the death of its former partner and/or a divorce). To overcome this problem, we used the MIXED procedure of SAS (with the Kenward-Roger degrees of freedom method, SAS Institute 2020), an equivalent of Generalized Linear Mixed Models which allows accounting for the correlations between observations concerning the same individual, can use data from individuals for which there are missing observations, allows within-individual effects to consist of continuous variables and to vary for the same individual, and analyses the data in their original form. To do this, we considered female (male) identity as a random effect.

To test whether pairing occurred at random with respect to genetic relatedness, we compared the relatedness of pair mates with that of male-female pairs drawn at random using a resampling procedure implemented in RESAMPLING PROCEDURES Version 1.3 (Howell 2001, cited in our MS), to account for non-independence of individual pairs. The procedure was repeated 5000 times.

To conduct parentage analyses, we compared chick genotypes with those of their social parents, and we excluded paternity (maternity) when the genotype of a chick mismatched that of its social father (mother) at two loci at least. A single mismatch between offspring and parental genotypes was interpreted as a mutation.

Only birds known to have made at least one breeding attempt in the past were used when calculating mate fidelity rates and determining the causes of divorce. Mate fidelity was defined as 1 minus the probability of divorce, the latter parameter being the total number of divorces divided by the total number of pair × years when both previous partners survive from one year to the next during the study period (Black 1996, cited in our MS).

To determine whether (1) reproductive performance (i.e., the probability of fledging  chick) increased with pair common experience and (2) whether the probability of divorce depended on pair common experience and previous reproductive performance, we performed logistic regerssions for repeated measures (GENMOD procedure of SAS, binomial distribution, logit link, with the pair as  the 'repeated' subject). Results from these logistic regressions were obtained from the models using generalized estimating equations (GEE).

More details are given in the main text of our MS.

Some individuals have missing values. However such individuals can be taken into account when using the GENMOD procedure as we did.