In Scotland this bird prefers Larch & Spruce Plantations (these are not native to Scotland) whilst the Parrot/Scottish Crossbill complex prefer Scots Pine. Scots Pine cones are bulkier & harder to open so Crossbills reliant on these need a larger more powerful bill.
Even in the hand it is difficult to separate Parrot & Scottish Crossbill as biometrics overlap & plumage is identical. So why treat them as separate species? Well the birds known as Scottish Crossbills only pair up with other Scottish Crossbills & their call/contact notes consistently differ. By call & behaviour these birds are effectively genetically isolating themselves from sympatric species effectively creating a new species!