A couple of pages that discuss the topic: http://www.deutschplus.net/pages/179 http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-N/Texte/s-und-es-Gen.html A rule of thumb I can give is that you should use -es if it is difficult to pronounce otherwise (e.g. certain consonants) and -s if the word already contains an e close to the end (e.g. Vogels). For some words, you can use both. I think grammatically, "Freunds" would also be correct (see e.g. https://de.wiktionary.org/wiki/Freund ) even though I would definitely prefer "Freundes". It also depends on the region you are in, some prefer -s, some -es.

More or less, you can say when a word consists only of one syllable, you must add -es, but of course, counting the single words also in compound words, e.g. 'des Hauses', but also 'des Hochhauses'.