Spanish „a/al“ with a second infinitive

Spanish „a/al“ with a second infinitive
 

My today‘s question has to do with a second infinitive in Spanish, introduced sometimes by „a“, sometimes by „al“. I came across this „problem“ while reading an unknown poem by a poet cuyo nombre no me acuerdo.
Example No. 1 is about „a colgar“. No.2 about „al contemplar“.
Is number 2 a kind of a hidden gerund, could „ al contemplar“ be replaced by
„ contemplando“ ? Thank you.

1.Volverán las oscuras golondrinas/En tu balcón sus nidos a colgar.

2. Pero aquellas que el vuelo refrenaban/Tu hermosura y mi dicha al contemplar.

The English gerund acts as a noun. Its Spanish equivalent is the infinitive.
I like swimming. = Me gusta nadar. In English, we have a full infinitive and a bare infinitive. The «a colgar» in your first stanza is the equivalent of an English full infinitive.

The Spanish gerundio is equivalent to the English present participle, which is a verbal that also ends in 'ing', like the English gerund. But unlike the gerund, which always acts as a noun, the participio is used as an adjective and, by the name gerundio, it is used to form the second part of the progressive/continuous tense.

Another way to use the infinitivo is with a preceding el. This, of course, also acts as a noun. The ending of your second stanza is an example of this, coupled with a preceding 'a' to produce the equivalent of an English full infinitive.

So, the "hidden gerund" is actually an alternate full infinitive, using English vernacular. You can't substitute the adjectival participio.

El infinitivo puede ser sustantivo y verbo a la vez en español !
This rule was new to me.
Thank you for the complexity of your reply. I am overwhelmed by it.