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Simple Summary: Monk Parakeets are invasive birds globally. Through the construction of communal nests, they shape urban bird life. Monk Parakeets were introduced in Chile in 1972 and are now widely distributed in urban environments. Through the description of ecological parameters, we aim to assess the state of the Monk Parakeet in Santiago of Chile after 50 years of invasion. We characterized 1458 Monk Parakeets' nests on 546 trees, 91% of which were introduced species. Tasmanian blue gum and black locust were the trees preferred by parakeets for nesting. The average nest height was at 14.2 m with an occupancy rate of 89.7% and associated to nest height. During two reproductive seasons, chambers had an average of 4.5 eggs and 4.2 nestlings. Results and conclusions obtained will help in understanding the ecology of this avian invasion. Other urban metropolises such as Washington DC, Mexico City, Rome, Berlin, Paris, London, Tokyo, Nairobi, Casablanca and City of Singapore also suffer Monk Parakeet invasion. To aid in the understanding of Monk Parakeet invasive biology, we aim to contribute to better informed decisions in invasive synanthropic species management. Monk Parakeets are considered one of the most invasive bird species given its unique capacity among psittacines to build their own communal nests. Originally introduced as pets in houses from where they escaped or were released, they are currently considered invasive in more than 20 countries worldwide. This is the case in Chile, where Monk Parakeets were introduced during the 1970s. Between 2016 and 2019 we searched Monk Parakeets' nests structures in the Santiago metropolis region. We identified 1458 Monk Parakeets' communal nests on 546 trees belonging to 34 tree species. Ninety-one percent of the occupied trees were also introduced. Paraná pine and cedar of Lebanon were the tree species with highest abundance of nests, averaging more than four nests/tree/species, with 23 and 18 maximum number of nests, respectively. Tasmanian blue gum and black locust were selected by parakeets more often than expected, based on availability. From all trees, 24.6% denoted health problems and 47.3% were pruned. The average nest height was 14.2 m and nests were observed mainly in secondary branches (59.3%). The occupancy rate was 89.7% and was associated to nest height and type of branch. During two reproductive seasons we quantified eggs and nestlings in chambers averaging 4.5 and 4.2, respectively. We provide a rough population size estimate and the characteristics of Monk Parakeets nest and tree selectivity, aiming to characterize several decades of a neglected urban invasion to warrant strategies for improved management measures. [ABSTRACT FROM AUTHOR]