Data collection

We carried out this study in three of the main markets in Mexico City, where wildlife is traded: the Sonora market, which is considered the busiest center of wildlife commerce (^{Anzures and Bolaños 1991}); the Nuevo San Lazaro market; and the Xochimilco market. The Xochimilco market is one of the oldest in the country (^{Colectivo Xochimilco no te mueras 2009}), where bird traders’ stalls have been bequeathed by their ancestors (^{Roldán-Clarà and Toledo 2017}). We visited each market 10 times (once per month) during two periods: the first period spanned from September 2012 to June 2013, and the second period from September 2015 to June 2016. We only carried out 10 visits to the markets due to the difficulty of gathering direct information from interviews with merchants who were fearful of providing data on protected or prohibited species. We thereby made a total of 520 visits to the stalls of all three markets: 260 visits during the 10 months of each period.

We recorded birds being sold at 18 permanent stalls in the Sonora market, five stalls in the Xochimilco market, and three stalls in San Lazaro market. Observations were conducted by GGA and SRRG. During each visit, we identified each bird species for sale, and whether they were native (distribution area includes Mexico) or non-native species. All bird species and the number of individuals were identified by direct observation using bird field guides (^{Howell and Webb 1995}; ^{Sibley 2001}). The scientific names were updated using the Birds of North and Middle America Checklist, from the American Ornithological Society (^{Chesser et al. 2019}).

We grouped bird species into three categories (^{Gómez-Álvarez et al. 2005a},^b): 1) ornamental and companion birds, which included members of the Psittaciformes (parrots) order, mainly the families of Cacatuidae, Psittacidae and Psittaculidae that are preferred by buyers because of their colors and ability to communicate with people; 2) ornamental birds including Columbiformes (Columbidae: doves) and Passeriformes (Corvidae, Sturdidae, Thraupidae, Cardinalidae, Strilidae), generally with colorful plumage; 3) songbirds (Turdidae, Mimidae, Ptilogonatidae, Emberizidae, Icteridae, Fringillidae) in demand because of their rich songs, and in some cases their attractive plumage.

Data analysis

Shapiro-Wilk test showed that data did not conform to a normal distribution. Therefore, to determine if there was an increase in the number of individuals and species traded in the markets over time, we used Wilcoxon tests to compare between periods the overall number of individuals, as well as the number of individuals of native species, endemic species, non-native species, Psittaciformes, songbirds, and number of individuals of ornamental species between periods. All statistical tests were performed with Megastat ver. 10.4, and Past ver. 4.11, and were considered significant at alfa of 0.05.

Number of species and individuals

We recorded a total of 74 bird species, 64 species in the September to June period of 2012-2013, and 62 species in the 2015-2016 period (Table 1). We recorded a total of 20,388 individuals comprising all species registered in 2012-2013 in the three markets, and 20,004 individuals of all species registered in 2015-2016 in the three markets. We found no significant differences in the overall number of individuals recorded for sale in markets between the two periods. The families with the highest number of species were Psittacidae with 16 species, Icteridae (blackbirds and orioles) with 12 species, and Cardinalidae (grosbeaks and buntings) with eight species. The families with the greatest number of individuals for sale were Fringillidae (n = 8,808 birds), Strildidae (n = 8,621 birds) and Psittacidae (n = 8,289 birds) The number of individuals by family also did not differ between periods for Psittacidae, Columbidae, Corvidae, Turdidae, Mimidae, Thraupidae, Emberizidae, Cardinalidae, Icteridae, Fringillidae, and Strildidae.

Companion, ornamental and songbirds

Regarding companion birds, we recorded a total of 22 species of Psittaciformes, most of which were from the Psittacidae family (17 species). The number of birds of parrot species for sale did not differ between the two survey periods (W = 117, P > 0.05). The non-native Budgerigar (Melopsittacus undulatus) had the largest number of 3,523 - 4,256 individuals for sale in markets (Table 1). Most of the individuals for sale in both periods were non-native species (Myiopsitta monacus, Melopsittacus undulatus and Agapornis spp.). However, we registered 11 native parrot species (Table 1), the sale of which are prohibited by Mexican law (^{DOF 2008}). Of these, three species are endemic to Mexico and considered nationally threatened or endangered (^{DOF 2010}): Red-crowned Parrot (Amazona viridigenalis; 4 birds), Lilac-crowned Parrot (A. finschi; 1 bird), and White-crowned Parrot (Pionus senilis; 1 bird). The most traded native parrot species was the Orange-fronted Parakeet (Eupsittula canicularis) with a total of 55 individuals recorded for sale (Table 1). Notably, the Thick-billed Parrot (Rhynchopsitta pachyrhyncha) was the second-most traded native parrot species (n = 20 birds; Table 1). I

