Intraspecific phenotypic variability is key to respond to environmental changes and anomalies. However, documenting the emergence of behavioral diversification in natural populations has remained elusive due to the difficulty of observing such phenomenon at the right time and place. Here, we investigated how the emergence of a new trophic strategy in a population subjected to high fluctuations in the availability of its main trophic resource (migrating songbirds) affected the breeding performance, population structure, and population fitness of a specialized color polymorphic predator, the Eleonora's falcon from the Canary Islands. Using long-term data (2007-2022), we found that the exploitation of an alternative prey (a local petrel species) was associated with the growth of a previously residual falcon colony. Pairs in this colony laid earlier and raised more fledglings than in the other established colonies. The specialization on petreles increased over time, independently of annual fluctuations in prey availability. Importantly, however, the positive effect of petrel consumption on productivity was stronger in years with lower food availability. This trophic diversification was further associated with the genetically-determined color morph, with dark individuals preying more frequently on petreles than pale ones, which might promote the long-term maintenance of genotypic and phenotypic diversity. We empirically demonstrate how the emergence of an alternative trophic strategy can buffer populations against harsh environmental fluctuations by stabilizing their productivity.

The Eleonora's falcon population from the Canary Islands was intensively monitored during July-October 2007-2022. Every year and for each nest-site, we recorded the color morph of both parents using a spotting scope.We visited nests (range = 73-113 nests/year) on at least 2 occasions each year, first to quantify clutch size, and posteriorly to determine the final number of fledged offspring. Laying date (where 1 = July 1^st) was calculated by substracting 30 days (the duration of the incubation period) to the hatching date of the older chick within a brood, which was calculated after measuring nestling wing length to estimate their age using the formula provided by Ristow and Wink (2004).The number of fledglings was estimated as the final number of fledged offspring from each nest. Mean annual productivity of the population was estimated as the number of fledglings divided by the number of breeding pairs monitored. We recorded the number of petrels hunted by quantifing the number of intact wing pairs that falcons do not consume within a 5-m radius of the nest-site by inspecting the nest and nearby larders.

We calculated food availability during September (the chick-rearing period in Eleonora's falcon) for every study year using the intensity of easterly winds across the migratory corridor that connects Europe (southwest Iberian Peninsula) and the Canary Islands. These wind patterns have proven to control the number os migratory birds arriving to Alegranza during the falcons' breeding season, explaining 70% of the variation in annual productivity (see Gangoso et al. 2020 for details). We calculated the mean easterly wind intensity as a proxy of overall food availability.