Yes indeed!
This can be fixed if the owner is approachable and shows willingness to work on the problem. If the owner is not approachable, you need to hand the problem over to the landlord.
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I hope you'll try talking to the neighbors again. From experience, problems are solved more quickly, more effectively, if friendly relations are maintained.
Keep it simple, just say that you can hear the dog barking from/during (timeframe). It would be a good idea to give concrete examples rather than 'barking too much' as that would A.) show the owner there is a real problem to be taken seriously and B.) when the dog barks might give the owner clues as to how to address the problem.
How I would steer the conversation next would depend on the response from the owners. If the owner opened up to me about having dificulties with the pup I would give them some tips, suggest area trainers to help, suggest day care options etc. But I'd only do that if the owners asked for advice, as unsolicited advice is never well received, and can often backfire.
As as side note:
Even with the best will in the world, with the most responsible owner who is getting help from skilled behaviorists, barking is not an easy thing to train out. It does take time. And time. And more time. Progress generally does not adhere to a checklist. Especially with an adolescent dog in the throes of puberty. If the owner is approachable and sincerely trying to improve the situation, maybe you two could come to some kind of agreement to give him the time needed to address the barking while seeking ways to minimizing the disturbance.
If the owner gets aggressive, is uncooperative or otherwise blows you off then going to the landlord is the right next step. Make it the landlord's problem, you should not have to deal with that kind of person.
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And now a background comment:
You mentioned your impression that the dog might be mixed with a bull breed. I bring this up because Basel is a BSL canton.
NOW it is notoriously difficult to guess breed mixes, especially when a young dog is still growing into it's adult body the head can look larger than it eventually will. Most people guess breeds wildly incorrectly, potentially condemning dogs unfairly.
BUT be aware that if the dog is mixed with a bull breed the owner should have applied for a permit to keep him, and is required to take specific training classes. If applicable I would hope, for the dog's sake, the owner knows this and is compliant.
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If you have concerns over the dog's welfare and if the owner is not approachable, an option is to speak with the BS Veterinäramt.
But never, ever, put yourself in a confrontational situation when you suspect an owner of animal abuse. These situations can escalate. Make a report, leave the rest to the professionals.
I don't know how responsive BS is to such reports. Some cantons/communites take it seriously, others turn a blind eye.
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I hope to doG that the owner is approachable, and is proactive in getting some help for that poor pup.
And I sincerely hope that you get the peace and quiet that you are entitled to.
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And yet again I am shaking my head at the 'throwing out the baby with the bathwater' that was the abolition of the SKN courses. Flawed as they were, at least they got everyone into minimal basic training and made owners aware of their responsibilities to their dogs, their neighbors, and society at large. These kinds of situations show why we need some kind of training mandate.