Our main result—AP management increases bird abundances—is robust to using corrected data that holds constant the share of observations in bird-abundant locations post-transference. The regression coefficients are quite similar to those from our primary specification (Figure S3b). The average effect remains large and statistically significant (ATT = 0.277 with a standard error of .072), which represents an increase in bird abundances of 32% (compared to 37% in our primary specification).

# **A.2.2 Robustness to Changes in Birder Seasonality (Figure S4)**

The second concern parallels the first, except rather than studying whether AP shifts _where_ birding trips occur we study whether AP shifts _when_ birding trips occur. Bird abundances naturally fluctuate over the course of a year due to seasonal bird migration. If AP increases the share of birding trips that occur in bird-abundant months of the year, then we would overestimate the increase in bird abundances due to AP management. To assess this possibility and its implications for our main result, we implement the identical procedure described in SI A.2.1, except now month of year is the dimension of interest rather than a 0.1<sup>_◦_</sup> grid cell. Month of year refers to the same month every year (e.g., “January”, as opposed to “January 2018”).

Prior to transference, there is little trend in the probability that the proportion of observations in bird-abundant months of the year is above the median (Figure S4a). However, there is a statistically significant increase on average over the eleven years of AP management we consider (ATT = 0.046 with a standard error of 0.015). It appears that part of the post-transference increase in bird abundances is due to a greater share of observations occurring in protected areas’ more bird-abundant months of the year.

We assess the magnitude of this upward bias in the same manner as in SI A.2.1. Since AP increases by 4.6 percentage points the share of observations in bird-abundant months of the year, we replace this share of observations with values from months of the year with below median bird abundance and then re-estimate our primary specification. For each protected area transferred to AP management, we randomly replace the log bird abundance and effort control variables with the mean values of the protected area’s below median abundance months of the year until the resulting share of post-transference observations “in” bird-abundant months of the year is 4.6 percentage points lower.

Our main result is robust to using corrected data that holds constant the share of observations in bird-abundant months of the year post-transference (Figure S4b). The average effect remains large and statistically significant (ATT = 0.240 with a standard error of 0.072), which represents an increase in bird abundances of 27%. Correcting for the compositional change in seasonality decreases the ATT by 0.078 log points (from 0.318 to 0.240). Recall from SI A.2.1 that correcting for the compositional change in location decreased the ATT by 0.041 log points (from 0.318 to 0.277). If the effects of the two compositional shifts were additive, we would obtain an ATT of 0.199, representing an increase in bird abundances of 22%.

# **A.2.3 Robustness to Changes in Birder Skill (Figure S5)**

The third concern relates to changes in who visits protected areas post-transference. If eBird observations are more likely to be submitted by individuals who are more 6