SOURCE: This is a Latin version of Aphthonius's Greek fables; you can see them side-by-side with the Greek text in Nevelet's 1660 edition of Fabulae Variorum Auctorum at GoogleBooks. This is fable 40 in Aphthonius; for other versions, see Perry 398.
READ OUT LOUD. Choose which marked text you prefer to practice with - macrons or accent marks - and read the text out loud until you feel comfortable and confident. Then, try reading the unmarked text at the bottom. It should be easy for you after practicing with the marked texts. :-)
MACRONS. Here is the text with macrons:
Cum cȳcnum vīdisset corvus, aemulārī colōrem voluit. Sed arbitrātus ē lavācrīs eum prōvenīre, relictīs ārīs, unde nūtriēbātur, ad lacūs et flūmina vītam ēgit, et cum lavāret quidem corpus, nōn mūtāvit, pastū vērō dēstitūtus mortuus est.
ACCENT MARKS. Here is the text with accent marks, plus some color-coding for the words of three or more syllables (blue: penultimate stress; red: antepenultimate stress):
Cum cycnum vidísset corvus, aemulári colórem vóluit. Sed arbitrátus e lavácris eum proveníre, relíctis aris, unde nutriebátur, ad lacus et flúmina vitam egit, et cum laváret quidem corpus, non mutávit, pastu vero destitútus mórtuus est.
UNMARKED TEXT. Here is the unmarked text - after practicing with the marked text that you prefer, you should not have any trouble with the unmarked text:
Cum cycnum vidisset
corvus,
aemulari colorem
voluit.
Sed arbitratus
e lavacris eum provenire,
relictis aris,
unde nutriebatur,
ad lacus et flumina
vitam egit,
et cum lavaret quidem corpus,
non mutavit,
pastu vero destitutus
mortuus est.
IMAGE. Here is an illustration for the story (image source) showing a very white swan!