Do helpers really help?

Bushtits have  the distinction of being one of the very first birds described as having "helpers at the nest." That was in a paper by Alexander Skutch in 1935 in which he described this unusual behavior in only three species.   And the bushtit was honored to be among them.  

A helper at the nest is a nonbreeding (so we think) feeder of the nestlings.  Sometimes they also defend the nest and, in the case of bushtits, also sleep in the nest at night.  The bushtit helpers Skutch described in Guatemala were young birds who were helping their parents raise another brood in the same season.  Since then, it's become abundantly clear that "helping behavior" is far more complex and confusing than Skutch had imagined.  I won't get into the details here.  

One of the most enduring questions about helping behavior is:  do helpers really help?  After all, more birds feeding at a nest may attract predators.  Young birds feeding nestlings may be bad at their job.  There are many reasons to believe "helpers" may be detrimental.  Or maybe their "help" is neutral.  Maybe they help but their help really doesn't do anything positive or negative.  Maybe they do it just for their own reasons too numerous to be listed here.  

This whole issue is very complicated, especially now that we have DNA to figure out if maybe there are even more complications to this interesting story.  So I'll stick to one interesting observation in April in Arizona that provided me with, I think, one possible answer for bushtits.  And I mean just one!   This is just the tip of a huge iceberg.  

One relatively cold day in Arizona, I had 4 nests that were ready for banding; the nestlings were all about 10 days old and the nests were easy to reach. We had been watching the nests almost every day and knew that one was attended by 3 birds (two adult males and one adult female).  The other three had only 2 birds:  one male and one female.  

The night before banding had been exceptionally cold for the season.  My recollection (without checking my notes which are buried in a box in my office) is that the evening low dipped well below 25 degrees.  That's very unusual for early April, even in the mountains.  Still, I expected nothing out of the ordinary.  Bushtit nests are nice, warm, down sleeping bags and the attendants sleep in the nest at night.  I fully expected all would be well. 

But it was not to be so.  The first nest contained a sad surprise:  5 cold, dead nestlings.  They were otherwise perfect.  They had obviously died overnight.  The parents were no longer in attendance, but that wasn't surprising as we were there mid-day and they must have had the morning to figure out that their kids were no longer hungry.  Very sad.  

We carried on and opened the next nest....with the same sad result.  A lovely brood of, you guessed it, 5 cold and dead nestlings -- all banding age and healthy-looking --- but dead.  And the same was true of the third nest.  

But the fourth nest was a different story.  This nest was the one nest with 3 birds attending.  At this nest, the nestlings were alive and well and we could carry on with our task of banding the little guys.  The attendants actively fed while we did this --- as I always remove only half the brood at a time to process so the attendants don't get upset by an empty nest.  (Fortunately, they can't count.)  

So why did this one brood survive the cold night?  I can think of some possibilities...all of which may be true and all of which involve the 3rd bird.  

The first possibility is that 3 adults sleeping in the nest during a cold night kept everyone warmer than in the nests with only two adults.  So these nestlings spent the night in relative warm and safe comfort. They weren't cold-stressed and that was due to the extra warmth provided by third bird.  

Another possibility is that 3 birds provisioned the nestlings the evening before and perhaps even in the early morning hours with more food --- and therefore energy --- giving them plenty of energy to deal with the unusual overnight cold.  

The final benefit the 3rd bird may have provided was extra food on a daily basis.  It may be that the surviving nestlings went into that cold night with greater reserves because they had been fed at a greater rate than the others with only 2 attendants.  (These are actually data I can look at once I extract those notebooks from the box!!)

Regardless of the mechanism and the small sample size, I am willing to suspect that this extra bird was indeed "helpful."  His actions may have saved the lives of the nestlings he was helping to feed.  And for him, sleeping in a warm nest most certainly saved his life as well.  

So, in this case, the helper was actually helping.  Chalk up one point for "helpers do help".......at least sometimes.