Contrary to common belief, learning signs (or signed words) is not easier than speech in infants.
Scientific studies show evidence that language development is maturationally brain-controlled, regardless of modality (speech or signlan).
Phonology is the study of how the smallest units of language (i.e. phonemes in speech and primes in signlan) are organized in a language and how they are learned.
There are about 40 phonemes or sound units in English speech. They are the building blocks of the language (English).
On the other hand, there are five sets (parameters) of primes (the smallest units of language) in sign language: handshape, movement, location, palm orientation, and non-manual marker.
Each parameter has a number of primes: over 40 handshapes, a number of movements, a number of locations, palm orientations, and a number of non-manual marker.
Below explains some ASL phonological processes in infants compared to speech.
For the first few months of life, babies produce vegetative sounds and later cooing sounds in speech. Likewise, they produce cooing visues in signlan.
Canonical (reduplicated) babbling begins to emerge at 6-7 months. Manual babbling stages occurs on the same timeline as vocal babbling (from marginal and canonical to variegated babbling).
Babies babble similar units all over the world. But, at 8 months, they babble a patterned set of units that are characteristic to a native language (whether ASL, English or any other language).
The reduplicated syllabic units "babababa" is a typical example in vocal canonical babbling. This parallels to "BABABA" (opening-and-closing movement of the hand) in manual babbling. Another common movement is batting -- moving up and down repeatedly.
The manual babbling sequence "BABABA" is often mistook for the first ASL word "milk". Manual babbling follows its own syllabic organization of a signed language.
The first handshapes in manual babbling that predominate in infants's early ASL productions are 5, index finger, B, A and fist (or in a linguistic term: 5, G, B, A, S respectively). Whereas, the early syllabic units (phonemes) in speech are: a, p, b, d.
Transition from manual babbling to first words occur at around 10 months, or as early as 8 months or as late as 16 months.
Toddlers begin with a small number of phonemes (sounds) in speech or primes (visues) in signlan to form a word.
For example, toddlers, who are exposed to a native signed language from birth, use these handshapes to form ASL words: B, C, O, A, S, 1 (index finger), and 5 (open handshape).
In speech, the consonant sounds are well established at this stage: m n p b t d w.
The following ASL handshapes of increasing complexity emerge at this stage: L, G, F, Q, D, Z, Y, I, J. This set may vary a bit from child to child.
More handshapes increase complexity as follows: V, H, W, U, T, H, K, P, X, Y, R, E, M, N. This set may vary a bit from child to child.
Not only the phonological processes in speech and signlan are somehow parallel, but also studies (Petitto) show that the milestones from babbling to multi-word are on the same timeline in both signlan and speech.
Regardless of the physical development, language milestones are controlled by the language-specific brain.
For more information about phonological acquisition by native-signlan children, below is a resource for further reading.
Boyes-Braem, P. (1973/1990). Acquisition of the handshape in American Sign Language. In V. Volterra & C. J. Erting (Eds.), From gesture to language in hearing and deaf children (pp. 107-127). New York: Springer-Verlag.
Conlin, K., Mirus, G. R., Mauk, C., & Meier, R. P. (2000). Acquisition of first signs: Place, handshape, and movement. In C. Chamberlain, J. Morford + R. I. Mayberry (Eds.), Language acquisition by eye (pp. 51-70). Mahwah, NJ: Lawrence Erlbaum.
Karnopp, Lodenir. (2002). Phonology acquisition in Brazilian Sign Language. In Gary Morgan and Bencie Woll (eds.), "Directions in Sign Language Acquisition". Amsterdam: John Benjamins. pp. 29-53.
Lillo-Martin, Diane, et al. "The Acquisition of Spatially Organized Syntax" August 1985. http://www.eric.ed.gov/PDFS/ED262568.pdf
Marentette, P. & Mayberry, R. (2000). Principles for an emerging phonological system: A case study of language acquisition by eye. In C. Chamberlain, J. Morford & R. Mayberry (Eds.), Language Acquisition by Eye. Mahwah, NJ: Lawrence Erlbaum.
Marentette, Paula F. (1989) "Babbling in Sign Language: Implications for Maturational Processes of Language in the Developing Brain."
Meier, Richard. (2005). The form of early signs: Explaining signing children's articulatory development. In Brenda Schick, Marc Marschark and Patricia Spencer (eds.), "Advances in the Sign Language Development of Deaf Children". pp. 202-230.
Gary Morgan, et al. "The first signs of language: phonological development in British Sign Language". City University, London. http://www.staff.city.ac.uk/g.morgan/Morgan%20BSL%20first%20signs%20final.pdf
"Wataru Takei. "How do Deaf infants attain first signs?"
Anne Baker, Benice Woll. Sign language acquisition. pp 41-? Chronology of sign language"
Laura Ann Petitto. "Bilingual signed and spoken language acquisition from birth: implications for the mechanisms underlying early bilingual language acquisition".
Wataru Takei. "How Do Deaf Infants Attain First Signs?" Developmental Science, 4: 71-78.