Abandoned Nestlings

[Axeman]

I was cleaning out my nesting boxes. One of my nest boxes is for tree swallows -- I hadn't cleaned it in 2 years. While cleaning it, I noticed 5 mumified nestlings. I couldn't tell if they were from this years next or last for sure, but it seemed like it was from the previous year...and about 15 m away is another nest box used by wrens and last year, I had noticed that activity had ceased...so assuming that they had fledged, I opened the box up to clean it and found 4 rotting nestlings...does anyone know of possible causes for the abandonment? Its particularly curious if both occurred in the same season.

[JTF]

I was cleaning out my nesting boxes. One of my nest boxes is for tree swallows -- I hadn't cleaned it in 2 years. While cleaning it, I noticed 5 mumified nestlings. I couldn't tell if they were from this years next or last for sure, but it seemed like it was from the previous year...and about 15 m away is another nest box used by wrens and last year, I had noticed that activity had ceased...so assuming that they had fledged, I opened the box up to clean it and found 4 rotting nestlings...does anyone know of possible causes for the abandonment? Its particularly curious if both occurred in the same season.

Here are some possible reason's:

1. House sparrows killed the young which has happened in my nest boxes.
2. Excessive Heat.
3. First time parent's, inexperienced.

[Axeman]

I doubt house sparrows because I don't see them where I live....cedar bush in grey bruce and the bodies didn't look traumatized...they looked kinda frozen in time as if they were in middle of begging for food from parents...

Excessive heat might be a possiblity for the swallow nest because its on my transformer pole...I have holes drilled in all my boxes for heat escape...but it appears that at least one clutch successfully fledged in both boxes...

Inexperience...well...another possibility.