The gemara discusses some of the parameters of the mitzvah of shiluach hakan. It is clear from the pesukim that if one would take the eggs and the mother, he would be in violation of a la'v and an a'seh - לא תקח האם על הבנים, שלח תשלח. If one would take only the mother, and leave the eggs behind - depends on the definition of על in the pasuk - is it like עם so that the torah is only saying that you can't take both, or is it an issur to take the mother while it is on the child? Rashi in chumash says that the pasuk means to say לא תקח האם בעודה על בניה, clearly holding that one will violate the issur even by taking the mother bird alone. The chacham tzvi (83) disagrees with rashi and holds that there is no l'av for taking the mother alone because the pasuk just forbids taking the mother "with" the children. The minchas chinuch (545) proves that rashi is correct from the gemara in chulin 141a that tries to find a case where there would be an aseh without the l'av. The only case the gemara was able to come up with is when you take the mother with the intent of sending it away, and then decide not to. From the fact that the gemara didn't say the simplest of cases such as where you only took the mother and left the children, implies that there is in fact a l'av even for just taking the mother.
When one takes only the children and leaves the mother there is definitely no l'av, but the gemara seems to learn out from the pasuk of שלח תשלח את האם, והדר, את הבנים תקח לך, that there is an issur d'oraysa (issur that develops from an aseh) to take the children in the presence of the mother. The minchas chinuch quotes the rambam (mechira 23:11) who implies that it is only a "gezeiras chachamim", but not an issur d'oraysa to take the children and leave the mother. The minchas chinuch asks that the doesn't understand where the rambam gets this idea from that it is only an issur d'rabonon? However, the strength of the question of the minchas chinuch is that the simple reading of the gemara implies that it is d'oraysa (as the shita mikubetzes writes explicitly), but fails to bring any proof from the gemara itself. It seems to me that the gemara itself strongly implies that it is d'oraysa, because the gemara asks, what is the case where there would only be gezel d'rabonon for taking the eggs - if he sent away the mother, the chatzer acquires the eggs and it would be gezel d'oraysa, and if he didn't send away the mother הא בעי שלוחה - he needs to send it away. Rashi explains that there is a chiyuv to send away the mother, why is it only a problem of gezel d'rabonon. Now, if the chiyuv of sending away the mother rather than taking the eggs is only d'rabonon, what is the question of the gemara - mabye the gemara prefers to say the issur gezel d'rabonon rather than the issur of taking the mother which is d'rabonon. It must be that it is a mitzvah d'oraysa to take the children without sending away the mother -then the question would be why do we consider taking the children to only be gezel d'rabonon, it is a d'oraysa violation of a לאו הבאה מכלל עשה. This proves that it is an issur d'oraysa like the minchas chinuch assumes, not just d'rabonon.