First off, let me say that you have purposed an excellent question. See this fine article. The quotation below is taken from it.

The Torah, in Parshas Shmini, when informing us of the kashrus status of animals, lists specific types of birds that are not kosher. Ultimately, through derivation of the various “l’mineihu” words found in that section, Chazal (Chullin 63b, Rambam, Ma’achalos Assuros 1:14) identify 24 classes of birds that are not kosher, each one with many species. Any bird that is not from one of these 24 classes is kosher (Rashi, Chullin 61a d”h oaf, Shulchan Aruch YD 82:1). The problem is, however, that we can no longer identify the non-kosher birds that are mentioned.

The simple answer is that we do not have a tradition that the blue-footed booby is kosher, so we obstain from eating it because there is a small chance that it might be one of the forbidden fowls.

First off, let me say that you have purposed an excellent question. See this fine article. The quotation below is taken from it.

The Torah, in Parshas Shmini, when informing us of the kashrus status of animals, lists specific types of birds that are not kosher. Ultimately, through derivation of the various “l’mineihu” words found in that section, Chazal (Chullin 63b, Rambam, Ma’achalos Assuros 1:14) identify 24 classes of birds that are not kosher, each one with many species. Any bird that is not from one of these 24 classes is kosher (Rashi, Chullin 61a d”h oaf, Shulchan Aruch YD 82:1). The problem is, however, that we can no longer identify the non-kosher birds that are mentioned.

The simple answer is that we do not have a tradition that the blue-footed booby is kosher, so we abstain from eating it because there is a small chance that it might be one of the forbidden fowls.