Variation in population size, nest distribution, colony extent, and timing of movements at the largest known parrot colony

Psittaciformes are among the most threatened birds, and population size and trend estimates are needed to aid conservation. The burrowing parrot (Cyanoliseus patagonus) is undergoing substantial changes in its population size, due to habitat degradation, trapping for the pet trade, and persecution as a crop pest. The species formerly had several large colonies, but these were severely reduced during a program of agriculture pest control. Currently, only one large colony remains, together with a decreasing number of smaller colonies spread throughout the Monte and adjacent ecoregions. The colony at El Cóndor, north-eastern Patagonia, Argentina, is of particular importance, concentrating 71% of the global breeding population of burrowing parrots. We aimed to determine changes in 1) breeding population size, 2) the extent of the colony, 3) the distribution of nests along the colony, and 4) the timing of movements from the nest to the foraging locations during the breeding season, over our long-term study (1998-2019). We found that the number of active nests declined from 37531 in 2001 to 21883 in 2005, and recovered in the following years, reaching 40671 nests in 2019. The decline observed in 2005 could be related to droughts and associated food shortage during the La Niña phase of El Niño Southern Oscillation. The colony expanded westwards, from 9 km length in 2001, to 18.1 km in 2019. During the same period, the part of the colony with the highest number of active nests shifted 5 km westwards from El Cóndor village. Both changes could be the consequence of human disturbance. As any serious threat affecting this colony could have severe consequences for the survival of the species, regular monitoring is needed, together with legal protection of the burrowing parrot colony at El Cóndor and the Monte ecosystem around it.

Methods
The dataset consist of photographs taken at the burrowing parrot Cyanoliseus patagonus colony at El Cóndor, village, adjacent to the estuary of the Río Negro in Río Negro, north-eastern Patagonia, Argentina, from 2001 to 2019. 

During the earlier years of current study (breeding seasons 2004-2005, and 2005-2006), we followed the methodology of Masello et al. (2006) to estimate of the breeding population size of El Cóndor colony (breeding season 2001-2002, hereafter 2001). We photographed the entirety of the densest part of the colony (easternmost; Table 1). This is a part of the cliff, up to 27 m high, where soft sandstone layers belonging to the Río Negro geological formation make the entire cliff. In this part of the colony, the parrots dig their nests between 3 and 26.5 m above sea level. However, for the westernmost part of the colony, sample photographs were taken every 450 m, and the total number of nests in the 450-m sector was extrapolated from these photographs (Masello et al. 2006). In more recent years (breeding seasons 2007-2008, 2008-2009, and 2019-2020), following advances in digital photography, we took a continuous series of digital photographs (3000 x 4000 pixels) covering the entire colony. This means, that the nest counts for those later years represent a complete census of the colony.

We took all photographs of the Burrowing Parrot colony from the sandy beach exposed at the bottom of the cliff during low tide. The colony is accessible from its easternmost end (kilometre 1), and from three man-made accesses cut in the cliff and situated at the easternmost part of kilometre 2 (car access called ‘Segunda Bajada del Faro’), in kilometre 15 (pedestrian access, ‘El Espigón’), and at the easternmost part of kilometre 17 (pedestrian access, ‘Playa Bonita’). Using those accesses, we were able to walk along the cliff, from the vicinity of El Cóndor village to up to three kilometres past the pedestrian access at kilometre 17; thus, we were able to survey the entire Burrowing Parrot colony. During low tide, we were able to shoot photographs from 150 to 170 m away from the cliff. This, resulted in larger sector of the cliff included in each photograph (approximately 200 m). During high tide, we had to take the photographs closer to the cliff (40 to 50 m), resulting in shorter sectors being captured per photograph (approximately 50 m). Due to the sheer size of the colony, we took the photographs over four to five days of fieldwork during each studied season. For this reason, the photographs show small differences in illumination. To minimize such differences, and to avoid strong shadows that could create false structures in the images, we took the photographs during overcast days.

Masello, J.F., Pagnossin, M.L., Sommer, C., Quillfeldt, P. 2006. Population size, provisioning frequency, flock size and foraging range at the largest known colony of Psittaciformes: the Burrowing Parrots of the north-eastern Patagonian coastal cliffs. Emu, 106: 69-79.