We also recorded a total of 24 species of songbirds, almost all of which were native species. The non-native songbird species that was most abundant in markets was the Island Canary (Serinus canaria), with 2,824 individuals for sale in 2012-2013 and 2,487 individuals for sale in 2015-2016 (Table 1). We found significant differences between both survey periods in the number of individuals of songbirds for sale (W = 228, P < 0.05), increasing from 3,625 individuals in 2012-2013to 4,109 individuals in 2015-2016. Five native songbird species are in a risk category, with the Slate-colored Solitaire (Myadestes unicolor) considered nationally Threatened (DOF 2022), and four species in Special Protection status (^{DOF 2010}): Icterus pustulatus (2012-2013: 51 birds, 2015-2016: 43 birds); I. spurius (2012-2013: 15 birds, 2015-2016: 93 birds); Turdus rufopalliatus (2012-2013: 20 birds, 2015-2016: 130 birds); and Myadestes occidentalis (2012-2013: 18 birds, 2015-2016: 151 birds).

On the other hand, we did not find significant differences between survey periods in the number of individuals of ornamental birds for sale. We recorded 10 native species, of which the Green Jay (Cyanocorax yncas) had the highest number of individuals with 153 and 183 individuals in the 2012-2013 and 2015-2016 survey periods respectively (Table 1). The non-native species, the Zebra Finch (Taeniopygia guttata), had the highest number of individuals for sale in the 2012-2013 and 2015-2016 periods, with 3,285 and 2,730 individuals respectively (Table 1). Regarding the Passeriformes we found significant differences between both survey periods in the number of individuals recorded for sale (W = 630, P < 0.05), with 2,058 birds in 2012-2013 and 2,917 birds in 2015-2016.

Conservation status

For both periods, most of the avian species registered for trade in markets were classified as Least Concern in the IUCN Red List (Fig. 1). Nevertheless, six species classified internationally as Endangered (^{IUCN Red List 2022}) were also traded, mainly parrots (Table 1). Of these, the Thick-billed Parrot stands out as having the most individuals for sale (n = 20 birds). Other internationally endangered species found for sale were the Java Sparrow (Lonchura oryzivora; 5 birds), Sun Parakeet (Aratinga solstitialis; 3 birds), and Yellow-headed Parrot (Amazona oratrix; 3 birds), as well as the Lilac-crowned Parrot and Red-crowned Parrot.

Figure 1 Number of bird species for sale in Mexico City markets during two survey periods, grouped by national at risk categories (NOM-059-2010), international IUCN Red List, and in the CITES Appendices. 

Another two parrot species are internationally considered Vulnerable (^{IUCN Red List 2022}), predominantly the Orange-fronted Parakeet with 55 individuals registered for sale, and one individual of the Salmon-crested Cockatoo (Cacatua moluccensis). Eight species were also nationally considered Threatened or Endangered (^{DOF 2010}), of which the Slate-colored Solitaire (Myadestes unicolor) stands out with the largest total number of 369 individuals for sale (Table 1). Finally, six species of Psittaciformes that are in CITES Appendix I prohibiting their trade were also for sale, where again the Thick-billed Parrot had the largest number of individuals for sale (Table 1).

Native and non-native birds

For native species, we found a significant increase in the number of individuals recorded for sale between the first and second survey period (W= 854, P < 0.05), from 1,892 individuals in 2012-2013 to 4,506 individuals in 2015-2016. The native species with the highest number of individuals in both periods was the House Finch (Haemorhous mexicanus) that increased from 576 individuals in 2012-2013 to 1,369 individuals in 2015-2016 (Table 1). The second-most abundant native species in 2012-2013 was the Slate-colored Solitaire with 202 individuals, while in the 2015-2016 period, the second-most abundant species was the Painted Bunting (Passerina ciris), increasing from 32 individuals in 2012-2013 to 473 individuals in 2015-2016.

On the other hand, although the number of individuals of endemic species increased between periods, from 234 individuals in 2012-2013 to 332 individuals in 2015-2016, this was not significantly different. In 2012-2013, the endemic species with the highest number of 141 individuals for sale was the Northern Cardinal (Cardinalis cardinalis), with other endemic species having an average 13.3 ± individuals for sale. However, in 2015-2016, the Rufous-backed Robin (Turdus rufopalliatus) increased from 20 individuals in the first period to 130 individuals for sale, and the Orchard Oriole (Icterus spurius) increased from 15 to 93 individuals for sale.

Regarding non-native species, no significant differences were found in the number of individuals between periods, with 18,148 non-native birds in 2012-2013 individuals and 15,166 individuals in 2015-2016. In this case, the species that had most individuals for sale in both periods were the Budgerigar with 4,256 individuals in 2012-2013, and 3,526 birds in 2015-2016, and the Zebra Finch with an overall total of 6,015 individuals for sale (Table 1).

