As far as I can tell, you don't actually need the date, name, address etc fields in Table B and you don't need the Donation field in Table A.
Define thie relationship between the two tables:
Donors::Donation > Mammals::constZero
define constZero as a calcualtion field with the constant number: 0 as its only term.
To get > instead of =, double clickt the relationship line between these two tables (table occurrences) in Manage | Database | Relationships to bring up a dialog box where you can make this change.
To see the data from Mammals appear when a donation is made, you can add the fields from Mammals directly to the Donors layout.
PS. It is also possible to set up looked up value field options that copy data from Mammals into specified fields in Donors whenever the value in the donation field is edited. I don't see the need for that here, but if you need to capture the current value of fields from a related table such that those values won't change when fields in Mammals are edited, that's how you'd set it up.