The picture below are some of the widow spider that I found in the backyard. I know these spiders are highly venomous and great harm can occur if get bitten. In the US, these little spiders can be found almost everywhere in the backyards, gardens, or any secluded, protected sites around the house. Total of 5 species of widow spider in the US and judging from the picture that I took, there are at least 2 species of widow spider: the infamous black widow spider and the brown widow spider. At night with long forceps and a flashlight, those spiders are easy catch. I usually catch around 20 adult widow spiders every couple of months and feed to all of my Shama Thrush on the next day. I killed the spider first before feeding to the bird and all of the birds just love to eat those spiders. So far, I have feed all of my shama thrush widow spider (few spiders every couple of months for each bird) for the last 2 years without any adverse effect to the bird. In fact, the bird just went crazy when seeing a spider and just can't wait to eat one just like giving candy to kid. :)
I don't know any benefit to the bird by eating spider but according to few researched paper, spider in general contains high level of taurine acid. Feeding baby bird with high taurine acid food, according to article in sciencedaily.com "... were on average much bolder and better at learning in adulthood, than their counterparts who were not fed the extra taurine." (http://www.sciencedaily.com/releases/2007/08/070824220328.htm) During the last 2 years of giving few spiders to the bird, I did not see parent shama bird feed their chicks any spider. The parents always eat all the spiders that I give them.