Sale of companion, ornamental and songbirds

Psittacidae (parrots), Icteridae (blackbirds and orioles), and Cardinalidae (grosbeaks and buntings) were the families with most species for sale, while finches (Fringillidae and Strildidae) and parrots had the highest numbers of̴̴ individuals for sale in Mexico City markets. These are mainly song and ornamental or companion birds that are likely to be the most exploited (^{Iñigo-Elias and Ramos 1991}), particularly song and ornamental birds (^{Roldán-Clarà et al. 2014}). ^{López-Medellín (2003)} also reported that one of the most commercialized families is the Cardinalidae (cardinals, grosbeaks and goldfinches).

We found a higher number of non-native parrots, clearly observed in the case of the Lovebird (Agapornis spp) from Africa, Budgerigar from Australia, and the Monk Parakeet (Myiopsitta monachus) originally from Uruguay, Bolivia, Brazil and Argentina (^{Forshaw 2010}). The case of Monk Parakeet is relevant because the species was probably introduced to the national trade after 2005, since it was not registered by ^{Gómez-Alvarez et al. (2005a)} as a captive parrot species in Mexico. The Monk Parakeet has since been registered as a free-living non-native species in Mexico’s central states (^{MacGregor-Fors et al. 2011}), in the southern state of Oaxaca (^{Pablo-López 2009}), and in the northern state of Sinaloa (^{Gurrola-López et al. 2023}) and has now been reported in 97 cities across all 7 biogeographical regions in Mexico (^{Hobson et al. 2017}). The predominant means of entry of the Monk Parakeet to Mexico may have been via the international pet trade, as over half a million Monk Parakeets were commercially imported to Mexico during 2000-2015 (^{Hobson et al. 2017}). Sale of the Monk Parakeet in national markets then facilitates introduction of this exotic species after birds are intentionally released or escape from their cages, and successfully become established in the wild (^{Tinajero and Rodríguez-Estrella 2015}).

Conservation status

Many parrot species that are considered internationally Endangered and included in CITES Appendix I prohibiting their trade were also found for sale in Mexico City markets. Notably, the Thick-billed Parrot and Orange-fronted Parakeet were sold in higher numbers (20 and 55 birds respectively). The nationally threatened Slate-colored Solitaire was also on sale (̴̴̴̴150 birds per period). These birds are sold as pets (^{Iñigo-Elias and Ramos 1991}) because of their colorful plumage, melodic songs, their attachment to humans, and because they can mimic the sound of human speach (^{López-Medellín 2003}).

We recorded 11 native species of Psittaciformes on sale in this study despite the prohibition in Mexico (^{DOF 2008}). The number of native parrots found for sale in markets during the present study may be lower than the real number captured, since there may be a mortality of 50% to 70% of birds within the commercial process (^{Cantú-Guzmán et al. 2007}, ^{SEMARNAT 2012}). Taking this into account, the number of native and endemic parrots that could have been extracted from the wild is not negligible. Furthermore, most native parrot species have low population densities, reproductive rates, and a more precarious conservation status (^{Gómez-Álvarez et al. 2005a}), making trade in these species detrimental to wild populations.

Supply and demand of native and non-native birds

We found no differences in the overall number of individuals and number of non-native birds sold in Mexico City markets between the 2012-2013 and 2015-2016 survey periods. These may represent birds that have remained unsold between the two survey periods, but the high numbers of non-native birds in both survey periods (̴15,000 - 18,000 birds) suggests that there is high demand for these species and sellers constantly obtain new birds for sale. A lack of difference between periods in the numbers of individuals for sale of avian families that represent songbirds, parrots and ornamental passerines in Mexico City markets also reflects the persistence of buyers for acquiring birds that provide company, are attractive, and have melodious songs (^{Corona-Martínez 2002}; ^{Nijman 2010}; ^{Alves et al. 2013b}, ²⁰¹⁶; ^{Nascimiento et al. 2015}; ^{Rocha et al. 2017}). The significant increase in numbers of native birds and of songbirds for sale in Mexico City markets further reflects a high demand for song and ornamental birds.

The high number of species and individuals of birds that we found for sale in markets demonstrates their continued high demand. Educational and outreach programs may help to reduce the trade of wild birds in markets, particularly for native or protected species with declining populations. Such campaigns have been implemented in Costa Rica (^{Nascimiento et al. 2015}) and Brazil (^{Rocha et al. 2017}) where they helped to raise awareness of the negative impacts of trade in native avifauna and reduce their demand in markets. Therefore, actions like this can be positive to decrease the commercial demand of wild species. The Center for Education and Training for Sustainable Development prepares and disseminates educational materials that contribute to the sustainable use of our natural resources and to the formation of an informed, responsible, and participatory environmental citizenry (^{SEMARNAT 2023}). However, such campaigns can be expensive, especially in big cities like Mexico City, therefore economic resources from the federal government and private institutions are needed as well as the participation of conservation and communications specialists.